The development of turbulence behind a shock wave front moving in an inhomogeneous region

Abstract

519Shock waves propagating in inhomogeneous media of various types are traditional objects for investigation in fluid dynamics. The presence of inhomogeneities in front of a shock wave can lead to modification of the flow structure, attenuation or enhancement of the wave, separation of the boundary layer, and curvature of the shock front. Another important effect is the for mation of new structures such as secondary shock waves, jets, precursors, and vortices. Artem'ev et al.[1] showed that, after passage of a shock wave along a warm channel, a significant vorticity of the flow is retained behind the shock wave front even after a long period of time, when a flat shock front is completely restored. This phenomenon can be used, in particular, for the enhancement of mixing during combustion. A basic research task is to study the interaction of shock waves with turbulence. Shock wave propagation through a turbulent region is accompanied by an increase in the fluctuations of thermodynamic param eters and the kinetic energy of turbulent pulsations [2,3].This Letter presents the results of an experimental investigation and numerical simulation of a nonsta tionary quasi two dimensional interaction of a plane shock wave with an inhomogeneous nonequilibrium layer formed by a distributed high current nanosecond pulsed surface discharge ("plasma sheet" [4]). By ini tiating a plasma sheet discharge on the bottom of a shock tube channel, it was possible to ensure a pulsed (almost instantaneous) energy supply to a thin (~0.5 mm thick) near surface gas layer without pre liminary heating of the surface.In our previous experiments [4-6], sliding dis charges on a dielectric surface were obtained by apply ing a pulsed voltage of 25-30 kV amplitude to a 10 cm long 3 cm wide interelectrode gap located on the bottom of a discharge section. The discharge cur rent direction was perpendicular to the direction of gasdynamic flow in the tube. The current pulse ampli tude reached 1-2 kA at a pulse duration on the order of 200 ns. The discharge section was a part of the low pressure chamber in a tube of 48 × 24 cm 2 cross section filled with air at room temperature and a pressure of p 0 = 20-80 Torr. Based on analysis of the dynamics of shock wave fields generated by the surface discharge, it was shown that a significant amount of the electric energy is converted into heat in a submillimeter near surface gas layer for a period of time shorter than 1 μs, which led in experiments to a fast heating up to 600⎯1000 K [6]. It was also found that, as the initial pressure in the discharge chamber was increased (>60-80 Torr), the plasma sheet glow became signifi cantly nonuniform in the flow direction, whereby sep arate bright channels were clearly manifested on a homogeneous background [5].A plane shock wave approached the beginning of the discharge gap in a preset time after termination of the discharge current pulse (the delay exceeded 40-50 μs). At this initial stage, intense relaxation pro cesses in discharge excited gas took place near the sur f...