Planar velocity measurements in compressible mixing layers

The efficiency of high-Mach number airbreathing propulsion devices is critically dependent upon the mixing of gases in turbulent shear flows. However, compressibility is known to suppress the growth rates of these mixing layers, posing a problem of both practical and scientific interest. In the present study, particle image velocimetry (PIV) is used to obtain planar, two-component velocity fields for Planar gaseous shear layers at convective Mach numbers Mc of 0.25, 0.63, and 0.76. The experiments are performed in a large-scale blowdown wind tunnel, with high-speed freestream Mach numbers up to 2.25 and shear-layer Reynolds numbers up to 106 . The instantaneous data are analyzed to produce maps of derived quantities such as vorticity, and ensemble averaged to provide turbulence statistics. Specific issues relating to the application of PIV to supersonic flows are addressed. In addition to the fluid-velocity measurements, we present double-pulsed scalar visualizations, permitting inference of the convective velocity of the large-scale structures, and examine the interaction of a weak wave with the mixing layer.;The principal change associated with compressibility is seen to be the development of multiple high-gradient regions in the instantaneous velocity field, disrupting the spanwise-coherent 'roller' structure usually associated with incompressible layers. As a result, the vorticity peaks reside in multiple thin sheets, segregated in the transverse direction. This suggests a decrease in cross-stream communication and a disconnection of the entrainment processes at the two interfaces. In the compressible case, steep-gradient regions in the instantaneous velocity field often correspond closely with the local sonic line, suggesting a sensitivity to lab-frame disturbances; this could in turn explain the effectiveness of sub-boundary layer mixing enhancement strategies in this flow. Large-ensemble statistics bear out the observation from previous single-point measurements that transverse turbulence intensity and Reynolds stress are suppressed as compressibility increases. The principal features of the wave-layer interaction are the imposition of compressibility effects even at low Mc, and more marked alteration of turbulence statistics. Structure convective velocities measured using the double-pulsed scalar images are lower than those predicted from isentropic mixing-layer theory, to a degree proportional to compressibility.