Control of Shock-Induced Boundary-Layer Separation Using Plasmas  
Why Separation Regions Are a Problem
Within supersonic inlets or combustors, when a shock impinges on a thick laminar or turbulent boundary layer, separation of the flow may occur and lead to a “separation bubble” of substantial size. This is problematic because these large separation bubbles may grow and shrink in an unpredictable manner and alter the path of the flow and the location of the shockwaves in the process. This leads to difficulties in designing optimally inlets and combustors of supersonic flight vehicles.
How Can Separation Bubbles Be Prevented?
The conventional approach to prevent separation bubbles is to “bleed” the thick boundary layer from the inlet or combustor. This is a non-optimal solution thus because it leads to a substantial loss in mass flow rate, hence decreasing the performance of the inlet or combustor.
Another approach that has received considerable interest in recent years is through the use of the MHD accelerator.
What is a MHD accelerator?
A MHD (magneto-hydro-dynamics) accelerator is a device used to impart momentum to a gas through the Lorentz force. The Lorentz force is a force acting on a gas due to electromagnetic effects, and is proportional to the current and the applied magnetic field.
How can current flow in air?
A current can flow in a gas only if the latter has some ionization (i.e., if the air becomes a plasma). Because self-ionization of air only occurs at temperatures in excess of 5000 K that are not encountered in typical flight conditions, the air needs to be ionized through a high electric field (using a discharge for instance), through potassium or cesium seeding, or through electron beams to permit flow of current.
Has the MHD Accelerator Been Successful So Far in Preventing Separation?
Experiments and preliminary “uncoupled” numerical simulations were performed showing a net reduction of the separation bubble when controlled by MHD [1-4]. However, the numerical simulations do not solve the plasma in coupled form with the neutrals, and the simulations are thus possibly tainted with significant physical error. For instance, they do not reproduce the striations within the plasma at high magnetic field observed in [5]. As well, I do have some concerns about the numerical error in some of those papers.
How You Can Help
We are seeking one or more Ph.D. students who would be interested in simulating numerically MHD control of shock-induced boundary-layer separation. We will first consider air ionization through a discharge and compare the simulations with the experimental data that has already been published in previous papers. These numerical simulations will be done using our in-house-developed WARP code. Because WARP has the unique capability to integrate a detailed plasma model in coupled form with the aerodynamics using aerodynamic-scale timesteps, the numerical results will be of significantly higher quality than any previous attempt. I suspect that there is a significant interaction between the plasma and the aerodynamics here, which WARP can capture. Thus, this will lead to a better understanding of the flow physics, which will itself lead to a better and more efficient design of the MHD device in preventing shock-induced separation of boundary layers.
[1]  Kalra, C. S., Zaidi, S. H., Miles, R. B., Macheret, S. O. “Shockwave–turbulent boundary layer interaction control using magnetically driven surface discharges,” Experiments in Fluids, Vol. 50, pp. 547-559, 2011.
[2]  Bisek, N. J., Rizzetta, D. P. and Poggie, J. “Plasma Control of a Turbulent Shock Boundary-Layer Interaction”, AIAA Journal, Vol. 51, No. 8, pp. 1789-1804, 2013.
[3]  Kalra, C. S., Shneider, M. N. and Miles, R. B. “Numerical study of boundary layer separation control using magnetogasdynamic plasma actuators”, Physics of Fluids, Vol. 21, 2009.
[4]  Atkinson, M. D., Poggie, J., and Camberos, J. A. “Control of separated flow in a reflected shock interaction using a magnetically-accelerated surface discharge”, Physics of Fluids, Vol. 24, 2012.
[5]  B Parent, SO Macheret, MN Shneider, N Harada, Numerical Study of an Electron-Beam-Confined Faraday Accelerator, Journal of Propulsion and Power, Vol. 23, No. 5, 2007.
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