Three-dimensional experimental study of compressibility effects on turbulent free shear layers

Turbulent planar mixing layers of varying compressibility levels have been studied experimentally via flow visualization techniques, pressure measurements, and stereoscopic particle image velocimetry. The experiments were conducted for five different convective Mach numbers (M_c 0.19, 0.38, 0.55, 0.69, and 0.88). For each case, inflow conditions were thoroughly documented with planar particle image velocimetry. Results indicate that the incoming boundary layers are thin (less than or equal to 9% of channel height) and fully developed. Schlieren and Mie scattering images show that for increased levels of compressibility the large-scale turbulent structures are less organized, and the Brown–Roshko rollers that are dominant in the incompressible case are not found. Three-component velocity measurements made on the spanwise-central plane (z 0) via stereo particle image velocimetry confirm the reduction in the normalized growth rate with increasing M_c that has been agreed upon widely. Turbulence statistics results show that the peak (normalized) streamwise normal kinematic Reynolds stress remains constant, while the spanwise normal, transverse normal, and primary shear kinematic Reynolds stress peak values all decrease with increasing M_c . In addition, knowledge of all three normal stress components allows for the calculation of the full Reynolds stress anisotropy tensor, a novel result for compressible mixing layer experiments. Each anisotropy component remains constant across the shear layer center. As compressibility in the mixing layer increases, the streamwise normal stress anisotropy increases, transverse and spanwise normal stress anisotropies decrease, and the shear stress anisotropy remains constant. Conversely, the Reynolds stress correlation coefficient remains constant across all M_c cases at approximately 0.47.