Nonequilibrium flow characterization in a mach 5 wind tunnel

The paper reports recent progress in experiments on supersonic flow characterization in a small scale, Mach 5 nonequilibrium flow wind tunnel. The wind tunnel generates nonequilibrium Mach 5 flows at steady state. Transverse repetitively pulsed nanosecond discharge, fully overlapped with transverse DC discharge, is used to load internal energy modes of nitrogen in the nozzle plenum, at P₀∼0.5 atm and at low translational/rotational temperatures. NO PLIF images on two single-line NO(X,v'=0→A,v"=0) transitions are used to infer two-dimensional rotational temperature distributions in NO-seeded nitrogen flows in the supersonic test section. These measurements are conducted in a cold flow (i.e. without the discharge in plenum) and in a flow vibrationally excited by the pulsed / DC discharge. The results do not indicate detectable temperature difference between these two cases, outside the experimental uncertainty. Single-line NO PLIF images on a NO(X,v'=1→A,v"=1) Q₁+P₂₁ (J=3.5) transition are used to infer the NO vibrational temperature in a nitrogen Mach 5 flow excited by the discharge in plenum and seeded with NO, T_V(NO)=1050 ± 170 K. The N₂ vibrational temperature, inferred from this results, is T_v(N₂)=850 K. Emission from NO γ bands and NO ₂ chemiluminescence are detected in a Mach 5 flow of nitrogen excited by the pulsed discharge in plenum over a cylinder model, with counterflow injection of NO-N₂ mixture through the model. NO electronic emission most likely occurs due to energy transfer from N₂(A ³Σ) state generated in the discharge to the NO(A ²Σ) state. Schlieren visualization is used to examine the effect of N₂ vibrational relaxation on a Mach 5 bow shock stand-off distance in the supersonic section, by seeding the nonequilibrium nitrogen flow downstream of the discharge with water vapor and hydrogen. The results suggest that shock stand-off distance change detected in these experiments is affected by water vapor condensation, and is unlikely to be related to accelerated N ₂ vibrational relaxation. Feasibility of Mach 5 flow control over a cylinder model by a surface dielectric barrier discharge (DBD) powered by repetitive nanosecond duration voltage pulses has been tested. Preliminary results demonstrate that the nanosecond pulse discharge initiated on the surface of the model generates shock waves upstream of the baseline Mach 5 bow shock, on a microsecond time scale. The shocks generated by the discharge pulses perturb the baseline shock and gradually merge with it. The results show that strong supersonic flow perturbations by using this approach can be generated at a repetition rate of up to 100 kHz.