The paper presents results of magnetohydrodynamic ͑MHD͒ 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 results show that as the Reynolds number increases, 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 ͑LDI͒ diagnostics, this behavior suggests that boundary layer transition occurs at stagnation pressures of P 0 ϳ 200-250 Torr. A crossed discharge ͑pulser+ dc sustainer͒ in M = 3 flows of air and nitrogen produced a stable, diffuse, and uniform plasma, with the time-average dc current up to 1.0 A in nitrogen and up to 0.8 A in air. The electrical conductivity and the Hall parameter in these flows are inferred from the current voltage characteristics of the sustainer discharge. LDI measurements detected the MHD effect on the ionized boundary layer density fluctuations at these conditions. Retarding Lorentz force applied to M = 3 nitrogen, air, and N 2-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 force͒.

A new blowdown nonequilibrium plasma magnetohydrodynamic (MHD) supersonic wind tunnel operated at complete steady state has been developed and tested at Ohio State. The wind tunnel can be operated at Mach numbers up to M = 3-4 and mass flow rates of up to 45 g/s at a stagnation pressure of 1 atm. Pitot tube and schlieren measurements in a M = 3 test section showed reasonably good flow quality, up to 80% inviscid core across the larger dimension and up to 50% inviscid core across the smaller dimension of the flow. Stable and diffuse transverse rf discharges (rf power up to 1 kW) have been sustained in M = 3 nitrogen flows, at magnetic fields of up to B = 1.5 T. Operation at higher magnetic fields produced a more uniform rf plasma in the MHD test section. Hall parameter and electric conductivity of the flow have been inferred from the dc (MHD) current and voltage measurements at different values of the magnetic field. At B = 1.5 T and rf power of 500 W, the Hall parameter is β ∼ = 3 and the conductivity is σ ∼ = 0.05 mho/m. At the rf power of 1 kW, the extrapolated conductivity is ∼0.1 mho/m. The results of the present work demonstrate the Lorentz force effect on the supersonic boundary layer in M = 3 flows of nitrogen ionized by a high-power transverse rf discharge in the presence of the magnetic field. Boundary-layer density fluctuation spectra are measured using the laser-differential-interferometry diagnostics. In particular, decelerating Lorentz force applied to the flow produces a well-reproduced increase of the density fluctuation intensity by up to 10-20% (1-2 dB), compared to the accelerating force of the same magnitude applied to the same flow. The effect is produced for two possible combinations of the magnetic field and MHD current directions producing the same Lorentz force direction (both for accelerating and decelerating force). The effect is observed to increase with the flow conductivity. On the other hand, the effect of Joule heating on the density fluctuation spectra appears insignificant.

This paper presents results of cold magnetohydrodynamic ͑MHD͒ flow deceleration experiments using repetitively pulsed, short pulse duration, high voltage discharge to produce ionization in M = 3 nitrogen and air flows in the presence of transverse direct current electric field and transverse magnetic field. MHD effect on the flow is detected from the flow static pressure measurements. Retarding Lorentz force applied to the flow produces a static pressure increase of up to 17%-20%, while accelerating force of the same magnitude results in static pressure increase of up to 5%-7%. The measured static pressure changes are compared with modeling calculations using quasi-one-dimensional MHD flow equations. Comparison of the experimental results with the modeling calculations shows that the retarding Lorentz force increases the static pressure rise produced by Joule heating of the flow, while the accelerating Lorentz force reduces the pressure rise. The effect is produced for two possible combinations of the magnetic field and transverse current directions producing the same Lorentz force direction ͑both for accelerating and retarding force͒. This demonstrates that the observed static pressure change is indeed due to the MHD interaction, and not due to Joule heating of the flow in the crossed discharge. No discharge polarity effect on the static pressure was detected in the absence of the magnetic field. The fraction of the discharge input power going into Joule heat in nitrogen and dry air, inferred from the present experiments, is low, ␣ = 0.1, primarily because energy remains frozen in the vibrational energy mode of nitrogen. This result provides first direct evidence of cold supersonic flow deceleration by Lorentz force.

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