Chiranjeev S. Kalra

Numerical study of boundary layer separation control using magnetogasdynamic plasma actuators

In this study, an efficient, time dependent, two-dimensional Navier–Stokes numerical code for shockwave boundary layer interaction in air is developed. Nonthermal surface plasma actuation is evaluated for effective shockwave induced boundary layer separation control within supersonic inlets. Specifically, high speed magnetogasdynamic plasma actuators are of interest. In these, localized ionization is produced close to the wall surface and then the flow is accelerated using strong magnetic fields. To replicate the experiments done at large boundary layer thickness, the code is divided into time independent and time dependent regimes to significantly reduce computation time. Computational results are in good agreement with experiments in terms of the flow structure as shown by Schlieren imaging, acetone planar laser scattering, and the static pressure profile on the test section wall.

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.

Non-Thermal Control of Shock-Wave Induced Boundary Layer Separation using Magneto-Hydrodynamics

This study investigates the non-thermal impact of a magnetically driven surface discharges (“snow-plough” 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 force so that it sweeps the gas near the surface either in the downstream direction or in the upstream direction. Initial results have been reported previously using a Mach 2.8 indraft wind tunnel show that upstream forcing of boundary layer induced separation in the interaction zone of an oblique shock, generated by a 10 degree wedge, with turbulent boundary layer. This interaction, of oblique shock with boundary layer, is shown not to induce a recirculation bubble or separation in the boundary layer without plasma actuation. A 14 degree wedge is used to generate an oblique shock that impinged on the turbulent boundary layer generating a separation zone and recirculation in the flow. Downstream plasma actuation, against the recirculating flow, in such a case resulted in modified geometry of interaction zone as seen in Schlieren images. Various new configurations of plasma – flow interaction are developed, mainly applying magnetic field at an angle such that plasma column forces the core flow into the boundary layer increasing the momentum and thus avoiding separation.

Effects of a conducting sphere moving through a gradient magnetic field.

We examine several conducting spheres moving through a magnetic field gradient. An analytical approximation is derived and an experiment is conducted to verify the analytical solution. The experiment is simulated as well to produce a numerical result. Both the low and high magnetic Reynolds number regimes are studied. Deformation of the sphere is noted in the high Reynolds number case. It is suggested that this deformation effect could be useful for designing or enhancing present protection systems against space debris.

Velocity measurements in synthetic jet using magnetically driven surface discharges

A magnetically driven DC surface plasma discharge has shown promise for supersonic boundary layer control. Previous experiments in supersonic flow performed using this plasma actuator have shown significant control over the shockwaveturbulent boundary layer interaction region. In this study, the velocity of the surface jet in quiescent air is measured and compared with predictions. A time gated schlieren technique is used to visualize the jet and determine the velocity. The surface plasma column appears as a transverse “arc” between two slightly diverging electrodes and is driven by j x B forces so that it sweeps the neutral gas near the surface creating a surface jet.The jet velocity generated by the magneto-gas dynamic actuator is measured in a 1 Tesla magnetic field at different actuation currents below 100mA in air at pressures in the range of 100 – 400 Torr

Effect of Vibrational Mode Excitation of N2 in Plasma Actuators on Shockwave Induced Boundary Layer Separation Control.

In this study analysis of the effect of energy deposition and subsequent release from the vibrational modes of N2 is done using an efficient numerical code for shockwave boundary layer interaction (SWBLI) that was developed earlier. Non-thermal surface plasma actuation using Lorentz force is evaluated further for effective shockwave induced boundary layer separation control within supersonic inlets using this modified numerical scheme. A significant part of the electrical power from the power supply is deposited in the vibrational modes of N2, 5 which may be released as heat over many molecular collisions. A time dependent computational 2D Navier-Stokes solver including the equation of state and parametric vibrational relaxation of N2 for shockwave boundary layer interaction is developed. To replicate the experiments done at large boundary layer thickness, the code is divided in time independent and time dependent regimes to significantly reduce computation time. Further, time and space dependent force and volumetric heating are included to account for effects of plasma actuation. Computational results show that vibrational mode excitation and subsequent energy release due to vibration-translation (VT) relaxation have a very low impact on the performance of plasma actuation due to high relaxation times, low residence time of excited species in the test section and small size of recirculation bubble at high actuation currents.