Seong-kyun Im

Dielectric barrier discharge control of a turbulent boundary layer in a supersonic flow

We demonstrate effective manipulation of a turbulent boundary layer at Mach 4.7 conditions using a surface dielectric barrier discharge (DBD) actuator. The freestream conditions of low static pressure (1 kPa) and temperature (60 K) are conducive to the visualization of flow features using Rayleigh scattering from condensed CO2 particles. The boundary layer thinning is observed when spanwise momentum is induced by the low power (6.8 W), low frequency (28 kHz) single actuator pair oriented parallel to the freestream flow.

The manipulation of an unstarting supersonic flow by plasma actuator

The manipulation of an unstarting supersonic flow is demonstrated using a dielectric barrier discharge (DBD). Experiments are carried out in a Mach 4.7 model inlet flow. Flow features, such as boundary layers and shockwaves at low freestream static pressure (1 kPa) and temperature (60 K) are visualized with Rayleigh scattering from condensed CO2 particles. Flow unstart, initiated by mass injection, is studied for three model inlet flow configurations, distinguished by the initial conditions (untripped or tripped, plasma actuated or not) of the boundary layers. Unstart in the presence of thick, tripped boundary layers is characterized by the formation of an oblique unstart shock just upstream of a separating and propagating boundary layer. The presence of plasma actuation of this tripped boundary layer seems to arrest the boundary layer separation and leads to the formation of a quasi-stationary pseudo-shock, delaying unstart. The flow generated with DBD actuation is more characteristic of what is seen when unstart is generated in a model flow in which thin boundary layers grow naturally. Planar laser Rayleigh scattering visualizations suggest that the DBD actuation thins the tripped boundary layer over the exposed electrode region.

Schlieren imaging investigation of successive laser-induced breakdowns in atmospheric-pressure air

Fast Schlieren imaging was performed to visualize the interactions between previously produced laser breakdown and a subsequent laser pulse. A pair of laser pulses was used to generate successive breakdowns in the quiescent standard air, and the interval between the pulses was varied from 50 ns to 100 μs to experimentally simulate various laser repetition rates. The incident laser energies ranged from 5 mJ to 31 mJ, and the energy absorbed by the breakdown of the second laser pulse was quantified by measuring the energies before and after the breakdown. The results indicate that the second laser pulse coupled to the background gas and produced a second laser breakdown only when the pulse interval was shorter than 250 ns or longer than 15 μs. For the shorter pulse intervals, the second breakdown occurred at the edge of the first breakdown region along the laser beam path, and its effect on the perturbation of the density field was found to be small. On the other hand, for the longer pulse intervals, the second breakdown occurred at the lens focal point, and the density field perturbations caused by the first and second breakdowns seemed to interact with each other inducing the Richtmyer-Mechkov instability. As a result, more significant turbulence in the density field was observed after successive laser pulse breakdowns than was observed following a single breakdown.

Nanosecond-Pulsed Discharge Plasma Splitting of Carbon Dioxide

This paper reports on the study of repetitive nanosecond-pulsed discharge splitting of carbon dioxide (CO2) for the production of CO. Gas chromatography is used to analyze the composition of the reformed gas when CO2 is exposed to high-voltage (15 kV) very short (10 ns) electrical discharges that deposit as much as 0.4 mJ of energy at a rate of 30 kHz. Conversion rate and energy efficiency are obtained while the discharge pressure is varied between 2.4 and 5.1 atm. At the tested conditions, the maximum conversion rate and energy efficiency are found to be 7.3% and 11.5%, respectively. The energy efficiency drops slightly with increased pressure because of the decreased electric field and electron energy per molecule. An energy balance analysis of a set of CO2 plasma reactions reveals that the dominant dissociation pathway under these conditions passes through the excitation of CO2 (10.5 eV) followed by autodissociation into CO and O, which are often in excited states.

A study of flow induced by laser induced breakdown-enhanced dielectric barrier discharges in air

The flow induced by an asymmetric dielectric barrier discharge (DBD) actuator in air together with laser induced breakdown (LIB) near the exposed electrode is investigated using particle image velocimetry. In this approach, the electrodes, driven by alternating current (8 kHz, 14 kVp-p) serve primarily to accelerate the ions generated by the laser pulse (532 nm, 15 mJ per pulse, and 2 Hz). The mean velocity fields suggest that this hybrid scheme leads to a significant enhancement in the wall-jet velocity and momentum flux generated by actuation.

Surface discharge plasma actuator driven by a pulsed 13.56 MHz - 5 kHz voltage waveform

The effect of incorporating pulses of radio-frequency (rf: 13.56 MHz) voltage into the driving waveform of a surface discharge plasma actuator is investigated. Rf voltage is applied to the actuator to increase the production of ions and thereby increase the thrust that is generated by the discharge. This waveform is coupled to the powered electrode in 5 µs pulses and combined with a relatively low-frequency (LF) 5 kHz sinusoid to form a pulsed 13.56 MHz–5 kHz (rf-LF) driving voltage. Measurements of the applied voltage, rf and LF currents, effective power, and velocity field of the surrounding air are undertaken at atmospheric pressure. The thrust that is generated using the rf-LF waveform is estimated from the velocity fields using a momentum balance and is found to increase for increasing rf voltage when the LF voltage remains constant. Maximum thrust is achieved when the rf pulses are positioned at the LF voltage minima and this suggests the importance of negative ions. The efficacy of rf-LF actuation is investigated by comparing the thrust that is generated per unit increase in peak voltage with that obtained using an LF-driven discharge.

Experimental Study and Plasma Control of an Unstarting Supersonic Flow

The unstart induced by a transverse jet in a Mach 4.7 model inlet flow is visualized by planar laser scattering from condensed CO 2 particles at low freestream static pressure (1kPa) and temperature (60K) for various boundary layer conditions. We find that the formation of an unstart shock is initiated on the relatively thick boundary layer side and its structure depends on the boundary layer conditions. The pseudo-shock or oblique shocks that precede unstart propagate upstream in symmetric and asymmetric boundary layer conditions, respectively. It is found that the pseudo-shock has a quasi-stationary mode resulting in longer overall unstart events. The results suggest that the unstart process can be influenced and possibly delayed by the control of the boundary layer conditions. In this paper, we present preliminary results on boundary layer manipulation of an unstarting supersonic flow using Dielectric Barrier Discharge (DBD) actuation. We find that the asymmetric boundary layer condition that generally leads to the formation and propagation of an oblique unstart shock, when actuated, can be transformed the flow to resemble that of a symmetric condition which leads to the generation of a pseudo-shock, extending the unstart duration by 22 %.

Plasma Actuator Control of a Lifted Ethane Turbulent Jet Diffusion Flame

A dielectric barrier discharge (DBD) actuator is used to stabilize the base of a lifted ethane turbulent jet diffusion flame by modifying the coflow velocity field. The velocity field and flame base are measured by particle image velocimetry and unfiltered flame chemiluminescence imaging, respectively. An axisymmetric DBD actuator, integrated onto the nozzle body and driven by 8 kHz, 10-12-kV peak-to-peak sinusoidal voltage, generates the directional flow that opposes the coflow that surrounds the jet nozzle. This flow induces a separation bubble that retards and diverts the surrounding fluid further away from the fuel jet. The turbulent jet diffusion flame liftoff height is found to be significantly reduced by this plasma actuation.

Dielectric barrier discharge induced boundary layer suction

Boundary layer downdraft suction and through-surface fluid removal is demonstrated with a streamwise oriented dielectric barrier discharges fabricated into a flat plate immersed in a 3.8 m/s freestream flow (Re = 72000). The velocity field is obtained by particle image velocimetry. Suction with through-surface fluid removal is found to thin the boundary layer and increases the velocity gradient in the vicinity of the wall by extracting slower fluid from the inner layer region. The effect of this suction on the boundary layer persists away from the center of the exposed electrode without inducing significant disturbances to the freestream flow.

Separation Control Using Dielectric Barrier Discharge-Induced Suction and Vortex Generation

We investigate the characteristics of vortices induced by spanwise forcing using a streamwise oriented dielectric barrier discharge (DBD) with and without a boundary layer suction channel, the flow of which is also driven by a DBD. The velocity field over various regions in the vicinity of the actuator is obtained by ensemble averaging particle image velocimetry. Increased boundary layer thinning and a stronger downward motion are seen near the powered flow-exposed electrode when the DBD is used in conjunction with a suction slot. The effect of varying the applied voltage and freestream velocity on the induced flow is examined. Studies are carried out on the ability for such a structure to control separation on an inclined flat plate and ramp. We find that DBD actuation together with boundary layer suction leads to more robust separation control in these adverse pressure gradient configurations.