International Journal of Applied Engineering Research

ISSN 0973-4562 Volume 5 Number 4 (2010) pp. 721–728

© Research India Publications
http://www.ripublication.com/ijaer.htm

Modification of Airflow along a Flat Plate Using

Positive and Negative DC Corona Discharge

1
Lt Cdr Sravan k Khuntia and 2TM Muruganandam
1
Asst Professor, Naval Institute of Aeronautical Technology, Dept: Aeronatical
Engineering (Aero Structure), Kochi-682004
Mail id- khuntia.sravan@gmail.com
2
Asst Professor, IIT Madras, Dept: Aerospace, Chennai-600036

Abstract

This paper studies the effect of Positive and Negative DC Corona discharge on
airflow over an inclined Flat plate made of insulating material. The discharge
is caused by a wire of small radius and aluminum foil changing their role as
anode and cathode. The electrodes are flush mounted such that the ionic wind
is created adjacent to the plate and thereby control the Boundary layer. In this
paper visualization of subsonic (0.5 m/s) airflow around the plate at different
Voltages (ranging from 10 kV–32 kV) for positive and Negative corona are
presented. Results show that at higher voltages the ionic wind created by both
positive and negative corona is strong enough to considerably affect the
viscous Boundary layer. Negative corona shows great potential to spoil the
flow which perhaps can be used as spoilers or Airbrake.

Introduction
In the past many experiments have been carried out to show the influence of Dc
corona discharge on subsonic flow. However effect of negative DC Corona discharge
on air flow has got less attention. Basically the effect of DC corona is to produce ionic
wind which can modify airflow around an obstacle. In general, the phenomena relate
to the direct conversion of electrical energy into kinetic energy, and vice versa.
Atmospheric plasma is a partially ionized gas that has a tendency to remain overall
electrically neutral over large volumes due to the atmospheric pressure induced
collision frequency
Gas discharge deals with interaction of electron with the atoms of a gas or air as in
our case. When electrons are acted upon by an electric field with frequency w, such
that E = E0 exp(jwt), in presence of a gas then collisions are assumed to act as
continuous viscous damping force.
722 Lt Cdr Sravan k Khuntia and TM Muruganandam

The total rate of change of momentum in electric field direction equals to sum of
rate of change of momentum due to field and the momentum change produced by
collision in the field direction. Hence the motion of each particle in a collection of
charged particle is determined by fields of other particle and also outside field if
present.
In case of polyatomic gases, inelastic collision occurs between electrons of
forming avalanche and polyatomic molecules. This results not in ionization but
increasing rotational and vibrational energy of the molecules. This effect tends to
decrease total photoemission of avalanche. In case of polyatomic gas, photons
(produced by vehicular gases) are prevented from reaching cathode by polyatomic gas
molecules (as they absorb them).The ionization process is much probable in high field
region because electron have greater energy. As high voltage is applied across the
electrodes electrons are attracted towards anode. They do not cause appreciable
ionization until they are quite close to anode wire. A sheath is build up in the
immediate vicinity of anode (electrons stick to the anode hence surrounded by
anions), thus effective radius of anode increases. The field intensity drops as effective
radius increases till a point when field intensity is too weak to support further
ionization. The formation of such space-charge sheath quenches the discharge called
self-quenching. In our case since polyatomic molecules are moving with high
velocity, sheath is temporary and of lesser deby length. Dead time is of order of 10-4
sec. Restabilizing the operating field takes around 10-5 sec by application of external
voltage called External Quenching.
Corona discharge has been used to modify the sub sonic airflow around obstacles
which could be a sphere or a flat plate or an aerodynamic shape as airfoil. Ionic wind
has been widely studied in the case of a corona discharge between a needle and plate
electrode in stagnant free gas. Yabe and Ballereau [2] observed that the velocity of the
corona wind increases with the square root of the current. Velkof et al [3]
demonstrated that the transition point on a flat plate could be affected by the
application of an electric field. They observed a downstream shift of the transition
location.
However polyatomic gases are self quenching- when a polyatomic ion reach
cathode surface it interacts with electron in metal and neutralizes itself by pulling an
electron from metal or cathode surface. Self quenching property implies lifetime of
excited state molecules for emission of photon is greater/longer than lifetime for
dissociation. So instead of releasing energy the excitation energy goes into
dissociation of molecules and increase of kinetic energy of dissociated fragment.
Hence this self quenching mechanism presents second avalanche.
Luc Léger [1], Eric Moreau, and Gérard G. Touchard have shown that its
properties depended highly on the ambient air humidity and on the electrode
geometry. The discharge current density ranged typically from 0.5 to 2 mA/m,
corresponding to electrical power of 400 W to 1600 W/m of plasma sheet. Flow
visualizations were conducted for airstream velocities up to 1.4 m/s . When the
electric field was applied, the airflow was reattached to the plate and the wake size
was reduced. This has been confirmed by PIV experiments for higher airstream
velocities (up to 11 m/s, ). Measurements of velocity profile along the plate wall were
Modification of Airflow along a Flat Plate 723

conducted using a Pitot tube . These measurements showed that an increase in

discharge current results in enhancement of the airflow within the viscous boundary
layer.

Experiment
The electric supply required for our experiment is obtained from Zeonics High
Voltage unit. The unit has two sub units regulator and table top capacitor. The HV
unit can supply maximum up to 50 kV and 300 µA. The arrangement is unipolar with
positive (red) end and a Neutral (black). The objective of the first phase of experiment
was to determine effect of a dc corona discharge (both negative and positive) on
aerodynamics over a flat plate. This study is an extension of work by Luc Léger, Eric
Moreau, and Gérard G. Touchard [1] regarding the effect of corona discharge on
subsonic flow over a flat plate. Two sets of experiments were conducted.
1. The first set consists of visualizing the two–dimensional (2-D) airflow around
a flat plate at low velocities (0.5 m/s) with wire electrode at the leading edge.
2. The second set consists of visualizing the two–dimensional (2-D) airflow
around a flat plate at low velocities with wire electrode on the surface of the plate.
In this Set up we have used a plexiglass flat plate (30*20*1 cm3) etched with
copper wire of dia 0.6mm (which acts as anode) and 0.1mm thick aluminum foil
(which acts as anode) as shown in fig for different set up explained earlier. This
configuration is shown in Fig. 1(a), and called Plate I. The anode is a copper wire
located at the leading edge of the plate. Its diameter is 0.6 mm. The cathode is a 0.1
mm-thick aluminum plate. Its width is 1.9 cm. The distance between the two
electrodes is 4 cm. The second configuration, referred to as Plate II (Fig. 1(b)),
consists of a 0.6-mm-diameter wire anode flush mounted on the surface of the wall, at
4 cm downstream of the leading edge. The cathode is still a 0.1 mm-thick aluminum
plate with a width of 1.9 cm. The distance between the two electrodes is kept at 4 cm.
The complete set of experiment was carried in the glow regime of corona. The
electrical power consumption of a generalized glow discharge corresponds to a
voltage difference of about 32 kV with a current of about 100 A (the electrode length
equals to 20 cm and the distance between both electrodes is 4 cm). Because the
electrode length is 20 cm, the current density is 0.5 mA/m. Then, the discharge needs
an electrical power of about 3Wfor a plasma sheet surface equals (electrode length, 20
cm) (distance between electrodes, 4 cm) 0.008 m . This induces an electrical power
consumption of 400 W/m
724 Lt Cdr Sravan k Khuntia and TM Muruganandam

Figure 1: Side view and plan of the two arrangements. (a) Plate I. (b) Plate II.

After the experiment was set up, the smoke visualization was carried out in order
to observe the change in flow pattern as well as the shift in separation point.

Test section
Subsonic wind tunnel
Laser light source

Inclined plate Smoke generator

Figure 2: Experimental set up.

Modification of Airflow along a Flat Plate 725

The experiment was carried out keeping in view the following conditions:
1. With discharge off.
2. With discharge on.
3. BY increasing discharge current.
4. With different Reynold number.
5. Changing the Polarity

Results and Discussion

After studying both the configurations (plate I and Plate II) we came to the conclusion
that the Plate II showed best glow discharge result as has been explained in [1]. So
complete set of experiments were carried out on Plate II at 0.5 m/s and a fixed angle
of attack of 12 deg. The effect of DC discharge was studied and following points were
observed.
(i) The flow gets visibly attached to the plate as the voltage reaches 15 kV. The
attachment starts at positive anodic wire.
(ii) As the voltage increases the flow remains attached to the plate for a longer
time downstream.
(iii)As the voltage increases beyond 30 kV there seems to be lot of turbulence at
the cathodic plate.
(iv) In negative discharge (i,e when the wire is connected to earth and the plate to
positive terminal) although the flow gets attached but it jumps across the wire
as if there is a sheath around the wire thereby not allowing the flow to come
closer.
(v) In negative discharge all the above mentioned phenomenon occur at a lower
value of voltage.
(vi) The flow attachment occurs at 12 kV and the turbulence in flow occur at 25
kV.

Note: Red circles show glow at anode wire/plate and green circle show glow at
cathode plate/wire for positive and negative corona respectively

Smoke visualization in positive voltage

Without Discharge 10 kV
(a) (b)
726 Lt Cdr Sravan k Khuntia and TM Muruganandam

15 kV 20 kV
(c) (d)

24 kV 30 kV
(e) (f)

31 kV
(g)

Figure 3: Visualization of the airflow at 0.5m/s for Plate II at an angle of attack 15

deg with Positive discharge with (a) discharge off (b)10 kV (c)15 kV (d)20 kV
(e)24kV (f)30kV (g)31kV.

Smoke visualization in negative voltage

Without Discharge -15 kV

(a) (b)

-18 kV -22 kV
(c) (d)
Modification of Airflow along a Flat Plate 727

-23 kV -24 kV
(e) (f)

-26 kV
(g)
Figure 4: Visualization of the airflow at 0.5m/s for Plate II at an angle of attack 12
deg with Negative discharge with (a)discharge off (b)-15 kV (c)-18 kV (d)-22 kV (e)-
23kV (f)-24kV (g)-26kV. The experiment has been carried out in the glow regime.

Explanation
Here air behaves as an dielectric material. A dielectric material is a substance that is
poor conductor of electricity but an efficient supporter of electrostatic field An
important property of dielectric is its ability to support an electrostatic field while
dissipating minimal energy in the form of heat. The dielectric strength of air at STP is
3KV/mm.When field intensity increases it affects the electrons in the atomic orbital
and may cause atoms or molecules to polarize or liberate electrons.
An electron that is excited by an electric field E will ionize an atom or molecule of
a gas if it has enough energy called Ionization Potential (IP) of the particular gas
expressed in eV thereby creating an extra electron. However if electron does not have
enough energy it can impart a certain amount of kinetic energy to the particle it
collide with. This KE gained by atom is released as photons having certain energy and
wavelength hence glow discharge. If the energy of photon is below that of IP it may
be absorbed by other atom and re-released. However if energy of photons exceeds IP,
one or more electrons may be released depending on energy level. This can result in a
chain reaction which produces ionic wind. Since the electrodes are flush mounted on
the surface it will modify the no-slip condition thereby changing the Boundary Layer
thickness.
Due to high electric field atoms in the vicinity of anode get excited and release
outermost orbit electron. These electrons form a sheath around the wire electrode
called space-charge sheath. The positive ions formed thus, move towards cathode
creating ionic wind. The ionic wind with velocity 2.6 m/s (for 30 kV) creates low
pressure at the surface, as a result of which, the flow turns towards the low pressure
area. As these ions reach cathode, self quenching occurs (with metal electrons) and
ionization energy is released. As the voltage increases the number of ions produced
increases and eventually the ionic wind velocity increases. Hence with increase in
voltage, the flow is pulled towards the surface and also remains attached for longer
728 Lt Cdr Sravan k Khuntia and TM Muruganandam

distance downstream. However as the voltage increase further, the second avalanche
occurs and as a result, the flow gets disturbed enough causing turbulence.
The negative corona effect can be explained as follows: - Due to high electric field
atoms in the vicinity of cathode get excited and release outermost orbit electron. As
per Morton’s condition there builds up a very strong potential gradient at point
cathode surface as discharge develops. So comparatively more electrons are released
for negative corona and also it is pertinent here to mention here that electrons have
more energy compared to those in positive corona. Due to extra energy, electrons
attach to neutral atom to form negative ions that get pulled towards the positive plate
thereby causing ionic flow at lower voltage. The effect of this ionic wind on the flow
is similar to that of positive corona. At higher negative voltages however, there was a
strange flow field observed, which needs to be understood through detailed
investigations, and has scope for future works.

Conclusion
The result of atmospheric pressure plasma-subsonic flow interaction can be summarized
as follows: Ionic wind strength is dependent on one single major factor that is applied
voltage. Also that negative corona discharge affects the flow at a lower voltage and
creates turbulence in the boundary layer at higher voltage. The Boundary Layer
energizing effect of Plasma can be utilized to make better control surfaces for aircraft as
in Flaps, ailerons, rudders, winglets etc. The negative DC discharge also has great
potential to change the Boundary Layer as well as the flow streamlines. Further
experiments can be carried out to control the discharge path in a desired manner which
will be of immense help in designing control surfaces using this concept.
Negative Corona seems to be spoiling the flow thereby detaching the flow from
the surface, the concept of which can be used to design an Airbrake or to spoil the Lift
during landing of an aircraft and in similar applications. The other application could
be that, the same Control surface with reverse polarity can be used as high lift device
as well as an airbrake. Thus the experiments hold promising results.

References
[1] Leger L, Moreau E, Artana G, Touchard G (2001) Influence of a DC corona
discharge on the airflow along an inclined flat plate.
[2] A. Yabe, Y. Mori, and K. Hijikata, “EHD study of the corona wind between
wire and plate electrode,” AIAA J., vol. 16, no. 4, pp. 340–345, 1978.
[3] H. R. Velkoff and J. Ketcham, “Effect of electrostatic field on boundary layer
transition,” AIAA J., vol. 6, no. 7, pp. 1381–1383, 1968.
[4] Leger L, Moreau E, Touchard G (2002a) Effect of a DC corona electrical
discharge on the airflow along a flat plate. IEEE Trans Ind Appl 38:1478–
[5] Roth J R 2003 “Aerodynamic Flow Acceleration using Paraelectric and
PeristalticElectrohydrodynamic (EHD) Effects of a One Atmosphere Glow
DischargePlasma,” Physics of Plasmas 10 5 pp. 2117-2126.