  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 wires and particle tracking. We realize
this objective by looking at the flow of a heavy gas (sulfur hexafluoride
$SF_6$), with a speed of sound almost three times lower than for air. By
switching between air and $SF_6$, we isolate the influence of the turbulent
Mach number (up to $M_t$ = 0.17) on turbulence statistics from the influences
of changes in the Reynolds number (up to $R_{\lambda}$ = 1600), and boundary
conditions, which we hold constant. A free shear flow is produced by a ducted
fan, and we show that it behaves like a turbulent jet in that the mean velocity
profiles approach self-similarity with increasing distance from the orifice (up
to $x/D_f$ = 9). The jet responds like a compressible shear layer in 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 Kolmogorov constant of the
turbulence are approximately invariant with respect to changes in either the
Reynolds or Mach numbers. We briefly report on instrumentation under
development that will extend the accessible Taylor-scale Reynolds and turbulent
Mach numbers to 4000 and 0.3, respectively.
