Investigation of plasma-generated jets for supersonic flow control

Venkat Narayanaswamy, Jichul Shin, Noel. T. Clemens and Laxminarayan. L. Raja

Center for Aeromechanics Research

The University of Texas at Austin

Austin TX, 78712

Abstract

The effectiveness of pulsed plasma jets for supersonic flow control applications iscurrently being investigated experimentally. A pulsed plasma jet (also termed a“spark-jet” in Ref. 5), is a high-speed synthetic-type jet that is generated by strikingan electrical discharge in a small cavity; the gas in the cavity expands owing to theheating is allowed to escape through a small orifice. The current studydemonstrated that typical jet velocities of about 250 m/s can be induced withdischarge energies of about 0.5 J. The maximum pulsing frequency is about 5 kHz,which seems to be limited by the recharge time of the chamber. The plasma jets arebeing explored as a means of controlling the unsteadiness of shock-inducedturbulent separation. An array of jets was issued from different locations upstreamof a 30-degree compression ramp in a Mach 3 flow. Furthermore, two different jetconfigurations were used: normal injection and pitched and skewed injection. Thepitched and skewed configuration was tried to see if the jets could act as high-bandwidth pulsed vortex generators. The interaction between the jets and theseparation shock was studied using phase-locked schlieren imaging. Results showthat the plasma jets cause a significant disturbance to the separation shock andclearly influence its unsteadiness. While all plasma jet configurations tested causedan upstream motion of the separation shock, pitched and skewed plasma jets causedan initial downstream shock motion before the upstream motion. Time-resolvedschlieren imaging was conducted to study the response of the separation shock toindividual pulses of the jets. At a pulsing frequency of 1 and 2 kHz the separationshock motion seems to be quite independent of the motion caused by the previouspulse. However at 5 kHz, there appears to be more coupling between pulses andpossible stabilization of the shock motion. It was also found that for small pulsewidths, the energy per unit pulse also determines the magnitude of the separationshock motion.

Introduction

Synthetic jets have found widespread use in low speed aerodynamics applications, particularly for the

control of boundary layer separation [1]. The most common methods for generating synthetic jets employ

acoustic resonators [2] or piezo-electric membranes [3]. The major disadvantage of these methods for

supersonic flow applications is that the stagnation pressure that is generated is relatively small and so the

peak velocity of the jet is typically too low and/or the maximum frequency that can be obtained is too

46th AIAA Aerospace Sciences Meeting and Exhibit7 - 10 January 2008, Reno, Nevada

AIAA 2008-285

Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

limited. A notle exception to this is the combustion-driven jet [4], which uses a premixed fuel-air mixture

to drive a high velocity jet at relatively high bandwidths. In a somewhat related technique, electric

discharges were used to obtain pulsed plasma jets (called ‘spark jets’) [5,6]. Reference 5 used numerical

simulations to suggest that jet velocities as high as several hundred m/s can potentially be achieved.

Preliminary experimental results described in Ref. 6 showed that the velocity several diameters away from

the jet exit is as high as 100 m/s.

The characteristics of steady jets-in-crossflow have been studied extensively in both subsonic [7] and

supersonic cross-flows [8,9]. For example, it has been demonstrated that pitching and skewing the injection

angle leads to increased streamwise vorticity and hence greater potential for energizing the boundary layer

[10]. Similarly, the evolution of transversely injected jets, at different injection angles, was studied in detail

for subsonic and transonic crossflows [11]. In agreement with low-speed flows, it was reported that

increased streamwise vorticity was created with pitched jets as compared to normal jets.

In supersonic / hypersonic flows, synthetic jets have seen less use owing to the difficulty of achieving

high momentum fluxes with fast acting micromechanical actuators. For this reason most jet control studies

in supersonic flows have used steady jets, relatively low-frequency fully modulated pulsed jets, or high-

frequency weakly modulated sonic jets. For example, Auvity et al [12] showed that a transversely injected

sonic continuous jet introduced large-scale organized structures downstream in an otherwise “disorganized”

boundary layer. Bueno et al. [13] showed that steady and pulsed pitched and skewed sonic jets upstream of

a Mach 2 cylinder interaction caused a downstream shift of the separation shock by about one-third of

cylinder thickness. Similar results [14] were also demonstrated using pulsed jets to control the SWBLI

generated by a 20-degree ramp. Selig and Smits [15] used a novel rotating slotted-drum to create a high

frequency, large mass flux transverse jet to control the unsteadiness of a shock wave / boundary layer

interaction. The pressure measurements made at different stations in the interaction region showed that at

sufficiently high pulsing frequency (2 kHz), the separation shock motion could be locked to the forcing

frequency. These previous studies have demonstrated the potential for using transverse jets or vortex

generator jets (pitched and skewed) to influence the unsteadiness of shock-induced turbulent separation.

However, these jets have been limited to low pulsing frequencies, which precludes their use to control

actively the separated flow unsteadiness.

In this work we are exploring the use of the spark jets to provide high bandwidth control of the

unsteadiness of shock wave / boundary layer interactions. These actuators are very similar to those

developed in Ref. 5, although we have tested a wider range of configurations including pitched, and pitched

and skewed jets. Furthermore, to amplify the actuation effect, the jets were arranged in a spanwise array of

three jets. The effect of the jets on the mean and unsteady characteristics of a Mach 3 compression ramp

interaction is investigated. In this preliminary study the diagnostic techniques are limited to phase-locked

and high-speed schlieren imaging.

Operation of the Pulsed Plasma Jet

The pulsed plasma jet operates by striking a short-duration, high-current electric arc in a closed cavity

containing a small orifice. The gas inside the cavity is electrothermally heated by the arc, which leads to a

rapid increase in pressure if the heating rate is higher than the rate of gas discharge. This high-pressure gas

issues through the orifice and forms the pulsed plasma jet. The velocity of the jet is related to the cavity

pressure, which in turn depends on the rate of deposition of energy. Once the jet issues from the cavity, it

creates a partial vacuum inside the cavity. Ambient gas is then drawn into the cavity, which recharges the

cavity for the next pulse. Thus the spark-jet actuator does not require an external gas feed, as is the case for

conventional plasma-jet generators [16].

Experimental Set upWind Tunnel facility

The experimental work was conducted in a Mach 3 wind tunnel located at The University of Texas at

Austin. The overall schematic of the apparatus is shown in Fig. 1. The wind tunnel test section has a cross

sectional area of 5 cm × 5 cm and a length of 0.4 m. An acrylic splitter plate extends from the plenum

section into the test section. The plasma actuator is placed at the trailing edge of the splitter plate.

Pressurized air from a 500-ft3 high-pressure tank is fed to the tunnel and is discharged into a 1000 ft3

vacuum tank. The test section static pressure is maintained at 35 torr for all the cases studied. The incoming

boundary layer is tripped several cm’s upstream of the jet orifice so as to obtain a fully developed turbulent

boundary layer. The boundary layer thickness, measured by previous workers [17] of this tunnel, is

approximately 4 mm.

Pulsed plasma jet actuator

The schematic and the circuitry of the pulsed plasma jet actuator are shown in Fig. 2. This design is a

modification of the design used in Ref. 5. A cylindrical cavity of 0.093 inch dia. was made in ceramic plate

and electrodes (0.093 inch dia.) were inserted from both ends to form a cavity bounded by electrodes. The

cavity material used for the present flow control application is boron nitride. Tests made in a stagnant

chamber (see results and discussion section) showed that boron nitride cavity generated a stronger plasma

jet as compared to a cavity made of MESCOR, which was used in our initial tests to characterize the

plasma jet. The gap between electrodes can be varied to provide different cavity volumes. A small orifice

of about 1.8 mm in diameter was made in the middle of cavity. The tip of the cathode was sharpened to

decrease the breakdown voltage. A capacitor (0.22µF) was charged by the DC power supply (Spellman,

SL2PN1200) until the discharge was formed between electrodes. Upon breakdown, the capacitor provided

the high current needed to sustain the non-equilibrium arc between the electrodes. A timing circuit

including BNC delay generators and MOSFET switches was made to repeat the charge-discharge cycle at

kilohertz rates. Instantaneous peak discharge currents of about 1.2 to 12 A have been tested. The pulse

width of the discharge employed in this work is 20 µs unless mentioned otherwise. Pulsing frequencies

upto 5 kHz has been achieved at 2 A discharge current. An array of 3 pulsed plasma jets placed 4 mm apart

was used for all the flow control applications reported in this work. The main restriction to the pulsing

frequency is the replenishment time required to fill the partial vacuum created in the cavity at the end of

each pulse. Initial tests were performed in a stagnant chamber maintained at 35 torr to determine the factors

that influence the strength of the spark-jet. Phase locked schlieren imaging was done to capture jet structure

at various time delays after the initiation of the discharge.

Phase locked schlieren Imaging

Low-repetition rate flash-lamp schlieren imaging was used to measure the contact surface velocity and

precursor shock velocity of the pulsed plasma jet. The flash lamp was pulsed at 60 Hz using a BNC delay

generator, and the pulse duration of about 2 µs was small enough to provide an instantaneous snapshot of

the flow. The flow was imaged through acrylic windows on each side of the test section. The light was

collimated and focused by 1m focal length concave mirrors. The schlieren images were captured using a

Photoron APX camera with a framing rate of 60 Hz, triggered internally, and an exposure time of 16 ms.

The images (1024×512 pixel resolution) were acquired for 10 seconds. Phase locking was achieved by

pulsing the lamp at a predetermined delay from the start of the discharge trigger.

Results

Pulsed Plasma Jet Characterization

Initial experiments were done in a stagnant chamber in order to characterize the contact surface velocity,

temperature of the contact surface and voltage-current characteristics of the plasma jet. The results shown

in this section were obtained using actuators with cavities made of MESCOR. In this section we discuss the

measurement of the velocity of the contact surface as it exits the orifice, as that is one way to characterize

the strength of the jet. Phase locked schlieren imaging was done to obtain the position of contact surface at

different time delays. The contact-surface velocity is measured from the slope of the contact surface

trajectory close to exit. It should be mentioned that the contact surface velocity is not necessarily the same

as the local jet fluid velocity. However the contact surface velocity is a definite indication of the magnitude

of the jet fluid velocity. With this caveat in mind, from now on we shall call the measured contact-surface

velocity the “jet velocity,” and the measured value is assumed to be a lower limit of the true jet velocity.

We found that the motion of the jet generated is sufficiently repeatable to enable us to make accurate

contact surface velocity measurements. Figure 3 shows the jet front at different time delays from the start

of discharge trigger for a discharge current of about 4 A. The velocity measurement was made by

calculating the displacement of the contact surface at each time delay. The measured distances, at a given

time delay, were repeatable to within about 10%. From this sequence the velocity of the contact surface is

estimated to be about 280 m/s. The same method was applied to the 1.2 A case and we found that the

velocity of the contact surface was about 250 m/s. Figure 4 shows the variation of jet velocity with

discharge current. It is seen that the jet velocity is a relatively weak function of discharge current since it

increases from 250 m/s to 320 m/s as the discharge current is increased from 1.2 A to 11 A. The velocity of

the spark-jet reported in Ref. 5 is about 100 m/s. However measurements in [5] were made at locations of a

few diameters downstream of the jet exit. Note that we observed the jet velocity increased by about 30% at

each discharge current when we used boron nitride as the cavity material instead of MESCOR, and so

boron nitride was used for the flow control experiments discussed in the subsequent sections. The jet

velocity was found to be relatively insensitive to other relevant parameters like orifice diameter, material of

the electrodes, volume of the cavity and pulsing frequency of the discharge for the frequency range

between 60 Hz and 5 kHz. For example, the jet velocity of a discharge pulsed at 60 Hz and 5 kHz carrying

the same instantaneous current was measured to be identical.

The current-voltage characteristic of the discharge showed that the discharge is an arc discharge. The

power input during the discharge is about 1 kW for 4 A discharge current. Optical emission spectroscopy

was performed to measure the gas heating due to the discharge. These measurements are not shown here

for brevity but it was found that the gas temperature of a 6.5 A discharge is about 800-1000 K while the gas

temperature of a 1.2 A discharge is about 600 – 700 K.

Effect on a Shock Wave / Boundary Layer Interaction

As a first test of the effectiveness of the plasma jet to influence the flow, the plasma jet was tested to

modify the separation shock generated by a 30-degree compression ramp in a Mach 3 supersonic flow. An

array of 3spark jet was injected normally approximately one boundary layer thickness (δ~ 4 mm) upstream

of the compression ramp, which is well downstream of the separation shock. In this region of the flow, the

static pressure is much higher than 35 torr. It should be noted in this pressure range, earlier work by Shin et

al. [18] showed that a surface glow discharge, with approximately the same average power, does not cause

significant flow actuation. The discharge current of the spark-jet was set at 1.2 A. Figure 5(a) shows an

instantaneous schlieren image without the injection. Several compression waves are seen to emanate from

the intermittent region (locations downstream of 2.5 δ from compression corner) to form a single shock

structure at an elevated location. Fig. 5(b) shows an instantaneous phase-locked schlieren image at a time

delay of 40 µs from the start of the discharge trigger. It can be clearly seen that the entire separation shock

is ‘bulged out’ due to the passage of the plasma jet. In fact, the shock foot moves about 1δ upstream as the

plasma jet propagates into the interaction. This clearly illustrates that the spark jet induces a significant

disturbance to the flow.

Experiments were conducted to study the effect of pulsed normal injection at different streamwise

locations upstream of the compression ramp corner in order to find the location of maximum disturbance.

The discharge current was fixed at 1.2 A. An array of 3 spark jets was injected from 1δ, 2.5δ (just

downstream of the separation shock), 4δ (upstream of the separation shock) and 9δ (far upstream of

separation shock) upstream of the ramp corner. The phase locked images were averaged to remove the

effects of turbulent fluctuations on the time sequence. Figure 6 (a) and (b) shows two frames of the phase-

averaged schlieren time-sequence for a plasma jet located at 1δ and 4δ, respectively. It is clear that the

impact of the spark jet array is to cause the separation shock to move upstream and parts of the shock bulge

out as the plasma jet passes through it. This trend was seen even for 1δ (Figs. 5(b) and 6 (b)) and 2.5δ (not

shown) cases; however the jet located at 1δ caused a greater shift in the separation shock. The qualitative

difference between the 1δ and 4δ case is that at 1δ, the jet is injected into the separation bubble, whereas at

4δ, the jet is injected upstream of the separation shock. The smaller shift in the separation shock for the 4δ

shows that the boundary layer largely recovers from its perturbed state by the time it reaches the

interaction. At very large distances from the compression corner (9δ) the upstream shift of the shock is

even smaller.

An attempt was made to estimate the time scale of the interaction – i.e. the time taken by the separation

shock to regain its average undisturbed location upon being perturbed by the spark jet. In the 4δ case, we

observed that it took longer than 100 µs for the shock foot to recover to its original position. This time scale

much longer than the pulse width of the jet (20 µs). This suggests that the pulsed jet may couple to an

instability in the separated flow, with the implication that pulsing frequencies of 5 to 10 kHz may be

sufficient to cause a permanent shift of the separation shock.

Given the ability of the jet to cause significant changes in separation shock, experiments were

performed to determine if: (i) the spark-jet can be configured to act as a pulsed vortex generator, and (ii)

the spark-jet can be pulsed at sufficiently high frequency in order to control the low frequency motion of

the separation shock. To investigate (i) an array of three jets were tested at different pitch and skew angles

and the resulting influence on the compression ramp interaction was visualized. To investigate (ii), high

speed schlieren was performed at 10 kHz in order to time resolve the shock motion after the firing of the

spark jet array.

Effect of geometric parameters

Experiments were done to investigate the effect of changing the jet pitch angle (without skew). It is

known that the pitched jet causes momentum addition to the flow and also creates streamwise vorticity. An

array of three pitched jets was used. The instantaneous discharge current of each spark jet was fixed at

1.2 A and the jet array was located 4δ upstream of the compression corner. Pitch angles of 0 (normal

injection), 30 and 60 degrees were tested. We observed that as pitch angle was changed, the global

character of the interaction remained the same as that of normal injection, although the shock moved

slightly smaller upstream distances with increase in pitch angle (0 to 60 degrees). It was concluded that,

pitch angle alone was not very effective at influencing the structure of the interaction. In order to increase

the streamwise vorticity produced by 4.5 A spark-jet, the jets pitch was fixed at 45 degrees and skewed at

90 degrees. An array of three jets was placed at 5δ upstream of the compression corner. This pitch and

skew angle configuration has been shown to be the most optimal in incompressible flow studies [8]. A

schematic illustration of the unskewed/pitched and skewed/pitched jet are shown in fig. 7(a) and (b)

respectively. Figures 7(c) and 7(d) show the phase-averaged shock position with an unskewed and skewed

the spark-jet at 35 µs after the start of discharge trigger. The average shock position and angle without

employing the spark-jet is also marked. In addition a linear extrapolation of the shock is made to the floor

in order to mark the position of shock foot. It is clear that there is an average downstream motion of the

shock foot, by about 0.3δ for the skewed jet. The downstream motion of the shock occurs for about 5 µs. At

longer times, the shock moves upstream with the passage of the low momentum fluid caused by the spark-

jet. The extent of the upstream motion is very similar to the unskewed jet.

In order to allow the streamwise vortex generated by the pitched/skewed jet to develop fully, the

compression corner was shifted to 9δ downstream of the jet orifice, while the discharge and geometric

parameters were kept the same. It was found that there was no apparent downstream shift of the separation

shock, which seems to indicate the vortex may have weakened too much at this upstream location. This

also shows that there is an optimal location of the jet, at which the downstream shift of the separation shock

is maximized and occurs for the longest duration. Note that after the shock shifts downstream, at later times

it appears more spread out (as would occur with a continuous compression), which is similar to that

observed with an unskewed jet.

Thus the global characteristics of the spark-jet / separation-shock interaction with changes in geometric

parameters of the plasma jet are as follows:

• The pitched and skewed jet seems to work as intended as a vortex-generator that energizes the

upstream boundary layer since it causes the separation shock to briefly move downstream. The

maximum downstream shift was observable for a jet with 45-deg pitch and 90-deg skew and 4.5 A

per jet.

• The pitched-only jet and the pitched and skewed jet (at longer times) both caused upstream motion

of the separation shock. The upstream motion may be due to the injection of low momentum fluid

into the boundary layer. It is also possible it is related to the local heating of the flow, which

causes the Mach number to be locally lower, and lower Mach numbers will tend to have larger

shock standoff distances. The extent of upstream motion is found to depend on the location and

the strength of the jet.

• The recovery of the shock to its undisturbed position after the jet fires is considerably longer than

the time required for the jet pulse to convect past the interaction at the freestream velocity.

Effect of input instantaneous power and energy per pulse

A study was made to assess the effect of pulse width of the discharge on the upstream displacement of

the separation shock. This is important because the exit velocity of the jet is mainly a function of the input

peak power, whereas the total momentum added to the flow depends on the pulse duration also (and hence

the energy per pulse). It is clear from the previous sections that the upstream separation shock motion is

caused by the spark jet independent of its geometry and hence it can be used a parameter to characterize the

-jet / separation shock interaction.

For an unskewed plasma jet with fixed pitch angle (30 degrees), and location (4δ), we studied the effect

of increasing the instantaneous discharge current from 1.2 A to 4.5 A. Figures 8 and 9 shows the phase-

averaged images at different instances. Comparing Figs. 8c and 9c we can observe an increase in the

upstream displacement of the separation shock as the current is increased. The extent of upstream motion is

larger for a current of 4.5 A, which shows that the upstream motion of the shock depends on the input peak

power, as would be expected.

Figure 10(a) and (b) shows the instantaneous upstream motion of shock caused by a 10 µs and 20 µs

spark jet pulse with peak current of 3.9 A. The jet is pitched at 45 degrees and skewed at 90 degrees. It can

be seen that the upstream motion caused by the 20 µs discharge is more than the 10 µs discharge. Thus it

appears that for small pulse widths the interaction also depends on the energy per pulse. However it is

expected that there is a maximum pulse width, depending on the volume of the spark-jet cavity and the

flow dynamics, beyond which the pulse width ceases to be important and the input power would determine

the spark-jet-separation shock interaction.

High-speed schlieren imaging

Schlieren imaging was performed at 10 kHz frame-rate in order to time resolve the response of the

separation shock to the incoming disturbance caused by the spark jet pulsed at high frequencies (few kHz).

The geometry of the jet is fixed to a spark-jet pitched at 45-degrees and skewed at 90-degrees located 5δ

upstream of the compression corner. The discharge parameters that were studied are the discharge current

and the pulsing frequency of the discharge. These results must be interpreted with caution since the

schlieren is line-of-sight integrated and therefore masks potential three-dimensional effects.

Before performing the 10 kHz schlieren imaging, phase locked schlieren at 2 kHz was performed to

verify the repeatability of the strength of a 2 A spark-jet pulsed at 2 kHz. No attempt was made to time

resolve the shock motion during this repeatability test. From the constant angle and position of the shock in

front of the jet orifice at a given time delay for various pulses, we inferred that the jet pulses are very

similar. Also it was observed that even at high frequency pulsing, a higher current spark-jet (3.9 A) created

a larger upstream shock motion than a lower current (2 A) one.

The response of the separation shock conditioned on its location 100 µs prior to the incoming

disturbance was studied. The disturbance was generated using a 3.9 A spark jet with a 10 µs pulse width.

The discharge was pulsed at 1 kHz. This ensured that there is no interaction between individual pulses. The

presence of the disturbance is characterized by the appearance of a weak oblique shock over the jet orifice.

From the phase locked schlieren imaging it is found that the interaction between the spark jet and the

separation shock takes place over about 100 µs. Hence there is atleast one frame, which captures the

interaction. The upstream (downstream) location of the shock foot is defined as the frame in which the

shock foot is upstream (downstream) of the mean shock location found from the schlieren movie of the

flow without spark-jet injection. It should be mentioned that the shock appears as a thick band (see Fig 10

(a)) and this causes some uncertainty in our ability to determine the location of the shock foot. Also the

shock foot is defined as the bottom-most point where the separation shock could be observed, which is

about δ/2 above the floor. The shock-upstream (downstream) case corresponds to an overall low (high)

momentum fluid in front of the separation shock. Figure 11 (a) and (b) shows the instantaneous separation

shock response corresponding to an upstream and downstream location respectively. The shock foot

location and shock angle found (by eye) in the previous frame is also marked. It is observed that the

separation shock moves to approximately the same upstream location for both cases. This implies that the

momentum of the incoming disturbed fluid does not depend on the momentum of the fluid before

disturbance. Hence with high frequency pulsing, it may be possible to anchor the shock at an upstream

location and hence arrest its low frequency motion.

The response of the separation shock with increased pulsing frequency was studied using a 2 A spark-

jet pulsed at 1, 2 and 5 kHz. The inferences made are just by looking at the schlieren movie and no statistics

were computed. At 1 kHz the response of the shock is similar to that with 3.9 A spark-jet albeit with lower

amplitude of upstream motion. The spark-jet / separation-shock interaction for a particular pulse is

independent of the previous pulse. However at 5 kHz, a definite dependence of the shock response for a

particular pulse with the previous pulse was noticed. An average upstream shift of about δ/2 was noticed

and the shock appeared more stable (i.e., exhibited smaller amplitude of motion). The most downstream

position was still similar to the unforced case; however the separation shock with forcing appeared near the

downstream position much less frequently compared to the unforced case. At this time, this is only an

tenuous inference. Pressure fluctuation measurements will be made in future to make quantitative

measurements of shock response.

Conclusions

The spark jet was discussed in Ref. 5 in the context of supersonic flow control and this device has been

tested and extended for the purpose of controlling a Mach 3 compression ramp interaction. The resulting jet

exhibits relatively high velocity ( ≈300 m/s) and can be pulsed at relatively high frequency (≈5 kHz). To

increase the perturbation to the flow, an array of three spanwise jets separated by 3 mm, were used. The

spark-jet was tested in different configurations including normal injection, streamwise pitched and pitched

and skewed. The latter was used in an attempt to induce a strong pulsed vortex generator. The results show

that the spark jet in all configurations caused significant modification to the separation shock. Essentially,

all of the configurations caused the separation shock to move upstream over some part of the pulse. The

pitched and skewed jets however, caused downstream motion of the separation shock for a brief time (~ 5

µs) demonstrating its potential to act as a vortex generator. Time resolved schlieren imaging was made to

study the response of the separation shock to the individual disturbances caused by a given pulse of the

spark-jet array. At frequencies below 2 kHz, the response of the individual pulses is quite independent of

the shock motion caused by the previous pulse. However at 5 kHz a quasi-steady upstream shift of the

separation shock was observed and the separation shock had very less tendency to move to the most

downstream position corresponding to unforced case. A study made on the effect of pulse width on the

interaction showed that for small pulse widths the magnitude of the disturbance induced by the spark jet

depends on both the peak power and the total energy deposited per pulse.

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Figure 1: Experimental setup

Figure 1: Experimental Set up

Figure 2: Mechanical and electrical schematic of the spark jet actuator.

(a) (b) (c)Figure 3: Instantaneous images of phase locked schlieren images of the pulsed plasma jet issuing in a

stagnant chamber at different time delays from the start of the trigger – 10 µs (a), 20 µs (b) and 30 µs (c)

Contact surface

Compression ramp

Pulsed plasma jetVacuum

High-pressure tank

Figure 4: Variation of jet exit velocity of the spark-jet with discharge current.

(a)

(b)

Figure 5: Instantaneous schlieren image showing the effect of normal jet injection on the Mach 3compression ramp interaction. Figure (a) is without injection and (b) is with injection captured 40 µs afterthe start of discharge trigger.

without actuation with actuation

(a)

Jet orifice

Separation shock

Jet orifice

Separation shockSeparation shock

FlowCompression ramp

Splitter plate

without actuation with actuation

(b)

Figure 6: Phase averaged schlieren of normal injection of spark-jet at two different upstream positions.Fig. (a) corresponds to a location of 1δ upstream of the compression corner, and (b) corresponds to 4δupstream.The average schlieren image without injection is shown at left and the case with injection isshown at right. In all cases the schlieren image is captured at 40 µs after the start of the discharge.

Jet orifice

Separation shockSeparation shock

(a) (b)

(c) (d)

Figure 7:Phase averaged schlieren images at 35 µs delay to compare unskewed and skewed 4.5 Aspark-jets on their ability to act as a vortex generator. Fig. (a) and (b) shows the schematic illustrationof a unskewed/pitched jet and skewed/pitched jet respectively. Fig. (c) corresponds to an unskewed 30-

degree pitched spark-jet and (d) corresponds to 90-degree skewed and 45-degree pitched spark-jet

With spark jet

Shock locationwithout spark jet

With spark jet

Shock locationwithout spark jetJet orifice

Spark jet

flow

Spark Jet

flow

(a)

(b)

(c)

(d)

(e)

Figure 8: Phase averaged schlieren images of a 30-degree pitched 1.2 A spark jet taken at differentdelays from discharge trigger (a) t = 0 µs, (b) t = 40 µs, (c) t = 45 µs, (d) t = 100 µs. (e) illustration of

schlieren image shown in (b)

Jet orifice

Separation shock

JetShock due to jet

(a)

(b)

(c)

(d)Figure 9: Phase averaged schlieren images of a 4.5 A 30-degree pitched spark jet taken at different

time delays from the discharge trigger (a) t = 0 µs, (b) t = 40 µs, (c) t = 45 µs, (d) t = 100 µs.

(a) (b)

Figure 10: Illustration of upstream motion of the shock foot for different jet pulse widths at a current of3.9 A. Fig. (a) corresponds to 10 µs pulse width and (b) corresponds to 20 µs pulse width.

Jet orifice

Shock due to jet

Separation shock

Jet orifice

Shock due to jet

Jet orifice

Shock due to jet

(a) (b)

Figure 11: Response of the separation shock foot to the incoming low momentum flow conditioned onthe shock location 100 µs prior to the disturbance. Fig. (a) corresponds to separation shock initiallylocated upstream and (b) corresponds to separation shock located downstream.

Shock locationbefore 100 µs

Shock locationbefore 100 µs