The Effect of Cylinder Type I-65° Staggered

Upstream Convex Blade on the Aerodynamic

Performance of the Savonius Turbine

Gunawan Sakti1,2, Triyogi Yuwono2(B) , and Wawan Aries Widodo2

1 Aviation Polytechnic of Surabaya, Surabaya, Indonesia
2 Sepuluh November Institute of Technology Nopember, Surabaya, Indonesia

tiryogi@me.its.ac.id

Abstract. The Savonius turbine has advantages, including a simple design, unfet-
tered incoming wind direction, can spin at low wind speeds, and start rotation
without additional equipment. However, this turbine design has low efficiency,
so many researchers have tried to improve it. The primary research in this report
is one of the efforts to upgrade the performance of the Savonius turbine in the
category of adding external devices experimentally. An I-65° cylinder type with
diameter d/D = 0.5 is placed upstream of the Savonius convex blade at a distance
S/D = 1.4 with a staggered angle −10° ≤ α ≤ 40° at Reynolds number Re =
10.3 × 104 correspondence to freestream velocity U = 5 m/s. The performance of
the standard Savonius turbine will be compared with a turbine that has installed a
cylinder type I-65° upstream convex blade based on the power and moment coef-
ficient. The static moment also is compared to identify the self-starting ability.
The experiment results show the stager angle α = 40° as the optimal angle with
an increase in the coefficient of power of 12%. For static torque measurement
parameters, the results also show an increase in performance where the turbine
rotation angle with negative torque decreases.

Keywords: Savonius · Coefficient of Power · Coefficient of Moment · Static

Torque · Cylinder I-65

1 Introduction

This project was carried out as a form of participation in the search for renewable energy
sources, which are currently very popular worldwide. Furthermore, environmentally
friendly energy consumption is prevalent because of human awareness of environmental
threats in the future [1]. Then this underlies the importance of research on various types
of renewable energy sources to reach their respective optimal points. This research report
is about improving the aerodynamic characteristic of the Savonius wind turbine. This
scope is widely assessed to achieve an optimal geometric design and extract maximum
wind energy. Nevertheless, the Savonius turbine is still a popular research object because
of its inefficiency compared to other turbine types [2].

© The Author(s) 2023

B. Bagus Harianto et al. (Eds.): ICATEAS 2022, AER 217, pp. 223–232, 2023.
https://doi.org/10.2991/978-94-6463-092-3_20
224 G. Sakti et al.

Savonius turbine is also defined as a drag-type turbine. Because it rotates based

on the distinction in positive pressure drag force at the advancing blade and negative
pressure drag convex blade [3, 4]. This research aims to increase turbine performance
and reduce negative pressure drag by installing cylinder-type I-65° staggered upstream
convex blade. The dominant drag force in the advancing blade area will be proven to
increase the drag-type performance of this turbine. Several previous studies are presented
in the following section to meet the sustainable research.
The researcher [5] studied a Savonius turbine with several blade shape variations, the
number of blades, and the stage numerically. 3D simulation is done with Ansys software
with an aspect ratio of D/H = 1. The findings are Cpmax up to 0.2, and the optimal blade
number and stages are two and three, respectively. The study by [6] focuses on high-
pressure drag and loss of lift in vertical-axis wind turbines. The double multiple stream
tube (DMST) approach is used numerically to solve the problem. The results show that
the DMST approach increases the torque coefficient for most turbine blade azimuth
angles. The torque coefficient is an indicator of self-starting capability that shows the
level of responsiveness of the turbine to the wind without the help of an external device.
The optimization of the power coefficient in the blade shape modification category is
studied numerically by [7]. The result achieved a 39% improvement of the Cp higher
than conventional Savonius.
Several efforts to improve efficiency that have been described above leave room
for further research in this study. That is by installing an additional configuration of
the cylinder type I-65° to reduce the drag force on the convex blade. This method is
determined based on research reports from [4, 8, 9], where the type I-65° cylinder can
reduce the main cylinder’s pressure drag by delaying the separation point. Finally, the
results of this study are applied experimentally to reduce pressure drag on the convex
blade with modifications to the stagger angle.

2 Methods
2.1 Experimental Arrangements
Figure 1 and Fig. 2 show the experimental arrangement Savonius turbine with the cylinder
I-65° staggered upstream convex blade. In this case, the I-type cylinder is small with
d/D = 0.5 in diameter, trimming each side 65° perpendicular to the horizontal axis.
The model was 3D printed with Ds = 320 mm in diameter and height H = 305 mm,
corresponding to the aspect ratio AR = 0.95. The turbine has no e-gap and is equipped
with endplate De/D = 1.10 compliance with the optimum geometry reported by [10].
The cylinder I-65° set within the distance fixed at S/D = 1.4 relative to the blade diameter
and staggered angle −10 ≤ α ≤ 40. The freestream velocity set at five m/s corresponds
to the Reynolds number Re = 10.3 × 104 and blows up from the exhaust of the wind
tunnel.
Figure 3 shows the experimental setup, where the turbine stands 230 cm from an
exhaust wind tunnel and 20 cm from it, the honeycomb installed with the task ensures
the airflow is as uniform as possible. This distance was calculated and tested based on
the uniformity assessment. Omega type HHF141 measures wind speed at five horizontal
and vertical points at 200–250 cm. This method verified that the uniform windspeed
 The Effect of Cylinder Type I-65° Staggered Upstream Convex Blade 225

Fig. 1. The Savonius Turbine configured with I-65° cylinder type staggered upstream convex
blade

Fig. 2. The experimental schematic diagram and the symbols to determine the dimension and
geometry of the Savonius turbine.

Fig. 3. The experimental conducted in the external flow using the wind tunnel outflow within a
specific distance to the Savonius turbine.

for both axes is 230 cm with an average of five m/s. Figure 4 illustrates the uniformity
assessment conducted in this study.
226 G. Sakti et al.

Fig. 4. The uniformity conducted at 5 points horizontally and vertical at 200–250 cm in front of
the exhaust tunnel.

Fig. 5. The M425 torque transducers installed at the end shaft of the turbine integrated with
friction brake for mechanical power measurements.

Figure 5 presents the M425 rotary torque sensor used in this experiment. This sensor
gives a high precision of 0.1%, torque accuracy range from 0–10 Nm, and sample rate
selectable from 1sps–4000 sps as standard. In addition, the Torque Sensor utilized a
built-in display Interface Module, providing harvest data for a moment (Nm), power
(rpm), and power (Watt).
 The Effect of Cylinder Type I-65° Staggered Upstream Convex Blade 227

2.2 Data Reduction

The Savonius shaft power is calculated by measuring the mechanical moment and rota-
tional per minute (rpm) at Reynolds number Re = 1.03 × 104 . The Reynolds Number
depends on freestream velocity V and the diameter of the Savonius turbine and is defined
by the following Equation:
ρVL ρV (2D − e)
= (1)
μ μ
L is the turbine’s length characteristic, D is blade diameter, and e is the overlap distance
(m). Furthermore, ρ is the mass of air per unit volume (kg/m3 ), μ is a viscosity dynamic
(kg/m.s), and V is wind velocity (m/s). Finally, the mechanical power is one of the sensor
outputs and can be quantified based on the following Equation:

Pm = T ω (2)

where T is a mechanical moment (N.m), then ω is the angular velocity (rad/s). If N refers
to the shaft rotation per minute (rpm), it can be obtained by the following formula:
2π N
ω= (3)
60
The tip-speed ratio λ depends on the turbine’s angular velocity, and it can be solved
with the following Equation:
ωD
λ= (4)
2V
The swept area A defines as the turbine diameter times the height of the turbine and
used for calculating the torque coefficient in the following Equation:
T
Cm = (5)
4 ρADV
1 2

The Savonius turbine efficiency is expressed in the power coefficient C p and can be
found from the Equation below:
Pm
Cp = (6)
Pw
The wind potential power Pw as mentioned in Eq. (6) determined based on Eq. (7).
1
Pw = ρAV 3 (7)
2
228 G. Sakti et al.

Fig. 6. The growth of static moment (N.m) as a function of azimuth blade angle (θ), Re = 10.3 ×
104 , conform between conventional and staggered I-65° cylinder Savonius Turbine −10° ≤ α ≤
40°.

3 Result and Discussion

3.1 Self-starting Capabilities

The self-starting capabilities of Vertical Axis Wind turbines are an essential deliberation
to define the turbine potential starting characteristics rotate without external device
assistance. Further evaluation of the turbine static moment at several azimuth angles
(θ ) has been determined by the M425 torque sensor experimentally. Figure 6 shows
the non-dynamic moment aligned between conventional and staggered I-65° Savonius
turbine −10° ≤ α ≤ 40° at specific Reynolds number Re = 10.3 × 104 . The azimuth
angle is only plotted up to 180 degrees because of its periodicity.
The maximum static moment obtained at staggered angle α = 0° at θ = 40° reaches
the value 0.19 N.m. This is because the static moment gradually gets closer to the conven-
tional turbine line as the staggered angle increases to an angle of α = 40°. This means
that increasing the positive staggered angle gives no improvement in turbine starting
capabilities. On the contrary, the negative static moment area reduced significantly at a
staggered angle α = −10° indicates the turbine starting capabilities improved. However,
this harmful staggering angle α = −10° disturbs an advancing blade airflow, leaving the
value at 0.145 N.m maximum, as seen in the graph.

3.2 Power and Torque Coefficient

Figure 7 shows the output experimental investigation on the power coefficient Cp against
the tip-speed ratio, λ. The staggered I-65° cylinder type at the range −10° ≤ α ≤ 40°
upstream convex blade gives a slight increment on Cp. The staggered angle α = 40°
reaches maximum Cp = 0.19 at tip-speed ratio λ = 0.8, or 12% higher than a conventional
turbine. The negative staggered angle α = −10° reaches maximum Cp = 0.156 at tip-
speed ratio λ = 0.8, or 7% below conventional turbine. The maximum TSR increases
directly to the staggering angle −10° ≤ α ≤ 40°. Finally, installing an I-65° cylinder
type upstream convex blade has proven to jack up the turbine power coefficient Cp.
 The Effect of Cylinder Type I-65° Staggered Upstream Convex Blade 229

Fig. 7. The growth of turbine Cp as a function of tip-speed ratio, λ, compared between the standard
turbine and staggered I-65° turbine Savonius at Re = 10.3 × 104 .

Fig. 8. The growth of turbine torque coefficient as a function of TSR, λ, at Re = 1.03 × 104
aligned between the conventional and staggered I-65° Savonius Turbine.

Figure 8 also presents a similar trend based on the Cm torque coefficient in line with
the Cp power coefficient. The Cm growth is directly proportional to installing an I-65°
cylinder type upstream convex blade. The staggered angle α = −10° indicated below
the conventional turbine, and the staggering angle α = 40° seem higher than other lines.
The more the staggering angle is given to the turbine configuration, the higher the Cm.
This consideration proved and enhanced previous hypotheses that installing an I-65°
cylinder type positively affects the Savonius wind turbine performance.
The analyzed turbine performance is based on the power and moment coefficients,
as summarized in Fig. 9. The graph presents the power coefficient maximum Cpmax
reached by every single staggered angle compared to the conventional turbine maximum
Cp0max . Figure 9 also shows the staggering angle α = 40° resulting in the Cp, which
is 1.12 times higher than a conventional turbine. The staggering α = 0° configuration
has a lower effect than another angle, only 1.05 times higher than the standard turbine.
The negative staggering angle α = −10° gives a contraindicative 0.95 times lower than
a standard turbine.
230 G. Sakti et al.

Fig. 9. The development of Cpmax /Cp0max as a function of cylinder I-65 staggered angle −10°
≤α ≤ 40°.

4 Conclusions
The Savonius wind turbine arrangement with the integration of I-65° cylinders type
staggered upstream convex blade is investigated experimentally. The investigation was
performed based on the dynamic torque coefficient, the power coefficient, and the static
moment consideration. The outcome indicated that the I-65° significantly strengthened
the turbine efficiency to 12% higher than the turbine standard. Furthermore, the optimum
configuration achieved within a staggered angle α = 40° at TSR λ = 0.8, Reynolds
number Re = 10.3 × 104 corresponds to wind velocity V = 5 m/s.
The starting capabilities of this turbine configuration were investigated under static
torque coefficient considerations. The result shows that the static torque reaches a maxi-
mum at a staggering angle α = 0° azimuth blade angle θ = 40°. The lower the staggering
angle, the more negative torque area disappears, which means a rise in starting capability
opposite to the static torque decreases.
According to the current investigation result, it still needs deep analysis and another
research area to reveal the optimum staggered angle. However, these predicted flow
characteristics could explain the behavior surrounding the turbine and need to be inves-
tigated numerically. The last important thing is revealing why static torque is inversely
proportional to the staggering angle.
 The Effect of Cylinder Type I-65° Staggered Upstream Convex Blade 231

Acknowledgments. This study was monetary supported by the Directorate General of Higher
Education Ministry of Education and Culture of the Republic of Indonesia in contract No.
008/E5/PG.02.00.PT/2022. In addition, the researchers were sponsored by the Department of
Mechanical Engineering, Institut Teknologi Sepuluh Nopember, Indonesia.

References
1. P. Javier and G. Montero, “Experimental Study of Flow Through a Savonius Wind Turbine.”
Accessed: Oct. 07, 2022. [Online]. Available: https://upcommons.upc.edu/bitstream/handle/
2117/99258/REPORT_45.pdf
2. H. A. Hassan Saeed, A. M. Nagib Elmekawy, and S. Z. Kassab, “Numerical study of improving
Savonius turbine power coefficient by various blade shapes,” Alexandria Engineering Journal,
vol. 58, no. 2, pp. 429–441, Jun. 2019. https://doi.org/10.1016/j.aej.2019.03.005.
3. P. A. Setiawan, T. Yuwono, and W. A. Widodo, “Numerical simulation on the improvement
of a Savonius vertical axis water turbine performance to advancing blade side with a circular
cylinder diameter variation,” IOP Conf Ser Earth Environ Sci, vol. 200, p. 012029, 2018,
https://doi.org/10.1088/1755-1315/200/1/012029.
4. G. Sakti, T. Yuwono, and W. A. Widodo, “Experimental and numerical investigation of I-
65-type cylinder effect on the Savonius wind turbine performance,” International Journal of
Mechanical and Mechatronics Engineering, vol. 19, no. 5, pp. 115–125, 2019.
5. D. M. Prabowoputra, A. R. Prabowo, S. Hadi, and J. M. Sohn, “Assessment of turbine stages
and blade numbers on modified 3D Savonius hydrokinetic, turbine performance using CFD
analysis,” Multidiscipline Modeling in Materials and Structures, vol. 17, no. 1, pp. 253–272,
2020, https://doi.org/10.1108/MMMS-12-2019-0224.
6. B. Elhadji Alpha, S. Lakshmi N., and J. Jechiel I., “An Assessment of Computational Fluid
Dynamics and Semi-empirical approaches for Vertical Axis Wind Turbine Analysis,” 2015.
https://doi.org/10.2991/iea-15.2015.29.
7. I. Marinić-Kragić, D. Vučina, and Z. Milas, “Computational analysis of Savonius wind tur-
bine modifications including novel scoop let-based design attained via smart numerical opti-
mization,” J Clean Prod, vol. 262, p. 121310, 2020, https://doi.org/10.1016/j.jclepro.2020.
121310.
8. S. Aiba and H. Watanabe, “Flow Characteristics of a Bluff Body Cut From a Circular
Cylinder,” J Fluids Eng, vol. 119, no. 2, pp. 453–454, 1997, https://doi.org/10.1115/1.281
9155.
9. Y. Triyogi, D. Suprayogi, and E. Spirda, “Reducing the drag on a circular cylinder by the
upstream installation of an I-type bluff body as passive control,” Proc Inst Mech Eng C J
Mech Eng Sci, vol. 223, no. 10, pp. 2291–2296, 2009, https://doi.org/10.1243/09544062J
MES1543.
10. N. H. Mahmoud, A. A. El-Haroun, E. Wahba, and M. H. Nasef, “An experimental study on
improvement of Savonius rotor performance,” Alexandria Engineering Journal, vol. 51, no.
1, Mar. 2012, https://doi.org/10.1016/j.aej.2012.07.003.
232 G. Sakti et al.

Open Access This chapter is licensed under the terms of the Creative Commons Attribution-
NonCommercial 4.0 International License (http://creativecommons.org/licenses/by-nc/4.0/),
which permits any noncommercial use, sharing, adaptation, distribution and reproduction in any
medium or format, as long as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons license and indicate if changes were made.
The images or other third party material in this chapter are included in the chapter’s Creative
Commons license, unless indicated otherwise in a credit line to the material. If material is not
included in the chapter’s Creative Commons license and your intended use is not permitted by
statutory regulation or exceeds the permitted use, you will need to obtain permission directly from
the copyright holder.