Investigation of subsonic compressible cylinder wakes using High-Speed Particle Image Velocimetry.

High-speed Particle Image Velocimetry (PIV) measurements were conducted to investigate the wake of a circular cylinder in a confined channel over a broad range of Reynolds numbers (Re = 9,750 to 289,500) and Mach numbers (M = 0.03 to 0.85). Across all conditions, the von Kármán vortex street is observed in the wake of the cylinder. Time-averaged velocity profiles in the wake, normalized by the inlet velocity, collapse onto a single curve in the incompressible regime, indicating self-similar wake behavior. In compressible regimes, the normalized profiles also collapse, but onto a distinct curve, highlighting a different self-similar structure. Spectral analysis of the centerline transverse velocity fluctuations shows that the measured vortex shedding frequency deviates from the predicted incompressible frequency as Reynolds and Mach numbers increase, exhibiting a pronounced downward deflection. This observation contrasts with prior results for cylinder wakes at Reynolds numbers greater than 200,000, where a sharp increase in frequency is typically reported due to the drag crisis associated with the boundary layer transition to turbulence. The present results suggest that compressibility effects in the channel flow are suppressing or delaying the turbulent boundary layer mechanisms responsible for the drag crisis. These findings provide new insight into the interplay between compressibility and Reynolds number effects in high-speed confined wake flows, advancing the fundamental understanding of how compressibility influences vortex shedding and wake stability in the classical cylinder wake configuration.