Low-Temperature Supersonic Boundary Layer Control Using Repetitively Pulsed MHD Forcing

Abstract

The paper presents results of magnetohydrodynamic (MHD) supersonic boundary layer control experiments using repetitively pulsed, short pulse duration, high voltage discharges in M=3 flows of nitrogen and air in the presence of a magnetic field of B=1.5 T. We also have conducted boundary layer flow visualization experiments using laser sheet scattering. Flow visualization shows that side wall boundary layers in the supersonic test section are considerably thicker near the center plane of the flow. The results also show that as the Reynolds number increases from Rex=2.7·10 5 to 8.1·10 5 , the boundary layer flow becomes much more chaotic, with the spatial scale of temperature fluctuations decreasing. Combined with density fluctuation spectra measurements using Laser Differential Interferometry (LDI) diagnostics, this behavior suggests that boundary layer transition occurs at stagnation pressures of P0~200-250 torr. Operation of a crossed discharge (pulser + DC sustainer) in M=3 flows of air and nitrogen demonstrated that such a discharge produces a stable, diffuse, and uniform plasma. The time-average DC current achieved in such discharges is up to 1.0 A in nitrogen (conductivity of σ=0.073 mho/m) and up to 0.8 A in air (σ=0.072 mho/m). The electrical conductivity and the Hall parameter in nitrogen and air flows are inferred from the current voltage characteristics of the sustainer discharge. LDI measurements detected MHD effect on the ionized boundary layer density fluctuations at these conditions. Retarding Lorentz force applied to M=3 nitrogen, air, and N2-He flows produces an increase of the density fluctuation intensity by up to 2 dB (about 25%), compared to the accelerating force of the same magnitude. The effect is demonstrated for two possible combinations of the magnetic field and current directions producing the same Lorentz force direction (both for accelerating and retarding force). Comparison with the LDI spectra measured with no MHD force applied showed that the effect on the density fluctuations is produced only by the retarding Lorentz force, while the Joule heat effect appears insignificant.