Alec W. Houpt

Optical Measurement of Transient Plasma Impact on Corner Separation in M=4.5 Airflow

The objective of this work is to perform a feasibility study on the steering effect of a weakly ionized transient plasma on the corner separation zone of a compression surface with a hypersonic boundary layer. The optical measurements, using a high-speed Shack-Hartmann sensor, have been successfully performed to quantify a characteristic frequency of flow perturbations at different locations in the flowfield and the plasma impact on the spectra of disturbances in the boundary layer and in the separation bubble. It was shown that the nearsurface plasma generation at a frequency of repetition higher than a dominant natural frequency of perturbations in the boundary layer (the first mode F1), f =100 kHz > F165 kHz, lead to a significant intensification of the amplitude of the high-frequency disturbances in the range of A A0 ⁄ = 2 − 8. The plasma effect was negligible or insignificant at excitation frequencies, f = 50 kHz, below F1.

Transient Plasma Impact on Spectra of Flow Disturbances in a Corner Separation Zone at Mach 4.5

This work was performed to study the effect on flow disturbances in the corner separation zone of a compression surface with a hypersonic boundary layer caused by a weakly ionized transient plasma generated upstream. Schlieren imaging was used to distinguish the corner separation zone for 15°, 20°, and 25° compression ramps at Mach 4.5 (nozzle exit). A Shack-Hartmann wavefront sensor was used to determine the dominant frequencies of flow oscillations at different locations in the flow field and the resulting effect of repetitively pulsed plasma actuators. A significant rise in amplitudes of high-frequency (>80 kHz) flow perturbations was found when pulsing the plasma at a frequency (100 kHz) higher than the natural dominant frequency of the boundary layer (~65 kHz). The plasma effect was negligible when operated below this frequency (50 kHz). PCB pressure sensors were used to determine the dominant frequencies of pressure oscillations present at the surface on the flat plate and compression ramp inside the separation zone. This technique can potentially be used for active control of the boundary layer condition and supersonic flow structure on the compression surface.

Ethylene Ignition and Flameholding by Electrical Discharge in Supersonic Combustor

This study examined the ignition and flameholding effects of the quasi-direct-current discharge on a directly injected hydrocarbon fuel (ethylene) in a supersonic combustion chamber without mechani...

Measurement of Plasma Induced Flow Perturbations Affecting a Mach 4.5 Corner Separation Zone

This study was performed to compare the effect of pulsed plasma actuators in Three Electrode Discharge (TED) and Shallow Cavity Discharge (SCD) configurations on downstream flow perturbations through the observation of the separation zone of a hypersonic boundary-layer occurring over a compression ramp surface. Tests were carried out in Mach 4.5 flow with varying Reynolds number to characterize naturally developing flow perturbations and the plasma-induced perturbations by different diagnostic tools. A high resolution Shack Hartmann wavefront sensor was used to measure flow density gradients and produce spectra of density perturbations before and during plasma actuation. High-speed schlieren imaging was able to distinguish separation of the boundary-layer and generate spectra of oscillations at the boundary-layer edge from extrapolated values of pixel intensity. Changes in the spectra of dominant flow perturbations due to plasma actuation were compared with spectra taken with high frequency PCB pressure sensors. The effects of plasma actuation at low (TED) and high (SCD) frequency configurations demonstrate a different pattern of interaction, appearing in a downward or upward shift of the dominant frequency of flow perturbations respectively.

Charge Transfer in Constricted Form of Surface Barrier Discharge at Atmospheric Pressure

This paper presents experimental data characterizing surface dielectric barrier discharge morphology and dynamics of electric charge transfer over a dielectric surface in atmospheric air using the single polarity and alternating-polarity of the supplied voltage. The experiments were performed using microsecond-scale bursts of high-voltage pulses. Diagnostics included electrical measurements, synchronized gate camera imaging, optical emission spectroscopy, and a set of original charge sensors. Two basic modes were analyzed: diffusive and filamentary, appearing in the result of the discharge contraction. In a single-polarity discharge, the surface charge accumulation limits energy coupled to the plasma by subsequent pulses, whereas the alternating-polarity pulsing, accompanied by the constricted form of plasma, leads to a significant extension in the charged surface area and an increase in power deposition.

Corner Separation Zone Response to Plasma Actuation in the Hypersonic Boundary Layer over a Compression Ramp

The characterization of the M=4.5 flow over a 2D model with a 30 compression ramp was performed at low enthalpy (T0=300K) and high enthalpy (T0=800-1250K) conditions for various Reynolds numbers. The impact of a pulse periodic plasma generated upstream of the ramp on the spectra of gas pressure/density oscillations was explored. Three measurement techniques were employed to characterize this oscillatory structure: a high frequency Shack-Hartmann wavefront sensor (aero-optical method), high frequency PCB pressure sensors, and a laser differential interferometer. Spectra of the flow oscillations measured by all three methods offer complementary results in characterizing dominant frequencies within the flow. Of these measurement methods, ShackHartmann is shown to be the most suitable tool for analysis of the flow spectra. Aero-optical measurements indicate flow structure modification due to variation of Re number and during plasma actuation. Under conditions of this test, further characterization by the Shack-Hartmann sensor at individual points in the flow has shown three modes of interaction depending on the Reynolds number: turbulent, transitional, and laminar, where the modes are mostly governed by freestream perturbations. The spectral measurements during plasma actuation indicate evidences of change in the dominant frequencies and amplitudes of perturbations from the natural state that occur over the separation region of the compression ramp.