Unsteady Dynamics in the Compressible Wake Behind a Cylinder

The transient, compressible wake behind a circular cylinder was captured using high resolution Particle Image Velocimetry (PIV) measurements and shadowgraph imaging. The measurements were conducted inside a high-speed wind tunnel for a large range of Reynolds numbers (Re = 6,500 - 289,500) with the corresponding Mach regime of M = 0.03 – 0.84. The high temporal resolution of the experimental data allowed for intermittent transient features to be observed and characterized when transitioning from an incompressible to compressible wake. Spectral analysis of the PIV data confirmed a dominant shedding frequency and an energy cascade consistent with turbulence statistics. The Strouhal number remained near 0.19 in the incompressible regime but decreased to approximately 0.16 in compressible conditions, indicating a clear influence of Mach number on vortex shedding dynamics. Shadowgraph imaging further validated these findings by showing the progression from mild pressure perturbations to pronounced compression waves at higher Reynolds numbers. These results demonstrate that compressibility alters both the frequency content and structural organization of the wake, reducing shedding frequency and introducing additional instability modes. The findings provide a foundation for improved modeling of compressible shear layer interactions in high-speed engineering applications.