Sohail Zaidi

Experimental investigation of dielectric barrier discharge plasma actuators driven by repetitive high-voltage nanosecond pulses with dc or low frequency sinusoidal bias

Experimental studies were conducted of a flow induced in an initially quiescent room air by a single asymmetric dielectric barrier discharge driven by voltage waveforms consisting of repetitive nanosecond high-voltage pulses superimposed on dc or alternating sinusoidal or square-wave bias voltage. To characterize the pulses and to optimize their matching to the plasma, a numerical code for short pulse calculations with an arbitrary impedance load was developed. A new approach for nonintrusive diagnostics of plasma actuator induced flows in quiescent gas was proposed, consisting of three elements coupled together: the schlieren technique, burst mode of plasma actuator operation, and two-dimensional numerical fluid modeling. The force and heating rate calculated by a plasma model was used as an input to two-dimensional viscous flow solver to predict the time-dependent dielectric barrier discharge induced flow field. This approach allowed us to restore the entire two-dimensional unsteady plasma induced flow ...

Flow field imaging through sharp-edged atomic and molecular `notch' filters

Sharp cut-off atomic and molecular notch filters simultaneously provide high spectral resolution and allow imaging by collecting light over a wide field of view. Many important properties of flow fields can be observed by imaging light elastically scattered from small particles, molecules or electrons. In order to extract information about the flow field from elastic scattering, the spectrum of the scattering must be resolved and the background scattering must be suppressed. Very high resolution, on the order of a few tens of megahertz, is usually required. The spectrum of the scattered light is broadened and shifted by the motion of the scatterers. For particles, which have relatively little thermal or acoustic motion, the spectral shift is only a function of the velocity. For molecules, the scattering spectrum is a function of the temperature, velocity and pressure of the gas as well as its composition. For electrons, the spectrum is a function of the electron temperature and electron number density in a plasma. In this paper, sharp edged notch filters made of rubidium, iodine or mercury vapour are used to image shock wave and boundary layer structure by Rayleigh scattering from particles, to image gas pressure, velocity and temperature by molecular Rayleigh scattering, and to measure electron temperature and electron number density by Thomson scattering. For molecular scattering, filter transmission is generally a function of velocity, temperature and pressure, but, under some circumstances, it is a function of only one or two variables, so a notch filter can provide single-pulse images of a specific flow field parameter.

Experimental Investigation of DBD Plasma Actuators Driven by Repetitive High Voltage Nanosecond Pulses with DC or Low-Frequency Sinusoidal Bias

Experimental studies were conducted of a flow induced in an initially quiescent room air by a single asymmetric dielectric barrier discharge driven by voltage waveforms consisting of repetitive nanosecond high-voltage pulses superimposed on DC or alternating sinusoidal or square-wave bias voltage. To characterize the pulses and to optimize their matching to the plasma, a numerical code for short pulse calculations with an arbitrary impedance load was developed. A new approach for non-intrusive diagnostics of plasma actuator induced flows in quiescent gas was proposed, consisting of three elements coupled together: the schlieren technique, burst mode of plasma actuator operation, and 2-D numerical fluid modeling. This approach allowed us to restore the entire two-dimensional unsteady plasma induced flow pattern as well as characteristics of the plasma induced force. The experiments and computations showed vortex flow structures induced by the actuator. Parametric studies of the vortices at different bias voltages, pulse polarities, peak pulse voltages, and pulse repetition rates were conducted. The significance of charge build-up on the dielectric surface was demonstrated. Based on the observations, a new voltage waveform, consisting of high-voltage nanosecond repetitive pulses superimposed on a highvoltage low-frequency sinusoidal voltage, was proposed. Advantages of the new voltage waveform were demonstrated experimentally.

Snowplow Surface Discharge in Magnetic Field for High Speed Boundary Layer Control

The performance of air breathing supersonic and hypersonic vehicles is strongly affected by the boundary layer that forms along the surface of the inlet. At offdesign operating conditions, a shockwave intersecting the boundary layer can cause the flow to separate which may lead to a drastic reduction in vehicle performance. The work presented in this paper explores a possible way to use magnetically driven surface contracted plasma column to accelerate the flow near the surface. The velocity of the plasma column in a Mach 2.8 in-draft tunnel was investigated at different magnetic field strengths (up to 2.0 Tesla) and was recorded using a high-speed camera. The discharge was observed to travel several times faster than the neutral flow and the velocity was seen to increase with the increasing magnetic field. Pitot probe measurements indicated that magnetically accelerated plasma column affected the flow near the wall and may have the potential of providing a method for flow boundary layer acceleration and flow control.

Virtual Shapes in Supersonic Flow Control with Energy Addition

This paper presents a short selective review of theoretical and experimental studies conducted by the authors and their collaborators during the past few years in areas related to supersonic and hypersonic flow regimes with applications such as drug reduction, inlet and effective vehicle geometry control in off-design flight regimes, and steering and sonic boom mitigation. Their results suggest a principal possibility to enable transitions between the propulsion modes and ramjet startup and to minimize the need for the traditional isolator stage, as well as to increase the inlet mass capture at Mach numbers below the design value, using active control based on virtual shapes created by energy addition upstream of the inlet throat. A common feature of all these substantially different applications and processes is a power deposition into a supersonic flow which results in the creating of virtual shapes, modifying flowlike solid obstacles immersed in it. The virtual shapes can be created by microwave plasma heating, magnetohydrodynamic forces, electron beams, and localized plasma-assisted surface combustion. The power necessary to operate plasmas can come either from the turbine at mode transition, from an auxiliary power unit, or, as suggested in a new bypass concept, from a magnetohydrodynamic generator either placed downstream of the combustor or collocated with it.

Magnetohydrodynamic Power Generation Using Externally Ionized, Cold, Supersonic Air as Working Fluid

Magnetohydrodynamic (MHD) power extraction from cold air has been demonstrated using short-duration, high-repetition rate, high-voltage pulses (2 ns, 100 kHz, 5 kV/cm) to ionize a Mach 3 (600 m/s), 0.04 kg/m 3 flow. Because the power used to ionize the flow using such a method was less than 1% of the total flow enthalpy, the flow was not heated significantly. A few tens of milliwatts were extracted from the 3-cm cube region of ionization, which scales to hundreds of kilowatts of power in higher velocity, larger-scale devices that would be appropriate for flight applications. Peak electron number densities between 5 × 10 11 and 10 12 cm -3 are reported from complementary measurements using microwave absorption in a variable magnetic field. The Hall parameter was estimated from the electrical properties of the MHD channel. Modeling predictions were found to be in agreement with experimentally extracted Faraday current measured between the high-voltage pulses. Modeling also confirmed that the electrons have low energy between the pulses and that the resultant cathode and anode voltage falls are quite low, on the order of 1 V.

Measurements of Hydrocarbon Flame Speed Enhancement in High-Q Microwave Cavity

In this work we demonstrate that a small amount of microwave power below its breakdown threshold can be locally absorbed into a flame combustion zone. The absorbed microwave power can significantly change the flame speed of both laminar and turbulent flames. PIV technique was employed to measure the laminar flame speed. It was found that microwave assisted flame speed enhancement was greatly dependent on Q of the microwave cavity. Due to the unsteady nature of interaction, microwave assisted flame speed measurements were difficult to make, however, preliminary observations of the flame luminosity indicated that there was energy addition occurring without microwave breakdown and the flame speed was increased.

Shockwave Induced Turbulent Boundary Layer Separation Control with Plasma Actuators

In this study we report Acetone Planar Laser Scattering (PLS) visualization of the boundary layer structure inside Mach 2.6 indraft wind tunnel at Princeton University. The aim is to better understand the surface plasma control of shockwave boundary layer interaction (SWBLI) region and separation. These experiments are designed to evaluate magnetically driven surface plasma actuators for effective shockwave induced boundary layer separation control within supersonic inlets. Static pressure measurements and Schlieren images of the shockwave boundary layer interaction region were reported earlier and it was shown that when a weak shock generator (10) is used to generate the impinging shockwave, while no separation occurs without plasma control, a small current plasma column (< 100mA) at 1-3 Tesla is enough to induce separation by flow retardation in the interaction region. Strong shockwave from a (14) generator was shown to induce separation and experiments are done at high currents 100-250 mA; for flow acceleration in the interaction region. At these relatively high currents, the plasma actuation is able to delay the incipient separation. Also, in the absence of magnetic field, no change in the flow structure is seen, indicating marginal role of joule heating in the process. Acetone PLS imaging provides boundary layer flow structure in relative detail and direct evidence of separation control.

Magnetically Driven Surface Discharges for Shock-Wave Induced Boundary-Layer Separation Control

This study investigates the impact of a magnetically driven surface plasma column (“snowplow arc”) on shock induced boundary layer separation. The surface plasma column appears as a transverse “arc” between two diverging electrodes which is driven by j x B forces so that it sweeps the gas near the surface either in the downstream direction or in the upstream direction. In the experimental setup, an oblique shockwave wave was generated using a ten degree wedge in a Mach 2.8 indraft tunnel. The shock wave impinged on the flat surface in close proximity to the plasma actuator. Experimental results revealed a coupling of the plasma column with the shock – boundary layer interaction region which resulted in a change in the location of the shock induced boundary layer separation point. In case of the body force j x B acting upstream, the separation point was seen to move upstream. In case of the downstream j x B body force, a very small coupling was observed and the separation point appeared largely unaffected. Various reasons for the absence of an interaction in the downstream direction are discussed, particularly including the ratio of the scale of the plasma column to the boundary thickness. A sapphire insert with embedded electrodes is under development to allow for a higher current which then may be more effective for the suppression of boundary layer separation.

Shock-Wave Mitigation Through an Off-Body Pulsed Energy Deposition

A model of dynamic shock interaction predicts that pulsed energy addition upstream of a bow shock can be used to weaken the time-averaged shock strength. To explore these shock interactions, tests have been conducted in a small-scale, blowdown facility with off-body energy addition upstream of an oblique shock wave that was generated by a wedge in a Mach 2.4 flow. The off-body energy addition was simulated by laser-induced breakdown (350 mJ/pulse with 10-ns pulse duration). The dynamics of the interactions were captured in both schlieren and shadowgraph images taken at 250,000 and 500,000 frames per second and compared with the computational model