Signatures of compressibility of an annular free-shear layer with increasing Mach number

The Variable Density and Speed of Sound Vessel (VDSSV) produces subsonic turbulent flows that are both compressible and observable at all scales with existing instrumentation including hot-wire anemometry. We raise the turbulent Mach number up to M_t = 0.17 at constant Reynolds numbers (up to R_λ = 1600) and while holding the boundary conditions fixed.  We do this by comparing the flow of air with the flow of the heavy gas sulfur hexafluoride (SF₆), with a speed of sound almost three times lower than for air, downstream of a ducted fan. We show that the mean velocity profiles of the resulting turbulence approach a self-similar shape with increasing distance from the source.  The jet responds like a compressible shear layer in the sense that it spreads more slowly at higher Mach numbers (up to M_j = 0.7) than at low Mach numbers. In contrast, the integral length scales and the Kolmogorov constant are approximately invariant with respect to changes in either the Reynolds or Mach numbers.