AIAA Low-Temperature Supersonic Boundary Layer Control Using Repetitively Pulsed MHD Forcing. Munetake Nishihara, Naibo Jiang,

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1 AIAA Low-Temperature Supersonic Boundary Layer Control Using Repetitively Pulsed MHD Forcing Munetake Nishihara, Naibo Jiang, J. William Rich, Walter R. Lempert, Igor V. Adamovich Dept. of Mechanical Engineering and Sivaram Gogineni Innovative Scientific Solutions, Inc.

2 Support AFOSR grant F Phase II SBIR of Air Vehicles Directorate of AFRL (with ISSI) Motivation Turbulent transition control in supersonic boundary layer Flow separation control Skin friction / wall heat transfer leverage May require weak actuating force near transition region, i.e. low interaction parameter: low electrical conductivity, modest magnetic field, reasonably small magnets This implies possible use of nonequilibrium, low-temperature plasmas

3 Background Salt water boundary layer data (Henoch & Stace, 1994): accelerating Lorentz force reduces turbulent fluctuations, decelerating force amplifies them. M=3 RF-ionized nitrogen flow (OSU, ): apparent MHD effect on BL density fluctuation spectra. Objectives Characterize the state of the boundary layer flow using flow visualization Characterize crossed discharge (Pulse discharge in DC and B fields) Detect Lorentz force effect on supersonic boundary layer at increased flow conductivity, using repetitively pulsed ionization

4 MHD test section schematic (M=3 and M=4) Optical access window Flow Pulsed electrode block DC electrode block Flow B Magnet pole Optical access window Contoured nozzle 12 cm long, 4 cm x 2 cm test section Equipped with pressure ports and Pitot ports Ceramic/copper Pulse and DC electrode blocks Stagnation pressure P 0 = atm Ionization: repetitively pulsed discharge

5 MHD wind tunnel (with high voltage pulse compression unit mounted) Developed in collaboration with Chemical Physics Technologies Peak voltage up to 30 kv, peak current up to A, pulse energy up to 20 mj, pulse duration nsec, pulse repetition rate up to 50 khz, load impedance matching up to 70-80% Much higher ionization efficiency at high E/N Much higher ionization efficiency at high E/N Much better plasma stability (duty cycle ~ 1:1000) Requires external magnetic field shielding

6 Laser Differential Interferometry (LDI): BL density fluctuation spectra measurements Magnet pole Flow He-Ne laser To vacuum Mirror Flow Magnet coil Photodiode Reference beam Probe beam

7 Boundary layer flow visualization: Laser sheet scattering using acetone vapor seeding Optical window Laser sheet Test section ICCD camera Distance to wall Scattering signal Flow Positive cylindrical lens λ/4 plate Mirror Negative cylindrical lens Nd:YAG laser

8 Boundary layer flow visualization M=3 air flows without plasmas, Re 5 5 x = to h=35/40 mm - 2 mm from the wall h=5/40 mm - x=21 cm x=19 cm P 0 =150 torr 200 torr 250 torr 350 torr 450 torr 4mmfrom the wall P 0 =170 torr 200 torr 250 torr 350 torr 450 torr

9 Comparison with 3-D laminar compressible Navier-Stokes Navier Stokes flow codes P0=150 torr P0=250 torr 2 mm from the wall 2 mm from the wall Lindsey, McMullan, AFIT, 2005 Rawat, Zhong, UCLA, 2005

10 LDI signal at 40 khz (at beginning of roll-off) M=3 air flows without plasmas, Re 5 5 x = to Intensity, db -35 Intensity, db mmfrom the wall 4mmfrom the wall Plenum pressure, torr Plenum pressure, torr Transition occurs at P 0 = torr?

11 Boundary layer flow visualization (cont.) M=3 air flows without plasmas 6 mm from the wall 8 mm from the wall P 0 =250 torr 500 torr P 0 =250 torr 500 torr

12 Repetitively pulsed discharge (40 khz rep rate) + DC sustainer in M=4 flows Nitrogen, B=0 P 0 =1 atm, P test =10 torr, U max =19 kv U DC =1.5 kv, R=2 kω, I DC max =0.8 A Air, B=1.5 T (with 6-layer magnetic shield) P 0 =1 atm, P test =13 torr, U max =11 kv, U DC =2.0 kv, R=0.5 kω, I DC max =1.7 A Plasma remains uniform and stable for run times of 2-3 seconds

13 Single-pulse voltage oscillograms in nitrogen 20 Voltage, kv Nitrogen, M=4 5 cm cables 15 cm cables 20 Voltage, kv Nitrogen, M= cm cables cm cables B=1.5 T B=0 B= Time, nsec M=4 nitrogen flows, P 0 =1 atm, P test =10 torr B=1.5 T B=0 B=1 T Time, nsec M=3 nitrogen flows, P 0 =1/3 atm, P test =10 torr.

14 Pulser / sustainer discharge in M=3 flows of N 2 and air: current and voltage traces 2.0 Voltage, kv Current, A 1.5 Current, A 1.5 Voltage Nitrogen 1.0 R=1 kω <U>=1.27 kv <I>=0.73 A 0.5 Current B=0 T B=1.0 T B=1.5 T Time, microseconds Time, microseconds <U>=1.27 kv, <I>=0.73 A <U>=1.6 kv, R=1.0 kω

15 Current-Voltage characteristics in M3 flows (P0=250 torr, Ptest=7.5 torr) 1.0 Average current, A Nitrogen 1.0 Average current, A Air 0.8 B=0T B=0.75T B=1.5T 0.8 B=0T B=1.5T Uc=260 V at B= 0 T Uc=500 V at B= 1.5 T Voltage, V Uc=350 V at B= 0 T Uc=520 VatB=15T Voltage, V I y σ σ = E A, I E A 2 y x = β 2 y 1+ β 1+ β. I = I 2 x + I 2 y = σ 2 1+ β ( U U c ) A h. <σ>=0.073 mho/m β=1.6 at B=1.5 T <σ>=0.072 mho/m β=2.1 at B=1.5 T

16 Accelerating vs. retarding Lorentz force Nitrogen and Air, M=3, P 0 =250 torr For different ballast resistors in nitrogen, For 4 different combinations of DC and B polarities in air, -35 Intensity, db R=1 kohm, <I>=0.65 A -35 Intensity, db -40 R=0.5 kohm, <I>=0.94 A Nitrogen, 6 mm away from the wall retarding force -50 accelerating force no MHD force Frequency, Hz -45 Air R=0.5 kohm, <I>=0.78 A 6 mm away from the wall -55 retarding force accelerating force no MHD force Frequency, Hz

17 Accelerating vs. retarding Lorentz force N2(80%)-He(20%) mixture, M=3, P 0 =250 torr The observed effect in nitrogen and air is consistent with the following results Adverse pressure gradient amplified turbulent intensities. (Fernando and Smits, 1990) Favorable pressure gradient reduced turbulent intensities, although the fluctuation of mass flux remained unchanged. (Smith and Smits, 1991) For different probe beam locations in N2-He mixture, Intensity, db 4mm, <I>=1.17 A 4mm, <I>=1.02 A N2-He, R=0.5 kohm Retarding force 6mm, <I>=1.14 A 6mm, <I>=1.02A Accelerating force Frequency, Hz

18 Summary Flow visualization: from P 0 =150 torr to 450 torr, the boundary layer flow becomes much more chaotic. Turbulent transition occurs at 250 torr. Crossed discharge is characterized. (I-V characteristics, Cathode voltage fall, electric conductivity, and Hall parameter) MHD currents at B=1.5 T are up to I=1.0 A in nitrogen (σ=0.073 mho/m) and up to I=0.8 A in air (σ=0.072 mho/m) In M=3 nitrogen and air flows, retarding Lorentz force produces an increase of the density fluctuation intensity with increasing currents, compared to accelerating force The effect on density fluctuation is detected only when retarding Lorentz force is applied In M=3 N2-He flows, the MHD effect is detected at multiple locations In M 3 N2 He flows, the MHD effect is detected at multiple locations across the boundary layer

19 Future work Improving flow quality: new M=3 andm=4 nozzle / test section with contoured side walls and side-wall diffuser angle steps This would reduce secondary cross flow and resultant boundary layer bulge Height 0.01 Height Contoured wall width Width Rawat and Zhong, 2005 Long-term objective: seeding the flow with NO and using laser sheet LIF on the 226 nm NO(X 2 Π, v=0 A 2 Σ, v=0) γ-band transition to map the temperature field in plasma flows with MHD forcing