Scaling Properties and Wave Interactions in Confined Supersonic Turbulent Bluff-Body Wakes
Results are presented from an experimental study that compares the large-scale structure, mean velocities, and scaling properties of confined, supersonic, planar, turbulent, bluff-body wakes at M∞ ≈ 2 and 3 with corresponding results from incompressible wakes. The large-scale structure of incompressible planar turbulent wakes is recovered at downstream locations x where the local relative Mach number Mr(x) has decreased to about 0.4. Interactions between these large-scale structures and reflected expansion waves from the near-field recompression region lead to substantial changes in the scaling properties of the flow. Wave interactions with the flow create local self-induced forcing as the large-scale structures pass through the reflected expansion waves. In the M∞ ≈ 2 wake, a subsonic upstream path exists from the first wave interaction point that allows upstream propagation of the wave-induced forcing, and consequently the measured wake scaling constants match values reported from forced incompressible wakes. In the M∞ ≈ 3 wake, no subsonic upstream path exists from the first interaction point, and as a consequence the measured scaling constants match values reported from unforced incompressible wakes. Downstream of each wave interaction point, the self-induced forcing leads to an increase in the wake growth rate and in the entrainment of free-stream fluid. The wake subsequently detrains this fluid and returns to its original growth rate. This local forcing mechanism and its effects on the wake scaling properties repeat at each downstream location where the reflected expansion waves intersect the wake.