American Institute of Aeronautics and Astronautics 092407

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An Investigation on the Application of DBD Plasma

Actuators as Pressure Sensors

Benjamin J. Chartier1, Maziar Arjomandi

2 and Benjamin S. Cazzolato

3

The University of Adelaide, Adelaide, South Australia, 5005, Australia

This paper investigates the possibility of using a Dielectric Barrier Discharge (DBD)

plasma aerodynamic actuator to measure ambient pressure conditions. Previous studies of

DBD actuators have focused on the use of plasma as a flow control device, which have

highlighted the change in the plasma formation at low ambient pressure and the mechanism

for plasma wind production. The modification of the plasma formation process has

significant influence on the plasma driving circuit and therefore can be used to extract data

regarding the local flow conditions. The time scales of the plasma formation orders of

magnitude smaller than that of the flow structures. Sufficient sensitivity of the plasma

driving circuit to the ambient pressure implies that this phenomenon could prove useful for

application of this technology as a pressure sensor, for relatively high bandwidth

measurements.

Nomenclature

d = diameter of molecules

NA = Avogadro’s number

P = ambient static pressure

R = gas constant

T = temperature

λ = mean free path

I. Introduction

ver recent years Dielectric Barrier Discharge (DBD) plasma actuators have received a great deal of interest

from academia and industry alike. One application of the glow discharge produced by DBD plasma actuators

has been as a flow control device, to inject momentum into the lower boundary layer and thereby promote desirable

flow conditions in a number of applications. There have been a number of investigations into this effect at varying

ambient pressure conditions. It has been demonstrated that both the formation of the plasma and the effectiveness of

the actuator as a flow control device are sensitive to ambient pressure conditions. This sensitivity may be

manipulated in order to use the DBD as a pressure sensor, and provide additional functionality. This motion forms

the basis for this paper.

The use of glow discharge, and other plasma sources, for flow measurement is not a new concept1,2. The first

published results of glow discharge for velocity measurement were in 1934 by Lindvall1 who utilized D.C. glow

discharge for the velocity measurements behind a cylinder in cross flow. Later, in 1949, Mettler2 developed a D.C.

glow discharge for measuring flow velocities in supersonic flow.

The plasma-based measurement concept has recently been further developed by Matlis et al.3 using an A.C.

plasma anemometer which they intend for use in hypersonic Mach number flows. However this research focused on

the complex computer control of an A.C. circuit in order to maintain sensitivity across a range of flow velocities,

using plasma generated from specifically designed electrodes, with the geometry containing no dielectric barrier.

The influence of the incident flow velocity between the charged electrodes producing a glow discharge was

investigated up to flow speeds of roughly 50 m/s. This concept works on the principle that gas traversing the gap

1 PhD Candidate, The School of Mechanical Engineering, AIAA Student Member 2 Lecturer, The School of Mechanical Engineering, AIAA Member 3 Associate Professor, The School of Mechanical Engineering, AIAA Member

O

47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition5 - 8 January 2009, Orlando, Florida

AIAA 2009-651

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American Institute of Aeronautics and Astronautics 092407

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between the electrodes, perpendicular to the flow, affects the plasma production cycle and thereby changes the

voltage-current behavior allowing flow measurements to be taken.

In addition to velocity measurements, glow discharge has been utilized for turbulence measurement in

applications where existing hot wire techniques were not viable, due to particulates in the flow and delicate hot wire

probe design4. Since time scales of the plasma formation are significantly smaller than that of the fluctuations in

fluid flows, a high temporal resolution may be achieved when utilizing this phenomenon as a flow sensor. Users of

this technology however found that significant velocity drift was apparent with this technology4.

Two recent studies5,6 have investigated the specific use of DBD plasma actuators, at varying ambient pressure

conditions. Abe et al.5 presented an experimental parametric study of a plasma actuator in which the momentum

transfer characteristics of the device to neutral gas at various ambient pressures was investigated. The authors found

that a variation of the static pressure had a significant effect on the formation of plasma and the induced momentum

from the actuator. It was also found that when the ambient gas pressure is reduced below 1 atm the momentum

transfer at first increases until a pressure of roughly 0.6 atm, then drops away significantly thereafter. From a

graphical analysis of the plasma voltage and current waveforms it was deduced that as the ambient gas pressure

drops, the pulse peaks in the current waveform associated with plasma decrease in magnitude. However the duration

of current development is increased, which suggests that the current is formed over a long time cycle at reduced

atmospheric pressure.

Further developments by Benard et al.6 have shown similar results related to the actuator performance on

ambient conditions. The authors of this work showed that the maximum velocity downstream of the plasma actuator

varies with ambient gas pressure. Results of this work showed that for small reductions of ambient pressure that the

maximum velocity increased at first, and peaked at 0.6 atmospheres, and below this ambient pressure the maximum

velocity magnitude dropped away, similar to the results of Abe, et al.5. Results were also presented for the voltage

and current waveforms, along with pictures of the plasma at low pressure. The study demonstrated the change in the

extent of the plasma over the covered electrode increases significantly for reductions in ambient pressure. The

increase in current pulse values with the reduction in ambient pressure, correlated with the plasma extent findings,

and resulted in a significant increase in power consumption. This implies that there is a significant possibility to

utilize this phenomenon, of the increase in pulse magnitude in the current waveform, in order to estimate the

ambient pressure from a given applied voltage to a plasma actuator.

The study presented aims to build on these works by further investigating the plasma voltage and current cycles

at reduced atmospheric conditions. More specifically the study aims to investigate the current signals from a plasma

actuator, at varying ambient conditions, in order to understand in greater detail the changes in the plasma formation

process, and how this might be used as a pressure sensor with practical application.

II. Theoretical Hypothesis

It is well understood that the formation of

DBD surface plasma is a result of high energy

electrons colliding with neutral gas atoms and

molecules in order to generate a partially

ionized gas7-9. The physical mechanisms are

governed by the properties of the ambient gas,

which depends on a multitude of variables,

such as pressure5,6, species

5, relative humidity

10

,,Mach number11 and the geometry of the

actuator. A schematic of a typical DBD plasma

actuator can be found in Fig. 1.

Gases, on a molecular level, are comprised of molecules with discrete energies, continually colliding into each

other and exchanging kinetic energy. The mean free path, is the statistical average distance between gas molecules

in a particular gas, and is a function of the pressure, temperature and the size of the constituent molecule. Eq. 1

shows the relationship of the mean free path of a given gas, and the ambient conditions. In this study the pressure is

varied significantly, while other operating conditions are held relatively constant, and thus with the reduction of the

pressure the mean free path increases significantly. This effectively reduces the ‘resistance’ of plasma formation and

causes a greater volume of plasma to form, as charged particles develop higher kinetic energies before collisions

with neutral air molecules. It is hypothesized that this increase in the energy of collisions may be the main

contributor to the growth in plasma formation.

Figure 1 Schematic of the Single DBD plasma actuator

Dielectric Material

~ AC

Exposed Electrode

Covered Electrode

Plasma Region

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22 PNd

RT

Aπλ = (1)

III. Experimental Setup

The plasma actuator was constructed using 2 mm plexiglass as the dielectric material, and tinned copper

electrodes of width of 12.7 mm, length of 100 mm and a thickness of approximately 25 microns. The electrodes

were arranged in a gapless geometric arrangement. The lower electrode was encapsulated using an epoxy, to ensure

that plasma is only formed on the upper electrode. The experiments were conducted in a vacuum dome, Figure 2, at

pressures ranging from 20 kPa (0.2 atm) to standard atmospheric pressure, of roughly 100 kPa (1 atm).

The plasma was formed using a proprietary

‘Minipuls2’ plasma generator12. This system has been

designed to generate high frequency AC high voltages

with an operational range from 5 to 30 kHz with

amplitudes up to 24 kVp-p. The system has an in-built high

voltage divider, and a current output block, from which the

experimental measurements were made, after calibration.

The measurements were recorded using a PicoScope

5204 PC based oscilloscope, to resolve the voltage and

current waveforms. This system has a sample rate of 1

GS/s, and a maximum record length of 128 MS, with 8 bit

resolution and a bandwidth of 250 MHz.

For this set of experiments an applied frequency and

voltage of 10 kHz and 18.5 kVp-p (6.54 kVRMS) were used

respectively, as this resulted in the most viable change in

plasma formation with ambient pressure variations, for the

specified plasma actuator geometry. The temperature was

monitored using a type-K thermocouple, to ensure that the

temperature did not drift significantly over the course of

the experiment. The thermodynamic effects were

negligible, as the temperature was allowed to stabilize

prior to measurements.

IV. Results

A. Plasma Formation

As the ambient pressure was reduced the plasma

was observed to become more filamentary, and

extends over a larger proportion of the covered

electroded. This can be seen by the comparison of

plasma developed for atmospheric pressure (Fig. 3a)

and 20% of atmospheric pressure (Fig. 3b).

As mentioned, the applied driving parameters, ,

were chosen such that at atmospheric conditions the

plasma formed as a uniform discharge, however at low

pressures the plasma was observed to not fully extend

over the entire encapsulated electrode,. A saturation of

the encapsulated electrode would negate any changes

in ambient pressure, as there would most likely not be

any observable increase in plasma formation.

Figure 2 Experimental set up showing the vacuum

dome, pump and test actuator

(a) (b)

Figure 3 Plasma actuator at atmospheric conditions (a) and

20% of atmospheric pressure (b).

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B. Frequency components

Fig. 4 shows a comparison of the plasma current cycle at three pressure levels. The plasma formation at lower

ambient pressures resulted in larger current spikes in the time series data. This phenomenon is believed to be

associated with the increase in micro-discharges at the reduced ambient pressure, and the growth in plasma

formation. At lower ambient pressures the plasma forms a greater number of micro discharges, of roughly the same

order of current magnitude. Through frequency analysis of the current waveform signal, a relationship between the

current waveform and the ambient pressure has been developed.

The current waveform, of a typical plasma driving cycle, has a number of distinctive features. There exist two

high frequency transient tonal bursts that are characteristic of the electronic circuit used to drive the actuator, and the

impedance mismatch for the specific plasma actuator. Fig. 5 shows an analysis of portions of the waveform, in both

the time and frequency domains. The top graphs show the current waveform in its entirety, with time and frequency

domain information presented on the left and right of the figure, respectively. The second and third sets of plots

analyze the two tonal burst at roughly 40 and 90 µs, respectively. The final plots analyze only the plasma

production component of the waveform; i.e. when the exposed electrode is the anode.

Upon analysis of these tones it was understood that these frequencies were of similar order of magnitude as that

of the micro-discharges associated with the plasma formation, with periods of roughly 20 ns. This implied that a

direct analysis of the frequency components in the plasma frequency will be affected by the driving circuitry, and

any impedance mismatch of the plasma actuator.

Analysis of the plasma production component of the waveform, bottom graphs in Fig. 5, suggests that there is a

sub-harmonic frequency component which is independent of the tonal transient signals, and may allow for further

analysis. This frequency component, in the frequency band of 3×105 to 3×10

6 Hz, results from the quasi periodic

production of plasma micro discharges, in the current waveform. It is hypothesized that the combination of a

number of plasma micro discharges, quasi-evenly spaced results in a current signal component in the specified

frequency band.

0 25 50 75 100−50

−25

0

25

501 Atm

Time (µs)

Cur

rent

(m

Am

ps)

0 25 50 75 100−50

−25

0

25

500.6 Atm

Time (µs)0 25 50 75 100

−50

−25

0

25

500.2 Atm

Time (µs) Figure 4 Comparison of the example current waveforms across the range of pressure levels

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0 20 40 60 80 100−50

0

50Filtered current

Time (µs)

Cur

rent

(m

Am

p)

106

107

108

0

0.5

1x 10

−6 Frequency domain

Spe

ctru

m (

Am

p2 /Hz)

90 90.5 91 91.5 92−50

0

50

Time (µs)

Cur

rent

(m

Am

p)

106

107

108

0

0.5

1x 10

−6S

pect

rum

(A

mp2 /H

z)

39 39.5 40 40.5 41−50

0

50

Time (µs)

Cur

rent

(m

Am

p)

106

107

108

0

0.5

1x 10

−6

Spe

ctru

m (

Am

p2 /Hz)

50 60 70 80−50

0

50

Time (µs)

Cur

rent

(m

Am

p)

106

107

108

0

1

2

3x 10

−7

Frequency (Hz)

Spe

ctru

m (

Am

p/H

z)

Figure 5 Comparison of waveform components in time and frequency domain, for atmospheric pressure levels;

time and frequency domain analysis on the left and right respectively. Top: full waveform, second and third:

tonal burst waveforms sections only, bottom: plasma production waveform only.

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C. Signal Analysis

In order to derive a relationship between the ambient

pressure and the plasma cycle the current waveform signal was

manipulated. The raw signal was first passed through a high

pass 4-pole Butterworth filter, to remove the actuation

frequency oscillation (of 10 kHz) from the analysis. The cut-off

frequency for this filter was chosen to be 100 kHz, as this was

sufficient to remove all underlying oscillations. The filter poles

were sufficiently low to still allow analysis of the high

frequency ‘noise’ associated with the formation of plasma. The

Butterworth filter was implemented using the MATLAB

command ‘filtfilt’ which essentially filters the data temporally

both forwards and backwards, to yield an 8 pole Butterworth

filter with zero phase offset. The micro-discharges developed

from the formation of plasma exist on the order of roughly 10

ns. This implies that the dominant time period is roughly 20 ns,

implying a signal frequency of roughly of the order of 50 MHz.

The effect of this filtering on an example current waveform can

be seen in Fig. 6.

Fig. 7 shows the result of a FFT taken on the filtered signal, for the five pressure levels tested, in order to

understand the signal spectrum associated with the formation of plasma in the frequency domain. Welch’s power

spectral density estimate was used on 10 continuous driving cycles divided up by 8 spectral Hanning weighting

functions, with 50% overlap. Further smoothing of the data was achieved using an 8-point moving average filter

conducted in a similar ‘filtfilt’ scheme. Fig. 7 clearly shows that an increase in RMS magnitude at the lower ambient

pressure levels, with the frequency range of 3×105 to 3×10

6 Hz. It is interesting to note that this increase is not at the

frequencies associated with the micro discharges. It is suggested that the greater number of micro-discharges

physically observed at lower pressures is relatively uniformly spread in the time domain. This is clearly shown in

Fig. 7 below. There is a noticeable change in current spectrum in the frequency band between 3×105 to 3×10

6 Hz.

This frequency band corresponds to the quasi-periodic nature of the plasma. As the plasma discharges grow in

magnitude the spectral components associated with the quasi-periodic nature of the micro-discharges likewise grow

in magnitude.

TFig. 8 shows the relationship between filtered waveform current RMS and absolute ambient pressure, for the

discrete frequency band of 3×105 to 3×10

6 Hz, highlighted previously. A quadratic curve has been fitted to the data

to show the relationship. At atmospheric pressure the current RMS is the lowest value, and as the pressure is reduced

0 20 40 60 80 100−50

−25

0

25

50

Time (µs)

Cur

rent

(m

Am

p)

Raw currentFiltered current

Figure 6 Filtered signal overlayed with the raw

current signal at atmospheric conditions

105

106

107

108

2

4

6

8x 10

−7

Frequency (Hz)

Cur

rent

Spe

ctru

m (

Am

ps/H

z)

1 Atm0.8 Atm0.6 Atm0.4 Atm0.2 Atm

Figure 7 Comparison of the frequency spectrum

with ambient pressure level.

0 20 40 60 80 100 1200.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Absolute Pressure (kPa)

Cur

rent

RM

S (

mA

mps

)

Figure 8 Band pass current RMS Vs absolute

pressure (kPa) (Frequency range from 3×105Hz to

3×106Hz). The blue dotted line is a curve fit

quadratic and error bars correspond to the 95%

confidence interval.

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the RMS values increases, as expected due to the increased concentration of current spikes from micro-discharges of

the plasma actuator. It should be noted that further reductions in pressure should result in an increased RMS signal,

on the graph of Fig. 8, however there exists a limitation as the covered electrode becomes saturated with glow

discharge. Speculatively, at higher pressures it is expected that the plasma is extinguished and therefore, apart from

underlying residuals, the current RMS becomes more or less zero.

V. Conclusions

The DBD plasma aerodynamic actuator is sensitive to atmospheric pressure. This study has investigated the

influence that ambient pressure has on the DBD current waveform. It was found that reductions in the ambient

pressure had an effect on the frequency component of the current waveform, drawn by the actuator. The most

sensitive frequency band for the current waveform, with respect to pressure is from 3×105 to 3×10

6 Hz. This was

attributed to the plasma formation process, which is closely linked to the ambient conditions.

The use of this technology as a pressure sensor is not without drawbacks, as changes to other parameters, such as

ambient flow velocity and humidity, as were not simultaneously investigated. Despite these limitations, the results

presented in this paper show that the DBD plasma actuator has promise as an ambient pressure sensor, with

potentially high bandwidth, due to the short time scale of the plasma formation process. Further work is required to

assess the sensitivity of these actuators to other conditions, such as humidity and local flow velocities, along with

electrode variations including length, operating parameters and dielectric material.

Acknowledgments

The authors would like to acknowledge The Sir Ross and Sir Keith Smith Fund, for their financial assistance and

support of plasma research. The authors would also like to acknowledge the support of Mr. Brad A. Gibson, of the

University of Adelaide, and Mr. Berkant Göksel, of Electrofluidsystems Ltd., for ongoing technical assistance and

advice regarding plasma aerodynamic actuators. Additionally, the authors wish to acknowledge the technical support

of electrical workshop of the School of Mechanical Engineering.

Disclaimer

Research undertaken for this report has been assisted with a grant from the Smith Fund (www.smithfund.org.au).

The support is acknowledges and greatly appreciated.

The Smith Fund by providing for this project does not verify the accuracy of any findings or any representation

contained in it. Any reliance in any written report or information provided to you should be based solely on your

own assessment and conclusions.

The Smith Fund does not accept any responsibility or liability from any persons, company or entity that may

have relied on any written report or representations contained in this report if that person, company or entity suffers

any loss (financial or otherwise) as a result.

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California Institute of Technology, 1949. 3Matlis, E., Corke, T., Gogineni, S. “A.C. Plasma Anemometer for Hypersonic Mach Number Experiments,” 44th AIAA

Aerospace Sciences Meeting and Exhibit, AIAA 2006-1245, Reno, Nevada, 2006. 4Bradshaw, P. An Introduction to Turbulence and its Measurement, 1st ed., Pergamon Press Ltd., Oxford, 1971. 5Abe, T., Takizawa, Y., Sato, S., Kimura, N. “A Parametric Experimental Study for Momentum Transfer by Plasma

Actuator”, 45th AIAA Aerospace Sciences Meeting and Exhibit, AIAA 2007-187, Reno, Nevada, 2007. 6Benard, N., Balcon, N. and Moreau E. “Electric wind produced by a surface dielectric barrier discharge operating in air at

different pressures: Aeronautical control insights”, J. of Phys: Appl. Phys, vol. 41, 042002 (5pp), 2007. 7Roth, J. Industrial Plasma Engineering; Vol. 2 Applications to non-thermal plasma processing, Institute of Physics

Publishing, 2001. 8Raizer, Y. Gas Discharge Physics, Springer Verlag, 1991. 9Mitchner, M. and Kruger, C. Partially Ionized Gases, John Wiley and Sons, Inc., 1992. 10Anderson, R. and Roy, S. 2006 “Preliminary Experiments of Barrier Discharge Plasma Actuators using Dry and Humid

Air”, 44th AIAA Aerospace Sciences Meeting and Exhibit, AIAA 2006-0369, Reno, Nevada. 11Pavon, S., Dorier, J-L., Hollenstein, Ch., Ott, P., Leyland, P., 2007 “Effects of high-speed airflows on a surface dielectric

barrier discharge”, J. Phys. D: Appl. Phys. Vol. 40, pp. 1733-1741. 12Goksel, B., 2007 “Minipuls2 Product Sheet”, Electrofluidsystems.