Compressibility effects on entrainment and mixing in supersonic planar turbulent wakes

Results are presented from an experimental investigation of compressibility effects on entrainment and mixing in the near- and far-fields of a supersonic, planar, turbulent, bluff-body wake formed by a two-dimensional slot jet nozzle aligned with a Mach 1.6 free stream. Planar laser Mie scattering (PLMS) and shadowgraph/schlieren imaging are combined with pitot and static pressure measurements to examine the mean flow scaling laws and the instantaneous structure of the near- and far-fields. Results show a classical vortex street-like large-scale structure in the wake far field, where the relative Mach number has decreased to clearly subsonic values. Where the relative Mach number is transonic, the far-field growth rate and velocity decay determined from PLMS imaging and mean velocity profiles follow the characteristic (δ/v) ~ (x/v})^1/2 and (u/U_∞) ~ (x/v)^-1/2 scaling of incompressible planar turbulent wakes. The virtual origin (x₀/v) is shifted downstream as a result of the expansion and recompression in the base flow region, by an amount that depends on the flow rate from the nozzle. Interaction of the reflected recompression shock with the large-scale vortical structures in the wake produces quasi-periodic forcing that affects the constants in the local flow width scaling and velocity defect scaling. Results for the scaling constants in the present supersonic wake agree with values reported for forced incompressible wakes.