Effect of Compressibility on Cylinder Wake Dynamics at Low Reynolds Numbers

Despite being historically overlooked, low Reynolds number bluff body flows in the compressible regime are of importance to several emerging engineering applications such as aircrafts in low-density extraterrestrial environments, microdroplet flows and ice crystal formation on high-speed vehicles. In this regime, the nature of bluff body wakes is significantly influenced by compressibility effects which may delay separation, elongate the recirculation region, or change the vortex-shedding behavior. Additionally, the value of the critical Reynolds number is itself subject to change with Mach number. In this study, we carry out a comprehensive parameter study to investigate the role of increasing Mach number on cylinder wake characteristics and instabilities. Direct numerical simulations based on a high-order viscous immersed boundary solver (ViCAS3D) are used for this purpose. A total of 110 simulations are carried out for Mach numbers ranging from 0.1 to 1.1 and Reynolds numbers from 20 to 300. Emphasis is placed on transonic speeds due to the rapidly changing flow characteristics in this regime. The effect of Mach number on the critical Reynolds number is quantified and the wake dynamics are characterized for the various cases.