Role of bulk viscosity on the flow physics past a rotating cylinder

The present study investigates the impact of bulk viscosity on the complex flow dynamics past a rotating cylinder, with particular emphasis on compressible and non-equilibrium effects that emerge in nitrogen (N-2) and carbon dioxide (CO2). By solving unsteady conservation laws obtained from the Boltzmann-Curtiss transport equation, the research focuses on key flow features such as vortex shedding, vorticity generation, enstrophy, kinetic energy dissipation, and the degree of thermal non-equilibrium. Numerical simulations are performed at a Mach number of 0.6 using the dbnsTurbFoam solver with unstructured meshes, and the computational model is verified using available data for flow past a rotating cylinder. The results reveal that bulk viscosity significantly affects vortex shedding, particularly suppressing vortex formation and reducing flow instability. In CO2, high bulk viscosity nearly eliminates vortex shedding, leading to a laminar wake, while in N2, vortex shedding is dampened but persists. Enstrophy and vorticity production through stretching and baroclinic effects are also reduced in both gases as bulk viscosity increases, with CO2 showing more dramatic reductions due to its higher inherent viscosity. The study further indicates that bulk viscosity enhances kinetic energy dissipation in both gases, with N-2 exhibiting sharper dissipation than CO2. Additionally, the role of rotational speed is explored, showing that higher rotational speeds amplify vorticity production and energy dissipation. While high-speed rotation induces more turbulence and instability in N-2, it stabilizes the flow in CO2, leading to a more organized wake. The findings demonstrate that bulk viscosity and rotational speed are crucial in controlling flow stability and energy dissipation, with significant variations depending on the gas properties.