CFD Simulation of VIV PDF

CFD Simulation of Vortex Induced
Vibration of a Cylindrical Structure
Muhammad Tedy Asyikin
Coastal and Marine Civil Engineering

Submission date: June 2012
Supervisor:
Hans Sebastian Bihs, BAT
Norwegian University of Science and Technology

Department of Civil and Transport Engineering
NORWEGIAN UNIVERSITY OF SCIENCE AND TECHNOLOGY

DEPARTMENT OF CIVIL AND TRANSPORT ENGINEERING
Report Title:
CFD Simulation of Vortex Induced Vibration
of a Cylindrical Structure
Date: June 11, 2012.

No. of pages (incl. appendices): 83
Master Thesis
Project
Work
Name:
Professor in charge/supervisor:
Hans Bihs
Other external professional contacts/supervisors:
-
Abstract:
This thesis presents the investigation of the flow characteristic and vortex induced vibration
(VIV) of a cylindrical structure due to the incompressible laminar and turbulent flow at
Reynolds number 40, 100, 200 and 1000. The simulations were performed by solving the
steady and transient (unsteady) 2D Navier-Stokes equation. For Reynolds number 40, the
simulations were set as a steady and laminar flow and the SIMPLE and QUICK were used as
the pressure-velocity coupling scheme and momentum spatial discretization respectively.
Moreover, the transient turbulent flow was set for Re 100, 200 and 1000 and SIMPLE and
LES (large Eddy Simulation) were selected as the pressure-velocity coupling solution and the
turbulent model respectively.
The drag and lift coefficient (Cd and Cl) were obtained and verified to the previous studies
and showed a good agreement. Whilst the vibration frequency (fvib), the vortex shedding
frequency (fv), the Strouhal number (St) and the amplitude of the vibration (A) were also
measured.
Keywords:
1. CFD Simulation
2. VIV
3. Cylinder

(signature)
i
NTNU
Norwegian University of Science
and Technology
Faculty of Engineering Science

and Technology
Department of Civil and
Transport Engineering
Division: Marine Civil

Engineering
Postal address:
Hgskoleringen 7A
7491 Trondheim
Phone: 73 59 46 40
Telefax: 73 59 70 21
Master Thesis
Spring 2012
Student: Muhammad Tedy Asyikin
CFD Simulation of Vortex Induced Vibration
of a Cylindrical Structure
Background:
This thesis presents the investigation of the flow characteristic and vortex induced
vibration (VIV) of a cylindrical structure due to the incompressible laminar and turbulent
flow at Reynolds number 40, 100, 200 and 1000. The simulations are performed by
solving the steady and transient (unsteady) 2D Navier-Stokes equation. For Reynolds
number 40, the simulations were set as a steady and laminar flow and the SIMPLE and
QUICK were used as the pressure-velocity coupling scheme and momentum spatial
discretization respectively. Moreover, the transient turbulent flow was set for Re 100,
200 and 1000 and SIMPLE and LES (large Eddy Simulation) were selected as the
pressure-velocity coupling solution and the turbulent model respectively.
The drag and lift coefficient (Cd and Cl) were obtained and verified to the previous
studies and showed a good agreement. Whilst the vibration frequency (fvib), the vortex
shedding frequency (fv), the Strouhal number (St) and the amplitude of the vibration (A)
were also measured.
iii
Objective of the thesis work

The main objectives of this thesis are:
1. To investigate the flow pattern and characteristic around a cylindrical
structure.
2. To investigate the vibrations of a cylindrical structure.
Scope of work (work plan)

The thesis work includes, but is not limited to, the following:
1. Familiarization with the concept of flow around cylindrical structure.
2. Understanding the vibration phenomena of cylindrical structure.
3. Determining the important features of the problem of flow around cylindrical
structure.
4. Performing the simulation of CFD
a. Defining the simulation goals.
b. Creating the model geometry and mesh.
c. Setting up the solver and physical model.
d. Computing and monitoring the solution.
e. Examining and saving the result.
f. Consider revisions to the numerical or physical model parameters, if
necessary.
5. Compare and discuss any findings and results.
General about content, work and presentation

The text for the master thesis is meant as a framework for the work of the candidate.
Adjustments might be done as the work progresses. Tentative changes must be done in
cooperation and agreement with the professor in charge at the Department.
In the evaluation thoroughness in the work will be emphasized, as will be documentation
of independence in assessments and conclusions. Furthermore the presentation (report)
should be well organized and edited; providing clear, precise and orderly descriptions
without being unnecessary voluminous.
Submission procedure
On submission of the thesis the candidate shall submit a CD with the paper in digital
form in pdf and Word version, the underlying material (such as data collection) in
digital form (eg. Excel). Students must submit the submission form (from DAIM) where
both the Ark-Bibl in SBI and Public Services (Building Safety) of SB II has signed the
form. The submission form including the appropriate signatures must be signed by the
department office before the form is delivered Faculty Office.
iv
Documentation collected during the work, with support from the Department, shall be
handed in to the Department together with the report.
According to the current laws and regulations at NTNU, the report is the property of
NTNU. The report and associated results can only be used following approval from
NTNU (and external cooperation partner if applicable). The Department has the right to
make use of the results from the work as if conducted by a Department employee, as
long as other arrangements are not agreed upon beforehand.
Start and submission deadlines

The work on the Master Thesis starts on January 16, 2012.
The thesis report as described above shall be submitted digitally in DAIM at the latest at
3pm June 11, 2012.
Professor in charge: Hans Bihs
Trondheim, June 11, 2012.
_______________________________________
Hans Bihs
ABSTRACT
This thesis presents the investigation of the flow characteristic and vortex induced
vibration (VIV) of a cylindrical structure due to the incompressible laminar and turbulent
flow at Reynolds number 40, 100, 200 and 1000. The simulations were performed by
solving the steady and transient (unsteady) 2D Navier-Stokes equation. For Reynolds
number 40, the simulations were set as a steady and laminar flow and the SIMPLE and
QUICK were used as the pressure-velocity coupling scheme and momentum spatial
discretization respectively. Moreover, the transient turbulent flow was set for Re 100,
200 and 1000 and SIMPLE and LES (large Eddy Simulation) were selected as the
pressure-velocity coupling solution and the turbulent model respectively.
The drag and lift coefficient (Cd and Cl) were obtained and verified to the previous
studies and showed a good agreement. Whilst the vibration frequency (f vib), the vortex
shedding frequency (fv), the Strouhal number (St) and the amplitude of the vibration (A)
were also measured.
Keywords :
1.
2.
3.
vi
CFD Simulation
VIV
Cylindrical Structure
ACKNOWLEDGEMENTS
This thesis is a part of curriculum of master program in Coastal and Marine Civil
Engineering and has been performed under supervision of Adjunct Associate Professor
Hans Bihs at the Department of Civil and Transport Engineering, Norwegian University
of Science and Engineering (NTNU). I highly appreciate for his guidance and advices,
especially for his willingness to spare his valuable time for discussions and encouraging
me.
I would like to thank Associate Professor ivind Asgeir Arntsen as a program
coordinator for the guidance and assistances, which make my study going well and
easier. I would also like to thank Mr. Love Hkansson (EDR Support team) for the help,
discussions and giving me enlightenment in my work, especially regarding to the Fluent
simulations.
I would also like to thank all my office mates Tristan, Arun, Oda, Nina, Kevin, Morten
and Jill for sharing the time together for last one year. Last but not least I would like to
thank Miss Elin Tonset for the assistance in administration.
vii
TABLE OF CONTENTS
ABSTRACT
vi
ACKNOWLEDGEMENTS
vii
TABLE OF CONTENTS
viii
LIST OF FIGURES
xi
LIST OF TABLES
xiii
LIST OF SYMBOLS
xiv
1 INTRODUCTION ..................................................................................................... I-1

1.1. Background ...................................................................................................... I-1
1.2. Scope of Work ................................................................................................. I-2
1.3. Project Objectives ............................................................................................ I-2
1.4. Structure of the Report..................................................................................... I-2
2 FLOW AROUND CYLINDRICAL STRUCTURE .................................................. II-1
2.1. Basic Concept ................................................................................................. II-1
2.1.1. Regime of Flow ......................................................................................... II-1
2.1.2. Vortex Shedding ........................................................................................ II-3
2.1.3. Drag and Lift Forces ................................................................................ II-4
2.1.4. Others Dynamic Numbers ......................................................................... II-5
2.2. Vortex Induced Vibration ............................................................................... II-7
2.2.1. Solution to Vibration Equation ................................................................. II-7
2.2.2. Damping of Fluid .................................................................................... II-10
2.2.3. Cross Flow and In-Line Vibration .......................................................... II-11
3 COMPUTATIONAL FLUID DYNAMIC .............................................................. III-1
3.1. Introduction ................................................................................................. III-1
3.1.1. Concervation Laws of Fluid Motion ....................................................... III-2
3.1.2. General Transport Equation ................................................................... III-3
3.2. Methodology of CFD .................................................................................... III-4
viii
3.2.1. Pre-processing Stage ............................................................................... III-5

3.2.2. Solving Stage ........................................................................................... III-8
3.3. Turbulent Flows .......................................................................................... III-10
3.3.1. Direct Numerical Simulations ............................................................... III-10
3.3.2. Large Eddy Simulation (LES) ................................................................ III-10
3.3.3. Reynolds Averaged Navier-Stokes (RANS) ............................................ III-11
3.4. Solution Algorithms for Pressure-Velocity Coupling Equation .................. III-11
3.4.1. SIMPLE ................................................................................................. III-11
3.4.2. SIMPLER ............................................................................................... III-11
3.4.3. SIMPLEC ............................................................................................... III-11
3.4.4. PISO ...................................................................................................... III-12
4 VALIDATION OF CFD SIMULATION ................................................................ IV-1
4.1. The Determination of Domain and Grid Type ............................................. IV-1
4.1.1. The Evaluation of the Grid Quality ........................................................ IV-1
4.1.2. The Result of the Evaluation of the Grid Quality.................................... IV-7
4.1.3. The Grid Indepence Study ...................................................................... IV-8
4.2. The Validations of the Results ..................................................................... IV-9
4.2.1. The Steady Laminar Case at Re 40 ......................................................... IV-9
4.2.2. The Transient (Unsteady) Case at Re 100, 200 and 1000 .................... IV-11
5 THE VORTEX SHEDDING INDUCED VIBRATION SIMULATIONS ............... V-1
5.1. Simulation Setup ............................................................................................ V-1
5.1.1. Turbulent Model ....................................................................................... V-1
5.1.2. Pressure-Velocity Coupling Scheme ......................................................... V-7
5.1.3. Momentum Spatial Discretization ............................................................V-2
5.2. The Result of the VIV Simulation .................................................................. V-2
5.3. Discussion ...................................................................................................... V-2
5.3.1. Effect of The Fluid Damping .................................................................... V-5
5.3.2. The Displacement of The Cylinder (CF Direction) .................................. V-5
5.3.3 The Displacement Initiation...................................................................... V-6
ix
6 CONCLUSION ....................................................................................................... VI-1

6.1. Conclusion .................................................................................................... VI-1
6.2. Recommendations ........................................................................................ VI-2
REFERENCES
APPENDIX A - Problem Description
APPENDIX B - UDF of 6DOF Solver
APPENDIX C - ANSYS Fluent Setup
LIST OF FIGURES
Figure 2.1. Strouhal number for smooth circular cylinder ........................................... II-3
Figure 2.2. Separation point of the subcritical regime and supercritical regime .......... II-4
Figure 2.3. Oscillating drag and lift forces traces ......................................................... II-5
Figure 2.4. Idealized description of a vibrating structure. ............................................ II-7
Figure 2.5. Free vibration with viscous damping ......................................................... II-9
Figure 2.6. Defenition sketch of vortex-induced vibrations ....................................... II-12
Figure 2.7. Fengs experimental set up....................................................................... II-13
Figure 2.8. Fengs experiment responses. .................................................................. II-14
Figure 2.9. In-line vibrations at Re = 6 x 104. ............................................................ II-15
Figure 3.1. Basic concept of CFD simulation methodology. .......................................III-4
Figure 3.2. A rectangular box solution domain (L x D). ............................................. III-5
Figure 3.3. A structured grid ....................................................................................... III-6
Figure 3.4. Block- structured grid ............................................................................... III-7
Figure 3.5. Unstructured grid ...................................................................................... III-7
Figure 3.6. Schematic representation of turbulent motion ......................................... III-10
Figure 4.1. Ideal and skewed triangles and quadrilaterals .......................................... IV-2
Figure 4.2. Aspect ratio for triangles and quadrilaterals ............................................. IV-3
Figure 4.3. Jacobian ratio for triangles and quadrilaterals .......................................... IV-3
Figure 4.4. Circular domain with quadrilateral grids .................................................. IV-4
Figure 4.5. Detail view of circular domain grids ........................................................ IV-4
Figure 4.6. Square domain with quadrilateral grids.................................................... IV-5
Figure 4.7. Rectangular domain with quadrilateral grids ........................................... IV-5
Figure 4.8. Detail view of the rectangular domain grids ............................................ IV-6
Figure 4.9. Wireframe arrangement of rectangular domain ....................................... IV-6
Figure 4.10. Rectangular domain with smooth quadrilateral grids ............................. IV-6
Figure 4.11. Detail view of the smooth grids close to the cylinder wall .................... IV-7
Figure 4.12. Result of the grid independence study ................................................... IV-9
Figure 4.13. Vortice features for Re = 40 ................................................................. IV-10
Figure 4.14. Simulation result of two identical vortices at Re = 40 ......................... IV-11
Figure 4.15. The time history of Cl and Cd for transient laminar flow case ............ IV-13
Figure 4.16. The time history of Cl and Cd for transient turbulent case (LES). ....... IV-14
xi
Figure 5.1. Lift coefficient and displacement (A/D) of the cylinder at Re = 100 ........ V-2
Figure 5.2. Spectrum of CF response frequencies (fv and fvib) at Re = 100 ................. V-3
Figure 5.3. Lift coefficient and displacement (A/D) of the cylinder at Re = 200 ........ V-3
Figure 5.4. Spectrum of CF response frequencies (fv and fvib) at Re = 200 ................. V-4
Figure 5.5. Lift coefficient and displacement (A/D) of the cylinder at Re = 1000. ..... V-4
Figure 5.6. Spectrum of CF response frequencies (fv and fvib) at Re = 1000 ............... V-5
Figure 5.7. The development of the displacement as function of flow time ................ V-7
xii
LIST OF TABLES
Table 2.1. Flow regime around smooth, circular cylinder in steady current ................ II-2
Table 4.1. Value of Skewness .................................................................................... IV-2
Table 4.2. Grid quality measurements ........................................................................ IV-7
Table 4.3. Result of the different grid size simulation at Re = 40 .............................. IV-8
Table 4.4. Vortice features measurements for Re = 40 ........................................... IV-10
Table 4.5. Experimental results of the Cl and Cd at Re 100, 200 and 1000 ............. IV-12
Table 5.1. Result of the VIV simulation ....................................................................... V-6
xiii
LIST OF SYMBOLS
Re
D
U
v
St
Fv,
L
Um
T
A
fn
KC
Fr
g
m
c
k
xiv
Reynolds number
cylinder diameter
the flow velocity
kinematic viscocity
Strouhal number
vortex shedding frequency
the lift force
the drag force
amplitudes of the oscillating lift
amplitudes of the oscillating drag
the mean drag
the phase angle
lift coefficient of the oscillating lift
drag coefficient of the oscillating drag
mean drag coefficient
fluid density
cylinder length
maximum flow velocity
period
displacement amplitude
true reduced velocity
nominal reduced velocity
natural frequency
Keulegan-Karpenter number
Froudes number
gravity force
cylinder mass
damping factor
stiffness
the total damping factor
Chapter I - Introduction
INTRODUCTION
1.1. Background
Vibration of a cylindrical structure, i.e. pipeline and riser, is an important issue in
designing of offshore structure. Vibrations can lead to fatigue damage on the structure
when it is exposed to the environmental loading, such as waves and currents. In recent
years, the exploration of oil and gas resources has advanced into deep waters, thousands
of meters below sea surface, using pipelines and risers to convey the hydrocarbon fluid
and gas.
For deep water, there will only be current force acting on the structure. As wave forces
reduce with depth, they become insignificant in very deep water. In this case, the
interaction between the current and the structure can give rise to different forms of
vibrations, generally known as flow-induced vibrations (FIV).
The availability of powerful super computers recently has given an opportunity to users
in performing simulations in order to obtain optimum results as well as in numerical
modeling of fluid dynamics. The numerical modeling in fluid dynamics, so-called
computational fluid dynamics (CFD), therefore, becomes very important in the design
process for many purposes as well as in marine industry.
By the need of offshore oil and gas production in deepwater fields, numerical simulation
of offshore structure has been an active research area in recent years. Experiments are
sometimes preferable to provide design data and verification. However, offshore
structures have aspect ratios that are so large that model testing is constrained by many
factors, such as experimental facility availability and capacity limits, model scale limit,
difficulty of current profile generation, and cost and schedule concerns. Under such
conditions, CFD simulation provides an attractive alternative to model tests and also
provides a cost effective alternative.
CFD Simulation of Vortex Induced Vibration of a Cylindrical Structure
I-1
1.2. Scope of Work

The thesis work includes, but is not limited to, the following:
1. Familiarization with the concept of flow around cylindrical structure.
2. Understanding the vibration phenomena of cylindrical structure.
3. Determining the important features of the problem of flow around cylindrical
structure.
4. Performing the simulation of CFD
a. Defining the simulation goals.
b. Creating the model geometry and mesh.
c. Setting up the solver and physical model.
d. Computing and monitoring the solution.
e. Examining and saving the result.
f. Consider revisions to the numerical or physical model parameters, if
necessary.
5. Compare and discuss any findings and results.
1.3. Project Objectives

The main objectives of the thesis work are:
1. To investigate the flow pattern characteristic around cylindrical structure.
2. To investigate the vibrations of cylindrical structure due to the flow (current).
1.4. Structure of the Report

The thesis is organized in five main chapters. Chapter 1 is an introduction. Chapter 2
consists of the theory and information regarding flow around cylindrical structure.
Chapter 3 explains the CFD theories and the simulation of CFD. Chapter 4 gives the
analysis and discussions from the result. Finally, Chapter 5 gives the conclusions and
recommendations.
Chapter 1 is an introduction of the thesis work. It describes a general overview of the
thesis work. The objectives and the structure of the report are also described in this
chapter.
Chapter 2 is the explanation of the theories regarding the flow around cylindrical
structure. This chapter also describes a similar experimental work that had been carried
out, as a comparison to the simulation results later on.
Chapter 3 is an explanation of CFD. This section describes a theory in CFD as well as its
simulation. The simulation of CFD consists of some procedures, which includes 1)
defining the simulation goals, 2) creating the model geometry and mesh, 3) setting up the
I-2
solver and physical model, 4) computing and monitoring the solution, 5) examining and
saving the result and 6) revisions, if necessary.
Chapter 4 is the result and discussion part. It presents the results of the CFD simulation
from Chapter 3. It also includes the discussion of the results by comparing to those of
similar previous experimental works. The last part of this chapter gives the summary of
the results.
Finally, Chapter 5 is the conclusions and recommendations part. This chapter gives the
conclusions from Chapter 4. The recommendations are given for any further work
related to this thesis topic.
I-3
Chapter II Flow Around Cylindrical Structure
2
2.1.
FLOW AROUND
CYLINDRICAL STRUCTURE
Basic Concept
When a structure, in this case, a cylindrical structure subjected to the fluid flow,
somehow the cylinder might experience excitations or vibrations. These vibrations
known as the flow induced vibrations can lead to the fatigue damage to the structure.
Hence, it is essential to take those vibrations into considerations whilst designing many
structures, particularly the cylindrical structure.
2.1.1. Regime of Flow
One of the non dimensionless hydrodynamic numbers that is used to describe the flow
around a smooth circular cylinder is the Reynolds number (Re). By the definition, the
Reynolds number is the ratio of the inertia forces to viscous forces and formulated as
(2.1)
in which D is the diameter of the cylinder, U is the flow velocity and v is the kinematic
viscosity of the fluid.
Flow regimes are obtained as the result of tremendous changes of the Reynolds number.
The changes of the Reynolds number create separation flows in the wake region of the
cylinder, which are called vortices. At low values of Re (Re < 5), there no separation
occurs. When the Re is further increased, the separation starts to occur and becomes
unstable and initiates the phenomenon called vortex shedding at certain frequency. As
the result, the wake has an appearance of a vortex street as can be seen in Table 2.1.
II-1
Table 2.1. Flow regime around smooth, circular cylinder in steady current, adapted from [14].
No separation
Creeping flow
A fixed pair of
symmetric vortices
5 < Re < 40
Laminar vortex street
40 < Re < 200
Transition to
turbulence
in the wake
200 < Re < 300
Wake completely
turbulent.
A. Laminar boundary
layer separation
A. Laminar boundary
layer separation .
B. Turbulent boundary
layer separation, but
boundary layer
laminar
B. Turbulent boundary
layer separation:the
boundary layer
part1y laminar partly
turbulent
C. Boundary layer
completely turbulent
at one side
C. Boundary layer
completely turbulent
at two sides
II-2
Re < 5
300 < Re < 3x105

Subcritical
3x105 < Re < 3.5 x 105

Critical (Lower
transition)
3.5 x 105 < Re < 1.5 x
l06
Supercritical
1.5 x 106 < Re < 4 x106
Upper transition
4 x l06 < Re
Transcritical
2.1.2. Vortex Shedding

The Vortex shedding phenomenon appears when pairs of stable vortices are exposed to
small disturbances and become unstable at Re greater than 40. For these values of Re,
the boundary layer over the cylinder surface will separate due to the adverse pressure
gradient imposed by the divergent geometry of the flow environment at the rear side of
the cylinder.
As mentioned in the previous section, vortex shedding occurs at a certain frequency,
which is called as vortex shedding frequency ( ). This frequency normalized with the
flow velocity U and the cylinder diameter D, can basically be seen as a function of the
Reynolds number. Furthermore, the normalized vortex-shedding frequency is called
Strouhal number (St), and formulated as:
(2.2)
The relationship between Re and St can be shown in Figure 2.1.
Fig. 2.1. Strouhal number for smooth circular cylinder, adapted from Sumer [14].
The large increase in St at the supercritical region is caused by the delay of the boundary
separation. It is known that the separation point of the subcritical regime is different
from that of the supercritical regime as shown in Figure 2.2. At the supercritical flow
regime, the boundary layers on both sides of the cylinder are turbulent at the separation
point. Consequently, the boundary layer separation is delayed since the separation point
moves downstream. At this point, the vortices are close to each other and create faster
rate than the rate in the subcritical regime, thereby leading to higher values of the
Strouhal number.
II-3
Fig. 2.2. Separation point of the subcritical regime and supercritical regime, adapted
Sumer [14].
When Re reaches the value of 1.5 x 106, the boundary layer completely becomes
turbulent at one side and laminar at the other side. This asymmetric situation is called the
lee-wake vortices. What happens next is that lee-wake vortices inhibit the interaction of
these vortices, resulting in an irregular and disorderly vortex shedding. When Re is
increased to values larger than 4.5 x 106 (transcritical regime), the regular vortex
shedding is re-established and St takes the values of 0.25 0.30.
2.1.3. Drag and Lift Forces
As the result of the periodic change of the vortex shedding, the pressure distribution of
the cylinder due to the flow will also change periodically, thereby generating a periodic
variation in the force components on the cylinder. The force components can be divided
into cross-flow and in-line directions. The force of the cross-flow direction is commonly
named as the lift force (FL) while the latter is named as the drag force (FD). The lift force
appears when the vortex shedding starts to occur and it fluctuates at the vortex shedding
frequency. Similarly, the drag force also has the oscillating part due to the vortex
shedding, but in addition it also has a force as a result of friction and pressure difference;
this part is called the mean drag. Both of the lift and drag forces are formulated as
follows:
(2.3)
(2.4)
and
are the amplitudes of the oscillating lift and drag respectively and
mean drag. The vortex shedding frequency is represented by
, and
is the
d is
the phase angles between the oscillating forces and the vortex shedding.
An experiment performed by Drescher in 1956 [5] which is described in Sumer [14]
traced the drag and lift forces from the measured pressure distribution as shown in
Figure 2.3. From the figure, it can be seen that the drag and lift forces oscillate as a
function of the vortex shedding frequency.
II-4
Fig. 2.3. Oscillating drag and lift forces traces, adapted from Sumer [14].
CD and CL are the dimensionless parameters for drag and lift forces respectively, and can
be derived as:
(2.5)
(2.6)
(2.7)
where , L, D and U are the fluid density, cylinder length, cylinder diameter and flow
velocity respectively.
2.1.4. Other Hydrodynamic Numbers

Other hydrodynamic numbers, which are dimensionless parameters that are often used to
study flow induced vibration, apart from Re and St that have been mentioned earlier, will
be described briefly in the following sections.
Keulegan-Carpenter Number, KC
The Keulegan-Carpenter number is used to predict the flow separation around a body
and whether the drag or inertia terms dominate in the Morison formula. It is also an
important parameter to describe harmonic oscillating flows and is formulated as:
II-5
(2.8)
where Um is the maximum velocity of the flow during one period T, D is the cylinder
diameter and A is the displacement amplitude of the fluid.
Reduced Velocity, Ured
Reduced velocity can be divided into two types, true reduced velocity (Ured,true) and
nominal reduced velocity (Ured,nom). The true reduced velocity is based on the frequency
at which the cylinder is actually vibrating (fn) whilst the nominal reduced velocity is
based on the nominal natural frequency (fn0), e.g. natural frequency in still water. Both
are formulated as follows:
(2.9)
(2.10)
This number is a useful parameter to present the structure response along the lock-in
range.
Froude Number, Fr
The Froude number is the key parameter in prediction of free surface effects, for
instance the effect of waves on ships. This number is always used in model testing in
waves and formulated as the following:
(2.11)
where the g is the gravity force and L is the structure length.
Roughness
The surface roughness is of importance in many ways. It will influence the vortex
shedding frequency. Increasing roughness will decrease Re at which transition to
turbulence occurs. Roughness is often measured as the ratio of the average diameter of
the roughness features, k, divided by the cylinder diameter D.
II-6
2.2.
Vortex Induced Vibration
In this section, the theory of the vortex induced vibration will be described, particularly
for cylindrical structure. It covers the solution to the vibration equation, structure and
fluid damping, vibration of cylindrical structure and suppression of vibrations.
2.2.1.
Solution to Vibration Equation
The sketch of the classic flow around cylindrical structure can be drawn as shown in
Figure 2.4. A free vibration of an elastically mounted cylinder is represented by an
idealized description of a vibrating structure.
y
Cylinder
F
Fluid Flow
c
Fig. 2.4. Idealized description of a vibrating structure.
In this thesis, the structure is conditioned as a vibration-free elastically cylinder. It means

that the cylinder is free to respond the vibrations. This condition is useful to find
amplitude, frequency and phase angle of a vibrating cylinder and helpful in studying
flow-visualization of the wake of the cylinder.
The differential motion equation of the system shown in Figure 2.4 is formulated as:
(2.12)
in which m is the total mass of the system and dot over the symbols indicates
differentiation with respect to time. For a free vibration system, i.e. no external forces
working on the system (F=0), the solutions can, therefore, be differentiated into two
conditions: without and with viscous damping.
II-7
Free vibrations without viscous damping

For free vibrations without viscous damping, the equation of motion will be
(2.13)
Since m and k are positive, the solution is
(2.14)
where
is the amplitude of vibrations and
and formulated as
is the angular frequency of the motion,
(2.15)
Free vibrations with viscous damping

In this case, the viscous damping in non-zero, therefore the equation of motion becomes:
(2.16)
The solution of Eq. (2.16) could be in an exponential form:
(2.17)
By inserting Eq. (2.17) into Eq. (2.16), an auxiliary equation will be obtained as
follows:
(2.18)
The two r values are calculated by
(2.19)
As the result, the general solution of this case is
(2.20)
The solution of Eq. (2.20) depends on the square root of (c2 - 4mk), i.e. c2 > 4mk (case
1) and c2 < 4mk (case 2).
For case 1, the values of r1 and r2 are real. Therefore, C1 and C2 must be determined
from the initial conditions, for example, for
. As a consequence,
C1 and C2 can be calculated as
II-8
(2.21)
Hence, the general solution of Eq. (2.20) becomes
(2.22)
For case 2, where c2 < 4mk, the roots r1 and r2 are complex:
(2.23)
The real part of the solution (Eq. 2.20) may be written in the following form
(2.24)
where
(2.25)
The solutions for both cases are illustrated in Figure 2.5.
Fig. 2.5. Free vibration with viscous damping, adapted from Sumer [14].
II-9
2.2.2.
Damping of Fluid
Damping is the ability of a structure to dissipate energy. In this case, the role of damping
in flow induced vibration is to limit the vibrations. There are three types of damping:
structural damping, material damping and fluid damping. The structural damping is
generated by friction, impacting and rubbing between the structures or parts of the
structures. The material damping is generated by internal energy dissipation of materials
such as rubber, while the latter (i.e., the fluid damping) is generated as the result of
relative fluid movement to the vibrating structure. The structural damping has been
described in the previous section. This section will, therefore, only focus on the fluid
damping description.
A system surrounded by fluid as shown in Figure 2.4 is considered to describe the fluid
damping. This system has not only damping due to the structure but also due to the fluid.
Under this situation, the structure will be subjected to a hydrodynamic force F.
Therefore, the equation of motion will be
(2.26)
in which F is the Morison force per unit length and formulated as
(2.27)
The second term on the right hand side,
, may be written in the form (
),
in which m is the hydrodynamic mass per unit length. Therefore, Eq. (2.26) becomes
(2.28)
From Eq. (2.28), it can be seen that the system has an additional mass m and resistance
force
. These changes will obviously affect the total damping. The solution
of Eq. (2.28) is
(2.29)
in which
and
respectively, and
are the total damping factor and damping angular frequency

is formulated
(2.30)
II-10
where
(2.31)
is called the undamped natural angular frequency. Since
small, Eq. (2.30) can then be written as
contribution is normally
(2.32)
The natural frequency of the structure, fn is formulated as
(2.33)
The total damping ratio,
damping component
, consists of the structural damping component (
and fluid
and is formulated as
(2.34)
(2.35)
(2.36)
2.2.3.
Cross-Flow and In-Line Vibrations of Cylindrical Structure
Vibrations of structure emerge as the result of periodic variations in the force

components due to the vortex shedding. The vibrations can be differentiated into crossflow and in-line vibrations. The cross-flow vibration is caused by the lift force whilst the
in-line one is caused by the drag force. Both vibrations are commonly called the vortexinduced vibrations.
II-11
Cyinder
Cross-flow
Vibration
Fluid Flow
Fluid Flow
In-line
Vibration
Fig. 2.6. Defenition sketch of vortex-induced vibrations.
The best description of the cross-flow vibrations was carried out by Feng [6]. He
mounted a circular cylinder with one degree of freedom and exposed it to an increased
air flow in small increments starting from zero. The vortex shedding frequency (fv), the
vibration frequency (f), the amplitude of vibration (A) and also the phase angle (),
which is the phase difference between the cylinder vibration and the lift force, were
measured in his experiment. The set up and the plots obtained from his experiment are
depicted in Figures 2.7 and 2.8 respectively.
II-12
Fig. 2.7. Fengs experimental set up, adapted from Feng [6].
From Figure 2.8a, it can be seen that the vortex shedding frequency, fv, follows the
stationary-cylinder Strouhal frequency, which is represented as dashed reference line,
until the reduced velocity, Vr, reaches the value of 5. When the flow speed increases, fv
does not follow the Strouhal frequency, in fact it begins to follow the natural frequency,
fn, of the system, which is represented by the full horizontal line f/fn = 1. This situation
takes place at the range of 5 < Vr < 7.
It can be concluded that in the range of 5 < Vr < 7, the vortex shedding frequency is
locked into the natural frequency of the system. This is known as the lock-in
phenomenon. At this range, fv, fn and f have the same values, therefore, the lift force
oscillates with the cylinder motion resulting in large vibration amplitudes.
For Vr > 7, the shedding frequency suddenly unlocks and jumps to assume its Strouhal
value again. This occurs around Vr = 7.3. Moreover, the vibration still occurs at the
natural frequency, thereby reducing the vibration amplitude as shown in Figure 2.8b.
This is caused only by the vortex shedding without the motion of the cylinder.
II-13
Figure. 2.8. Fengs experiment responses, adapted from Feng [6].
The in-line vibration of a structure is caused by the oscillating drag force and can be
differentiated into three kinds represented by the range of the reduced velocity, Vr. First,
at the range of
, which is called the first instability region. Second, at the
range of
, the so-called second instability region. The last occurs at higher
flow velocities where the cross-flow vibrations are observed. The first two kinds of the
in-line vibrations are shown in Figure 2.9.
II-14
Fig. 2.9. In-line vibrations at Re = 6 x 104, adapted from Sumer [14].
The first instability region in-line vibrations are caused by the combination of the normal
vortex shedding and the symmetric vortex shedding due to in-line relative motion of the
cylinder to that of the fluid. This vortex shedding creates a flow where the in-line force
oscillates with a frequency three times of the Strouhal frequency. Consequently, when
this frequency has the same value or close to that of natural frequency of the system,
the cylinder will start to vibrate. The velocity increases even further, the second
instability will occur when the in-line force oscillates with a frequency two times of the
Strouhal frequency. Hence, the large amplitude in-line vibrations will occur again when
the in-line frequency becomes equal to natural frequency of the system, fn.
II-15
Chapter III Computational Fluid Dynamics
3
3.1.
COMPUTATIONAL FLUID DYNAMICS
Introduction
Computational fluid dynamics, usually abbreviated as CFD, is a computer based

simulation or numerical modeling of fluid mechanics to solve and analyze problems
related to fluid flows, heat transfer and associated phenomena such as chemical
reactions. CFD provides a wide range application for many industrial and non-industrial
areas. CFD is a very powerful technique regarding the simulation of fluid flows.
The application of CFD began in 1960s when the aerospace industry has integrated
CFD techniques into the design, R&D and manufacturing of aircrafts and jet engines [7].
CFD codes are being accepted as design tools by many industrial users. Today, many
industrials, for instance, ship industry, power plant, machinery, electronic engineering,
chemical process, marine engineering and environmental engineering use CFD as a one
of the best design tools.
Unlike the model testing facility or experimental laboratory, in CFD simulations there is
no need for a big facility. Furthermore, CFD also offers no capacity limit, no model scale
limit and cost and schedule efficiency. Indeed, the advantages by using CFD compared
to experiment-based approaches can be concluded as follow:
a. Ability to assess a system that controlles experiments is difficult or impossible
to perform (very large system).
b. Ability to assess a system under hazardous conditions (e.g. safety study and
accident investigation).
c. Gives unlimited detail level of results.
This chapter describes basic theories and information of fluid dynamics and CFD. Given
theories and information are delivered in brief and general. For more detail, readers are
recommended to refer to the related sources.
III-1
Chapter III - Computational Fluid Dynamics
3.1.1.
Conservation Laws of Fluid Motion
As mentioned in the previous section, CFD is the science of predicting fluid flow, heat
and mass transfer, chemical reactions and other related phenomena. The CFD problems
are stated in a set of mathematical equations and are solved numerically. These set of
mathematical equations are based on the conservation laws of fluid motion, which are
conservation of mass, conservation of momentum, conservation of energy and etc. For
CFD problems related to fluid flow, the set of mathematical equations are based on the
conservation of mass and momentum.
A.
Conservation of Mass
The mass conservation theory states that the mass will remain constant over time in a
closed system. This means that the quantity of mass will not change and, the quantity is
conserved.
The mass conservation equation, also called the continuity equation can be written as:
(3.1)
Equation (3.1) is the general form of the mass conservation equation and is valid for
incompressible as well as compressible flows. The source
is the mass added the
system and any user-defined sources. The density of the fluid is and the flow of mass
in x, y and z direction is u, v and w.
B.
Conservation of Momentum
The conservation of momentum is originally expressed in Newtons second law. Like

the velocity, momentum is a vector quantity as well as a magnitude. Momentum is also a
conserved quality, meaning that for a closed system, the total momentum will not change
as long as there is no external force.
Newtons second law also states that the rate of momentum change of a fluid particle
equals the sum of the forces on the particle. We can differentiate the rate of momentum
change for x, y and z direction.
(3.2)
The momentum conservation equation for x, y and z direction can be written as follows:
(3.3)
III-2
(3.4)
(3.5)
The source
is defined as contribution to the body forces in the total force per unit
volume on the fluid. The pressure is a normal stress, is denoted p, whilst the viscous
stresses are denoted by .
3.1.2.
General Transport Equation
To derive the transport equation of viscous and incompressible fluids, the Navier-Stokes
equation is used. For a Newtonian fluid, which stress versus strain rate curve is linear,
the Navier-Stokes equation for x, y and z direction is defined as follows:
(3.6)
(3.7)
(3.8)
And the transport equation is formulated as:
(3.9)
Lamda ( ) is the dynamic viscosity, which relates stresses to linear deformation and is
the second viscosity which relates stresses to the volumetric deformation. The value of a
property per unit mass is expressed with .
Equation (3.9) consists of various transport processes, first is the rate of change term or
usually called the unsteady term (first term on the left side , second is the convective
term (second term on the left side), third is the diffusion term (first term on the right) and
fourth is source term (last term). In other words, the rate of increase of of fluid
element plus the net rate of flow of out of fluid element is equal to the rate of
increase of due to diffusion plus the rate of increase of due to sources.
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3.2.
Methodology of CFD
In general, CFD simulations can be distinguished into three main stages, which are 1)
pre-processor, 2) simulator or solver and 3) post-processor.
At the processing stage the geometry of the problem is defined as the solution domain
and the fluid volume is divided into discrete cells (the mesh). We also need to define the
physical modeling, parameter chemical phenomena, fluid properties and boundary
conditions of the problem.
The second stage is the solver. At this stage, the fluid flow problem is solved by using
numerical methods either finite difference method (FDM), finite element method (FEM)
or finite volume method (FVM).
The last stage is the post-processor. The post-processor is preformed for the analysis and
visualization of the resulting solution. Many CFD packages are equipped with versatile
data visualization tools, for instance domain geometry and grid display, vector plots, 2D
and 3D surface plot, particle tracking and soon.
Preprocessor
CFD
Simulation
Solver
Postprocessor
Solution domain
Grid generation
Physical modelling
parameters
Fluid properties
Boundary condition
Finite difference method

Finite element method
Finite volume method
Domain geometry and

grid display
Vector plot
2D and 3D surface plot
etc.
Figure 3.1. Basic concept of CFD simulation methodology.
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3.2.1. Pre-processing Stage

As mentioned in the previous section, at this stage we define the flow fluid problem by
giving the input in order to get the best solution of our problem. The accuracy of a CFD
solution is influenced by many factors, and some of them in this stage.
The pre-processing stage includes:
a.
b.
c.
d.
e.
A.
Solution domain defining.

Mesh generation.
Physical modeling parameters.
Fluid properties.
Boundary conditions.
Solution Domain
The solution domain defines the abstract environment where the solution is calculated.
The shape of the solution domain can be circular or rectangular. Generally, many
simulations use a rectangular box shape as the solution domain as shown in Figure 3.2.
Cylinder,
d diamater
D
Solution
domain
L
Figure 3.2. A rectangular box solution domain (L x D).
The choice of solution domain shape and size can affect the solution of the problem. The
smaller sized of domains need less iterations to solve the problem, in contrast to big
domains, which need more time to find the solution.
B.
Mesh Generation
After the solution domain has been defined, we shall generate the mesh within the
solution domain. The term mesh generation and grid generation is often interchangeably.
III-5
By definition, the mesh or grid is defined as the discrete locations at which the variables
are to be calculated and to be solved. The grid divides the solution domain into a finite
number of sub domains, for instance elements, control volumes etc [7].
Ferziger and Peric [7] divides grids into three types as follow:
a.
Structured (regular) grid

This regular grid consists of groups of grid lines with the property that members
of a single group do not cross each other and cross each member of the other
groups only once. This is the simplest grid structure since it has only four
neighbors for 2D and six neighbors for 3D. Even though it simplifies
programming and the algebraic equation system matrix has a regular structure, it
can be used only for geometrically simple solution domains.
Figure 3.3. A structured grid.
b.
Block-Structured grid
On this type of grid, the solution domain is divided into two or more subdivision.
Each subdivision contains of blocks of structured grids and patched together.
Special treatment is needed at block interfaces.
III-6
Figure 3.4. Block- structured grid.
c.
Unstructured grid
This type of grid can be used for very complex geometries. It can be used for any
discretization method, but they are best for finite element and volume methods.
Even though it is very flexible, there is the irregularity of the data structure.
Moreover, the solver for the algebraic equation systems is usually slower than for
structured grids.
Figure 3.5. Unstructured grid.
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C.
Boundary Condition
There are several boundary conditions for the discretised equations. Some of them are
inlet, outlet, wall, prescribed pressure, symmetry and periodicity [15].
a.
Inlet Boundary Condition

The inlet boundary condition permits flow to enter the solution domain. It can be
a velocity inlet, pressure inlet or mass flow inlet.
b.
Outlet Boundary Condition

The outlet boundary condition permits flow to exit the solution domain. It also
can be a velocity inlet, pressure inlet or mass flow inlet.
c.
Wall Boundary Condition

The wall boundary condition is the most common condition regarding in confined
fluid flow problems, such as flow inside the pipe. The wall boundary condition
can be defined for laminar and turbulent flow equations.
d.
Prescribed Pressure Boundary Condition

The prescribed pressure condition is used in condition of external flows around
objects, free surface flows, or internal flows with multiple outlets.
e.
Symmetry Boundary Condition

This condition can be classified at a symmetry boundary condition, when there is
no flow across the boundary.
f.
Periodic or Cyclic Boundary Condition

Periodic boundary condition is used when the physical geometry and the pattern
of the flow have a periodically repeating nature.
3.2.2. Simulation or Solving Stage

In the previous section, it is mentioned that the fluid flow problem is solved by using
particular numerical techniques. This technique or method is commonly called the
discretization method. The meaning of the discretisation method is that the differential
equations are approximated by an algebraic equation system for the variables at some set
of discrete location in space and time.
There are three main techniques of discretization method, the finite difference method,
the finite element method and the finite volume method. Even though these methods
have different approaches, each type of method yields the same solution if the grid is
very fine.
III-8
A.
Finite Difference Method (FDM)
The Finite Difference Method (FDM) is one of the easiest methods to use, particularly
for simple geometries. It can be applied to any grid type, whether structured or
unstructured grids.
FDM is very simple and effective on structured grid. It is easy to obtain higher-order
schemes on regular grid. On the other hand, it needs special care to enforce the
conservation condition. Moreover, for more complex geometry, this method is not
appropriate.
B.
Finite Element Method (FEM)
The advantage of FEM is its ability to deal with arbitrary geometries. The domain is
broken into unstructured discrete volumes or finite elements. They are usually triangles
or quadrilaterals (for 2D) and tetrahedral or hexahedra (for 3D). However, by using
unstructured grids, the matrices of the linearized equations are not as well ordered as for
structured grids. In conclusion, it is more difficult to find efficient solution methods.
FEM is widely used in structural analysis of solids, but is also applicable to fluids. To
ensure a conservative solution, FEM formulations require special care. The FEM
equations are multiplied by a weight function before integrated over the entire solution
domain. Even though the FEM is much more stable than finite volume method (FVM), it
requires more memory than FVM.
C.
Finite Volume Method (FVM)
FVM is a common approach used in CFD codes. Any type of grid can be accommodated
by this method, indeed it is suitable for complex geometries. This method divides the
solution domain into a finite number of contiguous control volume (CV), and the
conservation equations are applied to each CV. The FVM approach requires
interpolation and integration, for methods of order higher than second and are more
difficult to develop in 3D.
III-9
3.3.
Turbulent Flows
Turbulent flow can be defined as a chaotic, fluctuating and randomly condition of flow,
i.e. velocity fields. These fluctuations mix transported quantities such as momentum,
energy, and species concentration, and cause the transported quantities to fluctuate as
well. Turbulence is a time-dependent process. In this flow, the solution of the transport
equation is difficult to solve.
There are many methods that can be used to predict turbulence flow. Some of them are
DNS (direct numerical solution), RANS (Reynolds averaged Navier-Stokes), and LES
(large eddy simulation).
3.3.1.
Direct Numerical Solution (DNS)
DNS is a method to predict the turbulence flow in which the Navier-Stokes equations are
numerically solved without averaging. This means that all the turbulent motions in the
flow are resolved.
DNS is a useful tool in fundamental research in turbulence, but it is only possible to be
performed at low Reynolds number due to the high number of operations as the number
of mesh points is equal to
[1]. Therefore, the computational cost of DNS is very high
even at low Re. This is due to the limitation of the processing speed and the memory of
the computer.
3.3.2.
Large Eddy Simulations (LES)
The principal operation of LES is low-pass filtering. This means that the small scales of
the transport equation solution are taking out by apply the low-pass filtering. On the
whole, it reduces the computational cost of the simulation. The reason is that only the
large eddies which contain most of the energy are resolved.
Figure 3.6. Schematic representation of turbulent motion and time dependent of a velocity
component, adapted from Ferziger and Peric [1].
III-10
3.3.3.
Reynolds Averaged Navier-Stokes (RANS)
RANS equations are the time-averaged equations of motion of fluid flow. They govern
the transport of the averaged flow quantities, with the complete range of the turbulent
scales being modeled. Therefore, it greatly reduces the required computational effort and
resources and is widely adopted for practical engineering applications.
Two of the most popular models of the RANS are the k- model and k- model. The k-
was proposed for first time by Launder and Spalding [10]. Robustness, economy and
reasonable accuracy for a wide range of turbulent flows become its popularity. The k-
model is based on the Wilcox [16] k- model. This model is based on model transport
equations for the turbulence kinetic energy (k) and the specific dissipation rate ().
3.4.
Solution Algorithms for Pressure-Velocity Coupling Equation
To solve the pressure-velocity coupling equation, we need particular numerical

procedures called the solution algorithms. There are many algorithms that have been
developed, for instance SIMPLE, SIMPLEC, SIMPLER and PISO. These solution
algorithms are also called the projection methods.
3.4.1.
SIMPLE Algorithm
SIMPLE is an acronym for Semi-Implicit Method for Pressure Linked Equations. It is

widely used to solve the Navier-Stokes equations and extensively used by many
researchers to solve different kinds of fluid flow and heat transfer problems.
This method considers two-dimensional laminar steady flow equations in Cartesian coordinate. The principal of this method is that a pressure field (p*) is guessed to solve the
discretized momentum equations and resulting velocity component u* and v*.
Furthermore, the correction pressure (p) is introduced as the difference between the
correct pressure (p) and guessed p*. In summary, it will yield the correct velocity field (u
and v) and continuity will be satisfied.
3.4.2.
SIMPLER Algorithm
SIMPLER is a revised and improved method of SIMPLE. Note that R is stand for
revised. This method uses the SIMPLEs velocity correction to obtain the velocity fields.
3.4.3.
SIMPLEC Algorithm
SIMPLEC has the same steps as SIMPLE algorithm. The difference is that momentum
equations are manipulated so that the velocity correction equations of SIMPLEC omit
the terms that are less significant than those omitted in SIMPLE.
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3.4.4.
PISO
Pressure implicit with splitting operators, usually abbreviated as PISO, is a pressurevelocity procedure developed originally for the non-iterative computation of unsteady
flow [15]. This procedure has been successfully adapted for the iterative solution of
steady state problems. PISO consists of one predictor step and two corrector step.
At the predictor step, a guessed pressure (p) field is used to solved the discretized
momentum equation to give the velocity component (u and v). Furthermore, the first
corrector step of SIMPLE is used to give a velocity field which satisfies the discretized
continuity equation. Finally, second corrector step is applied to enhance the SIMPLE
procedure to obtain the second pressure correction field.
III-12
Chapter IV Validation of The CFD Simulations
VALIDATION OF THE CFD SIMULATIONS
This chapter describes the validation of the two dimensional steady flow simulation of
flow around a cylindrical structure using ANSYS Fluent. There are two main parts in
this chapter, first is the determination of the domain and grid type. Second is the
validation part. Many previous experiment results are given for validation and
comparison with the simulation results.
4.1. The Determination of Domain and Grid Type

The most important and crucial stage in the CFD simulation is the grids generation.
Moreover, the success of the CFD simulation depends on quality of the mesh. In this
section, several types of domains and grids have been generated and the best mesh
quality will be chosen for further simulation.
For problem of the flow around cylindrical structure, the possibilities of the domain
shape could be a circular, square or rectangular.
4.1.1. The Evaluation of the Grid Quality

The evaluations of the grid quality are based on the observation and criteria given by
ANSYS-Fluent [1]. Some of them are:
A.
Skewness
One of the major quality measures for a mesh is the skewness. It determines how
close to ideal a face or a cell is. It is expressed by values in a range between 0 1
as shown in Table 4.1. Highly skewed faces and cells are unacceptable because the
equations being solved assume that the cells are relatively equilateral/equiangular.
Figure 4.1 shows the ideal and skewed triangles and quadrilaterals.
IV-1
Table 4.1. Value of Skewness
Value of Skewness
Cell Quality
Degenerate
0.9 - <1
Bad (sliver)
0.75 0.9
Poor
0.5 0.75
Fair
0.25 0.5
Good
>0 0.25
Excellent
Equilateral
Equilateral Triangle
Equilateral Quad
Highly Skewed Triangle
Highly Skewed Quad
Figure 4.1. Ideal and skewed triangles and quadrilaterals.
B.
Element Quality
The element quality is expressed by a value in the range of 0 to 1. A value of 1
indicates a perfect cube or square while a value of 0 indicates that the element has a
zero or negative value.
C.
Orthogonal Quality
The range for orthogonal quality is 0 1, where a value of 0 is worst and 1 is the
best.
D.
Aspect Ratio
Aspect ratio is differentiated into two types which are the triangles and
quadrilaterals. Both are expressed by a value of number start from 1. A value of 1
indicates the best shape of an equilateral triangles or a square. Figure 4.2 shows the
aspect ratio for triangles and quadrilaterals.
IV-2
20
20
Figure 4.2. Aspect ratio for triangles and quadrilaterals
E.
Jacobian Ratio
The Jacobian ratio is computed and tested for all elements and expressed by a value
of number start from 1. An illustration for different values of Jacobian ratio is
shown in Figure 4.3.
Figure 4.3. Jacobian ratio for triangles and quadrilaterals
Several domains have been generated and compared in order to obtain the best option.
Generated domains and grids types are differentiated into four types. Each type will be
presented and the evaluation of the grid quality is presented in tabulation form as shown
in Table 4.2.
IV-3
A.
Circular Domain with Quadrilateral Grids
The circular domain has a uniform grid distribution along the geometry. It is the easiest
domain to be generated. The cylinder wall is located at the center of the domain with D
diameter. Figure 4.4 shows the typical circular domain with 1 m of cylinder diameter and
64D of domain diameter.
Figure 4.4. Circular domain with quadrilateral grids.
This domain is divided into 192 circumferential divisions and 192 radial divisions and
results in 73.728 elements. However, we can observe that the grid quality is not
uniformly distributed along the geometry face, especially for area close to the cylinder
wall (center). Detail view on Figure 4.5 indicates that the elements around the cylinder
wall have variation in grid quality.
Figure 4.5. Detail view of circular domain grids.
IV-4
B.
Square Domain with Quadrilateral Grids
This domain uses a square shape with dimension 60D length and 60D width. The
cylinder is located at the middle of the geometry as indicated in Figure 4.6.
Figure 4.6. Square domain with quadrilateral grids.
Domain face is divided into 8 equal parts and each inner line is divided into 160
divisions. Furthermore, cylinder wall is divided into 400 circumferential divisions.
Hence, 64.000 elements are created. In spite of we divide the domain into 8 equal parts,
the grid distribution is not uniformly distributed along the domain face.
C.
Rectangular Domain with Quadrilateral Grids
This domain has 50D x 30D dimension. The cylinder position in x-direction is located at
1/5 of the length and for the y-direction is of the width as indicated in Figure 4.7.
The establishment of the grid uses an automatic feature by Fluent called mapped face
meshing with 100% relevancy. The grids distribution is indicated in Figure 4.7. Even
though this method is the simplest and the easiest way to create the grids, in contrast it
produces low level of the grid quality as indicated in Figure 4.8.
Figure 4.7. Rectangular domain with quadrilateral grids.
IV-5
Figure 4.8. Detail view of the rectangular domain grids.
D.
Rectangular Domain with Smooth Quadrilateral Grids
This domain has 60D x 90D dimension. The domain face is divided into 10 divisions and
has a wireframe configuration as indicated in Figure 4.9. This wireframe arrangement
leads the smooth transition between two adjacent faces and generates uniform grid
distribution as indicated in Figure 4.10 and 4.11. Finer grids are needed in area close to
the wall in order to obtain precise results.
Figure 4.9. Wireframe arrangement of rectangular domain.
Figure 4.10. Rectangular domain with smooth quadrilateral grids.
IV-6
Figure 4.11. Detail view of the smooth grids close to the cylinder wall.
4.1.2. The Result of the Evaluation of the Grid Quality

By assessing the grid quality measurements as explained in the previous section, a
tabulation of grid quality for different type of domains is given in Table 4.2. The table
indicates the values of different grid quality criteria for each domain.
Table 4.2. Grid quality measurements.
Criteria
Circular Domain
Min
Max
Avrg
Square Domain
Min
Max
Avrg
Ractangular Domain
Min
Max
Smooth Rectangular
Avrg
Min
Max
Avrg
0.267
0.041
Skewness
0.005
0.265
0.066
3.604
0.500
0.190
2.922
0.964
0.371
1.3E10
Element Quality
0.243
0.994
0.807
0.306
0.996
0.697
-0.001
0.993
0.503
0.349
0.999
0.923
Orthogonal
Quality
0.915
0.999
0.989
0.702
0.999
0.932
3.298
0.999
0.769
0.918
0.994
Aspect Ratio
1.002
7.931
1.848
1.002
5.010
2.107
1.002
123.63
3.983
4.660
1.283
Jacobian Ratio
1.002
1.033
1.025
1.004
1.040
1.029
1.002
31.348
1.132
1.36
1.038
Min = Minimum; Max = Maximum; Avrg = Average
Referring to the values given in Table 4.2, it can be concluded that the best domain and
grid quality is the rectangular domain with smooth quadrilateral grid. Therefore, this
domain will be used for further simulations.
IV-7
4.1.3. The Grid Independency Study

The objective of the grid independency study is to precisely determine the grid size to
produce an accurate result. The grid independency is considered to be achieved when the
solution is not affected anymore by the size of the grid.
In this study, 2-dimensional steady flow at Re = 40 simulations has been carried out for
8 different sizes of grids. It is noted that the simulations use a rectangular domain with
smooth grids as proposed in the previous section. The first simulation uses 2 384
elements and yields Cd was equal to 1.431. The second simulation uses 9 728 elements
(308% higher than first simulation) increases Cd by 10.83% which was 1.586. However,
increasing the element number by 125% in fifth simulation yields a very small change to
the Cd, which only rises up to 0.1%. The results of the 8 different grid sizes are given in
Table 4.3. Figure 4.12 shows the result of the grid independence study.
Table 4.3. Result of the different grid size simulation at Re = 40.
Simulation Number
No. Of Element
Cd
S1
2 384
1.430
S2
9 728
1.586
S3
21 744
1.595
S4
38 912
1.601
S5
87 552
1.602
S6
136 620
1.602
S7
196 560
1.602
S8
442 908
1.606
As indicated in Figure 4.12, solution starts to converge at the 4th simulation which the
grids number is equal to 38.912. In conclusion, the minimum number of grids in order to
produce an accurate solution is 38.912.
IV-8
1.650
Drag Coefficient (Cd)
1.600
Initial point of convergence
1.550
1.500
1.450
1.400
-
100 000 200 000 300 000 400 000 500 000
Number of Grid Elements
Figure 4.12. Result of the grid independence study.
4.2. The Validations of the Results

This section describes the validation of the CFD simulations by comparing the
simulation solutions to the previous studies. The case of simulation is a 2-dimensional
case for difference values of Re = 40, 100 and 200.
4.2.1. The Steady Laminar Case at Re = 40

At Re = 40, two-attached recirculating vortices will be formed at the wake region. Apart
from the coefficient of drag (Cd), other features will be validated as indicated in Figure
4.13. Linnick and Fasel [5] did an experiment of a steady uniform flow past a circular
cylinder for Re was equal to 40. They measured the Cd value, length of recirculation
zone (L/D), vortex centre location (a/D,b/2D) and the separation angle (). These
measurements were also carried out by Herfjor [9] and Berthelsen and Faltinsen [3]. In
addition, the measurement of the separation angle () also was conducted by Russel and
Wang [13], Xu and Wang [17] and Calhoun [4]. The summary of the measurements is
given in Table 4.4.
IV-9
Figure 4.13. Vortice features for Re = 40, adapted from [10].
The simulation is set at Re = 40 and a steady laminar flow condition. The SIMPLE is
used as the pressure-velocity coupling scheme and QUICK is used as the momentum
spatial discretization. The QUICK scheme will typically be more accurate on structured
meshes aligned with the flow direction [2]. Moreover, 3000 iteration is set for the
simulation.
The solution is converged at 884 iterations and yields Cd is equal to 1.6002. At this Re
value, two identical vortices is formed behind the cylinder wall as indicated in Figure
4.14. Furthermore, the result of the measurement of the vortice features is shown in
Table 4.4.
Table 4.4. Vortice features measurements of a steady flow past a circular cylinder for Re = 40.
L/D
a/D
b/D
(deg)
Cd
Linnick and Fasel [11]
2.28
0.72
0.6
53.6
1.540
Herfjord [9]
2.25
0.71
0.6
51.2
1.600
Berthelsen and Faltinsen [3]
2.29
0.72
0.6
53.9
1.590
Russel and Wang [13]
2.29
53.1
1.600
Xu and Wang [17]
2.21
53.5
1.660
Calhoun [4]
2.18
54.2
1.620
Present study
2.27
0.73
0.6
49.5
1.600
Experiment by
Based on the results shown in Table 4.4, it can be concluded that the present value of the
simulation for Re = 40 is in a good agreement to the other measurements.
IV-10
Figure 4.14. Simulation result of two identical vortices at Re = 40.
4.2.2. The Transient (unsteady) Case at Re 100, 200 and 1000

For the 40 < Re < 200, the laminar vortex shedding will be formed behind the cylinder
wall as the result of the instability of the wake region. Furthermore, the vortex street
experiences the transition from laminar to turbulence and moves toward the cylinder
wall as Re is increased in the range 200 < Re < 300. When Re further increases (Re >
300), the wake region behind the cylinder wall becomes completely turbulent. The flow
regime at this Re value is described as the subcritical region (300 < Re < 3.5 x 10 5).
In this case, the cylinder is exposed to the transient laminar flow at Re = 100, 200 and
1000. Moreover the cylinder is also exposed to the transient turbulent flow at Re = 200
and 1000. The comparison of the results will be presented. For the transient laminar
flow case, the PISO is selected as the pressure-velocity coupling solution, since it gives a
stable solution for transient applications [2]. The large eddies simulation (LES) is
selected for the turbulent model in modeling the transient turbulent flow case.
To capture the shedding correctly, 25 time steps were chosen in one shedding cycle for
St = 0.2 (average estimation for flow past cylinder). In this case, for D = 1 m and U = 1
m/s, the vortex shedding frequency will be 0.2 Hz. Therefore, the time step is equal to
0.2 seconds. Figure 4.15 shows the time history of Cl and Cd at Re 100, 200 and 1000
and the Strouhal frequency for the transient laminar flow case. The transient turbulent
flow is shown in Figure 4.16.
The comparison of the Cd and Cl values are given in Table 4.5. It can be seen that the
application of the LES turbulent model yields the good agreement with previous studies.
This is due to the capability of the LES model to resolve all eddies. On the other hand,
the results from the transient laminar case show a wide discrepancy to the other studies.
This might be caused by the limitation of the laminar model to resolve the momentum,
mass and energy equations that are transported by the large eddies.
IV-11
Table 4.5. Experimental results of the Cl and Cd at Re 100, 200 and 1000.
Re = 100
Re = 200
Re = 1000
Experiment by
Cd
Cl
Cd
Cl
Cd
Cl
Linnick and Fasel [11]
1.34
0.333
1.34
0.69
Herfjord [9]
1.36
0.34
1.35
0.70
Berthelsen and
Faltinsen [3]
1.38
0.34
1.37
0.70
Russel and Wang [13]
1.38
0.30
1.29
0.5
Xu and Wang [17]
1.42
0.34
1.42
0.66
Calhoun [4]
1.33
0.298
1.17
0.668
Franke, et al [8]
1.31
0.65
1.47
1.36
Rajani, et al [12]
1.335
0.179
1.337
0.424
Present study*
1.28
0.13
1.20
0.29
0.80
0.37
Present study**
1.42
0.38
1.29
0.48
1.40
1.22
* Simulation results for the transient laminar flow

**Simulation results for the transient turbulent flow (LES model)
IV-12
Cl and Cd History
Strouhal Frequency
Figure 4.15. The time history of Cl and Cd for transient laminar flow case.
IV-13
Cl and Cd History
Strouhal Frequency
Figure 4.16. The time history of Cl and Cd for transient turbulent flow case (LES).
IV-14
Chapter V The Vortex Induced Vibrations Simulations
THE VORTEX INDUCED VIBRATION

SIMULATIONS
This chapter describes the vortex induced vibrations (VIV) simulations at Re 100, 200
and 1000. To simulate the vibrations of the cylinder due to the flow, the dynamic mesh
method is performed and a user defined function (UDF) was introduced to define the
motion of the cylinder. To capture the displacement of the cylinder clearly, the cylinder
is set to freely vibrate in the cross-flow direction (y direction) by defining the mass per
length of the cylinder is set equal to 1 kg, while the natural frequency (fn) is set equal to
0.2 Hz. There is no structural damping included in the motion of the cylinder, the
damping is only provided by the fluid due to the viscosity. The purpose of the VIV
simulation is to measure cross-flow displacement of the cylinder due to the flow. In
addition, the real vortex shedding frequency (fv), the vibration frequency (fvib), Strouhal
number (St) and the amplitude of the vibration (A) were also calculated.
This chapter is divided into three parts. First, the description of the VIV simulation
setup. Secondly, the result of the VIV simulation for Re 100, 200 and 1000. And finally,
the discussion of the results will be given on the last part.
5.1.
Simulation Setup
In order to obtain the accurate and stable result numerically, some procedures had been
applied. The procedures include the choice of the turbulent model, the pressure-velocity
coupling scheme and the momentum spatial discretization.
5.1.1.
Turbulent Model
In this VIV simulation, the large eddy simulation (LES) is selected to resolve the
turbulent flow. LES is able to resolve the large eddies directly, while the small eddies are
modeled.
5.1.2.
Pressure-Velocity Coupling Scheme
Two schemes have been tested in this VIV simulation. First is the PISO and second is
the SIMPLE. Even though the PISO gives a stable solution for transient application of a
fixed cylinder (Chapter 4), it produced unstable solution for the freely vibrating cylinder.
Here the SIMPLE gives a stable solution.
V-1
5.1.3.
Momentum Spatial Discretization
The Bounded Central Differencing is set for the momentum spatial discretization
scheme. This is the default scheme for LES model and is available only in the pressurebased solver. Other momentum spatial discretization is also tried in this simulation, the
QUICK. Although the QUICK scheme will typically be more accurate on structured
meshes aligned with the flow direction, it produces an error due to the negative cell
volume. This error occurs due to the mesh deformation becomes too large in one time
step.
5.2.
The Result of the Simulation of the VIV
In Figure 5.1, the displacement history of the freely vibrating cylinder at Re = 100 is
shown. The simulation is conducted for 100 second flow time. The vertical axes indicate
both the lift coefficient (Cl) and the non-dimensional magnitude of the cylinder
displacement (A/D). From the figure, it can be seen that the cylinder response rises few
second after the force.
Figure 5.1. Lift coefficient and displacement (A/D) of the cylinder at Re = 100.
The Fast Fourier Transform (FFT) is implemented to calculate the frequencies spectrum
of fv and fvib as indicated in Figure 5.2. The vortex shedding frequency (fv) has the value
of 0.166 Hz whilst the vibration frequency (fvib) is 0.176 Hz. The frequency ratio
between fvib and fn,
V-2
is equal to 0.88.
Figure 5.2. Spectrum of CF response frequencies (fv and fvib) at Re = 100.
The displacement history of the cylinder at Re = 200 is indicated in Figure 5.3. The
figure shows that the displacement of the cylinder grows larger than the force.
The frequencies spectrum at Re = 200 is shown in Figure 5.4. The fv is equal to 0.186
whilst fvib is equal to 0.195. Therefore the
is equal to 0.98.
V-3
Figure 5.5 indicates the displacement history of the cylinder at Re = 1000. In this case
the U is set equal to 0.5 m/s. The reason is to maintain the stability of the numerical
calculation. As the consequence, the fluid viscosity must be set to 0.0005 kg/m-s. It can
be seen from the figure that the magnitude of the displacement is much lower than the
displacement for Re 100 and 200.
The value of the frequencies shows the same value as indicated in Figure 5.6. Both f v
and fvib have the frequency of 0.103 Hz. This means that ratio of
V-4
is equal to 0.52.
5.3.
5.3.1.
Discussion
Effect of The Fluid Damping
As mentioned in the previous subchapter, there is no structural damping included in the

motion of the cylinder, the damping is only provided by the fluid due to the viscosity.
The fluid damping is the result of energy dissipation, as the fluid moves relative to the
vibrating structure, the cylinder. The interaction of the cylinder and the fluid produces
the additional mass (added mass) and this obviously affects the total damping.
In a vacuum environment, the response of the structure is stationary and no decrement.
Indeed, in this case the decrement are exist as we can observe if Figure 5.1 and 5.3. By
contrast, at Re 1000 the decrement seems does not exist (Figure 5.5). It is presumably
caused by the small effect of the added mass due to the small displacement of the
cylinder.
5.3.2.
The Displacement of The Cylinder (Cross-Flow Direction)
Table 5.1 indicates the result of the measurements of the VIV simulation at Re 100, 200
and 1000. The value of amplitude is calculated by using the root mean square (RMS)
method. The true reduced velocity, Ured,true, is based on the frequency at which the
cylinder is actually vibrating (fvib).
From the table, it can be seen that the magnitude of the dimensionless displacement
(A/D) at Re 100 and 200 does not change even though the Re doubles. This is caused by
the vibration frequency which for both Re values is quite same. However, there is a
significant decrease in the displacement magnitude for Re 1000, which is 55%
V-5
decrement. The reason behind this decrement is that the vibration frequency (fvib) quite
differs to the value of the natural frequency (fv) of the cylinder, as indicated in Table 5.1.
Table 5.1. Result of the VIV simulation.
Re
fv
St
fvib
Ured,true
A/D
fvib/St
fvib/fn
100
0.166
0.166
0.176
5.68
0.379
1.06
0.88
200
0.186
0.186
0.195
5.13
0.380
1.05
0.98
1000
0.103
0.206
0.103
4.85
0.172
0.5
0.52
To explain the displacement of the cylinder in cross-flow direction, Figure 2.8 can be
referred. From the figure, it can be concluded that the vortex-shedding frequency follows
the cylinders Strouhal frequency until the velocity Ured reaches the value of 5. Beyond
this point, however, the vortex-shedding frequency begins to follow the natural
frequency of the system (fvib/fn = 1) until it reaches Ured = 7. This phenomenon is known
as the lock-in phenomenon. In this range, the vortex-shedding frequency, the vibration
frequency and the natural frequency coincide (fv = fvib = fn). This means that, in this
range the lift force (the shedding) oscillates in sympathy with the cylinder motion,
results in the largest amplitude. The same thing occurs in this case. The maximum
displacement happened at Ured,true = 5.13. At this point, the value of fv, fvib and fn
relatively have the same value. By contrast, the smallest displacement happens at Ured,true
= 4.85, which have the fv and fvib 50% smaller than fn.
5.3.3.
The Displacement Initiation
The time required to initiate the displacement of the cylinder is different for each case.
For Re 100, the initiation of the displacement starts at the flow time was equal to 32.2
second (Figure 5.1), for Re 200 the initiation starts at the flow time was equal to 23.3
second (Figure 5.3), whilst for Re 1000 the initiation starts after 68 second (Figure 5.5).
These initiations are strongly associate with the frequency of the vibration (fvib). The
closest fvib to the fn value, the faster the displacement initiation. From the Table 5.1, at
Re 200 the ratio of fvib/fn is 0.98, which means that the value of fvib is close to the fn of
the cylinder. Figure 5.7 shows the development of the displacement of the cylinder as a
function of the flow time.
V-6
Figure 5.7. The development of the displacement as a function of flow time

V-7
Chapter VI Conclusion
CONCLUSIONS
The main part of this thesis focused on the characteristics of the flow pattern and
vibration of the cylindrical structure due to the incompressible laminar and turbulent
flow. The software package, FLUENT, has been used to perform the flow simulations
for Re 40, 100, 200 and 1000.
This chapter is divided into two sections. The first section describes the most important
conclusions that are obtained from the performed simulation. The second section
presents some recommendation for further work within this subject.
6.1. Conclusion
The next lines present the conclusion of the thesis work as follow:
1.
The flow characteristics of the steady laminar case at Re = 40, which is

represented by the Cd value, length of recirculation zone, vortex centre location
and the separation angle, shows a good agreement with the other studies.
2.
The drag coefficient (Cd) at Re 100 shows a good agreement to the other studies.
However, the lift coefficient (Cl) has a slight discrepancy compared to the other
studies.
3.
The implementation of the transient (unsteady) laminar flow for Re 200 and 1000
are perceived not accurate enough. The result shows that the Cd and Cl value has
a poor agreement to the other studies. The use of the Large Eddy Simulation
(LES) model improves the result significantly.
4.
The largest cylinder displacement occurs when the lift force oscillates in
sympathy with the cylinder motion. It means that the the vortex-shedding
frequency, the vibration frequency and the natural frequency coincide (f v = fvib =
fn). In this simulation, the maximum displacement occurs at Ured,true = 5.13 (Re =
200).
VI-1
Chapter VI Conclusion
6.2. Recommendations
VI-2
1.
The success of the CFD simulation depends on many factors, for instance the
domain shape, mesh/grid shape and size, solver and etc. The choices of these
factors influence the simulation time and result. For that reasons, specific
studies to optimize setup are recommended.
2.
The simulation set up in this thesis can be used for further analysis of vibration
due to the flow effect. For instance, to assess the galloping, drag crisis or flow
interaction in group of cylinders.
3.
Even though flow around cylindrical structures is a classical subject and many
studies have been carried out, it is still an interesting and relevant field to
investigate and study.
REFERENCES
[1] ANSYS. 2010. ANSYS Meshing Users Guide. ANSYS, Inc. Canonsburg, USA.
[2] ANSYS. 2010. ANSYS Fluent Theory Guide. ANSYS, Inc. Canonsburg, USA.
[3] Berthelsen, P.A. and Faltinsen, O.M. 2007. A local directional ghost cell
approach for incompressible viscous flow problems with irregular boundaries.
Journal of Computational Physic 227 (2008) 4354 4397.
[4] Calhoun, D. 2002. A Cartesian grid method for solving the two-dimensional
streamfunction-vorticity equations in irregular regions. Journal of
Computational Physic. 176 (2) 231275.
[5] Drescher, H. 1956. Messung der auf querangestriimte Zylinder ausgeubten
zeitlich veriinderten Drucke. 2. f. Flugwiss, 4(112):17-21.
[6] Feng, C.C. 1968. The measurements of the vortex induced effects in flow past
stationary and oscillating circular and D-section cylinders. Thesis, The
University of British Columbia.
[7] Ferziger, J.H and Peric, M. 1997. Computational Methods for Fluid Dynamics.
Springer. Berlin-Germany.
[8] Franke, R. et al. 1990. Numerical Calculation of Laminar Vortex Shedding
Flow Past Cylinders. Journal of Wind Engineering and Industrial Aerodynamics,
35 (1990) 3237-257. Elsevier. Amsterdam.
[9] Herfjord, K. 1996. A study of two-dimensional separated flow by a combination
of the finite element method and NavierStokes equations. Dr. Ing.-Thesis,
Norwegian Institute of Technology, Department of Marine Hydrodynamics.
Trondheim, Norway.
[10] Launder, B.E. and Spalding, D.B. 1972. Lectures in Mathematical Models of
Turbulence. Academic Press. London, England.
[11] Linnick, M. N and Fasel, H.F. 2005.A high-order immersed interface method
for simulating unsteady compressible flows on irregular domains. J. Comput.
Phys. 204 (1) 157192.
[12] Rajani, B.N. et al. 2008. Numerical Simulation of Laminar Flow Past a Circular
Cylinder. Journal of Applied Mathematical Modelling 33 (2009) 1228-1247.
Elsevier.
[13] Russel, D. and Wang, Z.J. 2003. A Cartesian grid method for modeling multiple
moving objects in 2D incompressible viscous flow. Journal of Computational
Physic. 191 (1) 177205.
[14] Sumer B.M. and Fredse J. 2006. Hydrodynamics Around Cylindrical

Structure. World Scientific Publishing Co Pte. Ltd. Singapore.
[15] Versteeg H.K. and Malalasekera, W. 2007. An Introduction to Computational
Fluid Dynamics : The Finite Volume Method. Pearson Prentice Hall. England
[16] Wilcox, D.C. 1998. Turbulence Modelling for CFD. DCW Industries, Inc. La
Canada. California.
[17] Xu, S. and Wang, Z.J. 2006. An immersed interface method for simulating the
interaction of a fluid with moving boundaries. Journal of Computational Physic.
216 (2) 454493.
APPENDIXES A
Problem Description
Consider a fluid flowing pass a cylinder, as illustrated below. The cylinder is considered
as a fixed or a free oscillating body. The flow is expressed by the Reynolds number 40,
100, 200 and 1000.
Diameter of the pipe,
D=1 m
Flow velocity,
U = 1 m/s
3
Density of the fluid,
=1
kg/m .
Thus, the dynamic viscosity must be set,
= 0.025
kg/m*s (Re = 40)
= 0.01
kg/m*s (Re = 100)
= 0.005
kg/m*s (Re = 200)
= 0.0001
kg/m*s (Re = 1000)
Figure 1. Simulation illustration.

The simulation will be carried out as indicated in following table.
Re
Steady Unsteady Laminar

x
40
100
200
1000
Simulation objectives
Try to obtain :
1.
2.
3.
4.
5.
Turbulent
Fixed
Free
(LES)
Cylinder Oscillating
x
Lift and Drag Coefficient (Cd and Cl)

The vortex shedding frequency (fv)
Strouhal Number (St)
Vibration frequency (fvib)
The Amplitude of the vibration (A)
Solution Domain
A rectangular domain will be used for the simulation. The dimension of the domain is
60D x 90D.
Cylinder,
d diamater
D
Solution
domain
Figure 2. SolutionL domain sketch.

Boundary Conditions
First, we will specify a velocity inlet boundary condition. We will set the left side
boundary as a velocity inlet with a velocity of 1 m/s in the x direction. Next, we will use
a pressure outlet boundary condition for the right side boundary with a gauge pressure
of 0 Pa. Lastly, we will apply a no slip boundary condition to the cylinder wall. The
aforementioned boundary conditions are illustrated below.
Velocity inlet, Ux = 1 m/s)

Cylinder wall
no friction
Pressure outlet,
P = 0 Pa
Figure 3. Boundary condition sketch.
APPENDIXES B
User Define Function
This is the user define function (UDF) used in Fluent to define the motion of the
cylinder due to the flow. It is the six degree of freedom (6DOF) solver, which is used
when the movement of the rigid body is determined by the forces of the flow.
#include "udf.h"
DEFINE_SDOF_PROPERTIES(sdof_props, prop, dt, time, dtime)
{
real cg;
real k = 1*1.26*1.26;
cg = DT_CG(dt)[1];
prop[SDOF_MASS] = 1;
prop[SDOF_LOAD_F_Y] = -k*cg;
prop[SDOF_ZERO_TRANS_X] = TRUE;
prop[SDOF_ZERO_ROT_X] = TRUE;
prop[SDOF_ZERO_ROT_Y] = TRUE;
}
APPENDIXES C
Fluent Simulation Setup
This is the summary of the Input Setup of the simulation of the Steady Laminar Case, Re
= 40.
FLUENT
Version: 2d, dp, pbns, lam (2d, double precision, pressure-based, laminar)
Release: 13.0.0
Title:
Models
-----Model
Settings
------------------------------------Space
2D
Time
Steady
Viscous
Laminar
Heat Transfer
Disabled
Solidification and Melting Disabled
Species
Disabled
Coupled Dispersed Phase Disabled
NOx Pollutants
Disabled
SOx Pollutants
Disabled
Soot
Disabled
Mercury Pollutants
Disabled
Material Properties
------------------Material: aluminum (solid)
Property
Units Method Value(s)
--------------------------------------------------Density
kg/m3 constant 2719
Cp (Specific Heat) j/kg-k constant 871
Thermal Conductivity w/m-k constant 202.4
Material: air (fluid)
Property
Units Method Value(s)
-------------------------------------------------------------Density
kg/m3 constant 1
Cp (Specific Heat)
j/kg-k constant 1006.43
Thermal Conductivity
w/m-k constant 0.0242
Viscosity
kg/m-s constant 0.025
Molecular Weight
kg/kgmol constant 28.966
Thermal Expansion Coefficient 1/k
constant 0
Speed of Sound
m/s
none
#f
Cell Zone Conditions

-------------------Zones
name
id type
------------------------surface_body 2 fluid
Setup Conditions
surface_body
Condition
Value
--------------------------------------------------------------Material Name
air
Specify source terms?
no
Source Terms
()
Specify fixed values?
no
Fixed Values
()
Frame Motion?
no
Relative To Cell Zone
-1
Reference Frame Rotation Speed (rad/s)
0
Reference Frame X-Velocity Of Zone (m/s)
0
Reference Frame Y-Velocity Of Zone (m/s)
0
Reference Frame X-Origin of Rotation-Axis (m)
0
Reference Frame Y-Origin of Rotation-Axis (m)
0
Reference Frame User Defined Zone Motion Function
none
Mesh Motion?
no
Relative To Cell Zone
-1
Moving Mesh Rotation Speed (rad/s)
0
Moving Mesh X-Velocity Of Zone (m/s)
0
Moving Mesh Y-Velocity Of Zone (m/s)
0
Moving Mesh X-Origin of Rotation-Axis (m)
0
Moving Mesh Y-Origin of Rotation-Axis (m)
0
Moving Mesh User Defined Zone Motion Function
none
Deactivated Thread
no
Embedded Subgrid-Scale Model
0
Momentum Spatial Discretization
0
Cwale
0.325
Cs
0.1
Porous zone?
no
X-Component of Direction-1 Vector
1
Y-Component of Direction-1 Vector
0
Relative Velocity Resistance Formulation?
yes
Direction-1 Viscous Resistance (1/m2)
0
Direction-2 Viscous Resistance (1/m2)
0
Choose alternative formulation for inertial resistance? no
Direction-1 Inertial Resistance (1/m)
0
Direction-2 Inertial Resistance (1/m)
0
C0 Coefficient for Power-Law
0
C1 Coefficient for Power-Law
0
Porosity
1
Boundary Conditions
------------------Zones
name
id type
------------------------------------------inlet
10012 velocity-inlet
cylinderwall
10013 wall
outlet
10014 pressure-outlet
wall-surface_body 5
wall
Setup Conditions
inlet
Condition
Value
-------------------------------------------------Velocity Specification Method
1
Reference Frame
0
Velocity Magnitude (m/s)
0
Supersonic/Initial Gauge Pressure (pascal) 0
X-Velocity (m/s)
1
Y-Velocity (m/s)
0
X-Component of Flow Direction
1
Y-Component of Flow Direction
0
X-Component of Axis Direction
1
Y-Component of Axis Direction
0
Z-Component of Axis Direction
0
X-Coordinate of Axis Origin (m)
0
Y-Coordinate of Axis Origin (m)
0
Z-Coordinate of Axis Origin (m)
0
Angular velocity (rad/s)
0
is zone used in mixing-plane model?
no
cylinderwall
Condition
Value
---------------------------------------------------------Wall Motion
0
Shear Boundary Condition
0
Define wall motion relative to adjacent cell zone? yes
Apply a rotational velocity to this wall?
no
0
X-Component of Wall Translation
1
Y-Component of Wall Translation
0
Define wall velocity components?
no
X-Component of Wall Translation (m/s)
0
Y-Component of Wall Translation (m/s)
0
Rotation Speed (rad/s)
0
X-Position of Rotation-Axis Origin (m)
0
Y-Position of Rotation-Axis Origin (m)
0
X-component of shear stress (pascal)
0
Y-component of shear stress (pascal)
0
Specularity Coefficient
outlet
Condition
Value
--------------------------------------------------------Gauge Pressure (pascal)
0
Backflow Direction Specification Method
1
X-Component of Flow Direction
1
Y-Component of Flow Direction
0
X-Component of Axis Direction
1
Y-Component of Axis Direction
0
Z-Component of Axis Direction
0
X-Coordinate of Axis Origin (m)
0
Y-Coordinate of Axis Origin (m)
0
Z-Coordinate of Axis Origin (m)
0
is zone used in mixing-plane model?
no
Specify Average Pressure Specification
no
Specify targeted mass flow rate
no
Targeted mass flow (kg/s)
1
Upper Limit of Absolute Pressure Value (pascal) 5000000
Lower Limit of Absolute Pressure Value (pascal) 1
wall-surface_body
Condition
Value
---------------------------------------------------------Wall Motion
0
Shear Boundary Condition
0
Define wall motion relative to adjacent cell zone? yes
Apply a rotational velocity to this wall?
no
0
X-Component of Wall Translation
1
Y-Component of Wall Translation
0
Define wall velocity components?
no
X-Component of Wall Translation (m/s)
0
Y-Component of Wall Translation (m/s)
0
Rotation Speed (rad/s)
0
X-Position of Rotation-Axis Origin (m)
0
Y-Position of Rotation-Axis Origin (m)
0
X-component of shear stress (pascal)
0
Y-component of shear stress (pascal)
0
Specularity Coefficient
0
Solver Settings
--------------Equations
Equation Solved
----------------Flow
yes
Numerics
Numeric
Enabled
--------------------------------------Absolute Velocity Formulation yes
Relaxation
Variable Relaxation Factor
------------------------------Pressure 0.3
Density
1
Body Forces 1
Momentum 0.7
Linear Solver
Solver Termination Residual Reduction
Variable Type
Criterion Tolerance
-------------------------------------------------------Pressure V-Cycle 0.1
X-Momentum Flexible 0.1
0.7
Y-Momentum Flexible 0.1
0.7
Pressure-Velocity Coupling
Parameter Value
-----------------Type
SIMPLE
Discretization Scheme
Variable Scheme
------------------Pressure Standard
Momentum QUICK
Solution Limits
Quantity
Limit
--------------------------------Minimum Absolute Pressure 1
Maximum Absolute Pressure 5e+10
Minimum Temperature
1
Maximum Temperature
5000

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CFD Simulation of Vortex Induced

Vibration of a Cylindrical Structure

Muhammad Tedy Asyikin

Coastal and Marine Civil Engineering

Norwegian University of Science and Technology

NORWEGIAN UNIVERSITY OF SCIENCE AND TECHNOLOGY

Date: June 11, 2012.

Muhammad Tedy Asyikin

Faculty of Engineering Science

Division: Marine Civil

Objective of the thesis work

Scope of work (work plan)

General about content, work and presentation

Start and submission deadlines

1 INTRODUCTION ..................................................................................................... I-1

3.2.1. Pre-processing Stage ............................................................................... III-5

6 CONCLUSION ....................................................................................................... VI-1

CFD Simulation of Vortex Induced Vibration of a Cylindrical Structure

1.2. Scope of Work

1.3. Project Objectives

1.4. Structure of the Report

CFD Simulation of Vortex Induced Vibration of a Cylindrical Structure

CFD Simulation of Vortex Induced Vibration of a Cylindrical Structure

Chapter II Flow Around Cylindrical Structure

CFD Simulation of Vortex Induced Vibration of a Cylindrical Structure

Chapter II Flow Around Cylindrical Structure

Laminar vortex street

40 < Re < 200

200 < Re < 300

300 < Re < 3x105

3x105 < Re < 3.5 x 105

CFD Simulation of Vortex Induced Vibration of a Cylindrical Structure

Chapter II Flow Around Cylindrical Structure

2.1.2. Vortex Shedding

CFD Simulation of Vortex Induced Vibration of a Cylindrical Structure

Chapter II Flow Around Cylindrical Structure

mean drag. The vortex shedding frequency is represented by

CFD Simulation of Vortex Induced Vibration of a Cylindrical Structure

Chapter II Flow Around Cylindrical Structure

2.1.4. Other Hydrodynamic Numbers

Chapter II Flow Around Cylindrical Structure

CFD Simulation of Vortex Induced Vibration of a Cylindrical Structure

Chapter II Flow Around Cylindrical Structure

Vortex Induced Vibration

Solution to Vibration Equation

Fig. 2.4. Idealized description of a vibrating structure.

In this thesis, the structure is conditioned as a vibration-free elastically cylinder. It means

CFD Simulation of Vortex Induced Vibration of a Cylindrical Structure

Chapter II Flow Around Cylindrical Structure

Free vibrations without viscous damping

is the angular frequency of the motion,

Free vibrations with viscous damping

CFD Simulation of Vortex Induced Vibration of a Cylindrical Structure

Chapter II Flow Around Cylindrical Structure

CFD Simulation of Vortex Induced Vibration of a Cylindrical Structure

Chapter II Flow Around Cylindrical Structure

are the total damping factor and damping angular frequency

CFD Simulation of Vortex Induced Vibration of a Cylindrical Structure

Chapter II Flow Around Cylindrical Structure