Non-equilibrium effects on flow past a circular cylinder

This paper studies a comprehensive investigation of non-equilibrium gas flow past a circular cylinder by discrete unified gas kinetic schemes, focusing on the Knudsen number (Kn) range from 0.001 to 0.5 and the Mach number (Ma) range from 0.1 to 1.1. The primary objective is to examine the effects of rarefaction and compressibility on flow patterns and properties. In the Kn-Ma space, six types of flow patterns are observed, especially for the transonic flow near the continuum regime, we observe that the vortices transition to a pair of symmetrical vortices after the initial shedding, and eventually evolve into alternating small-scale vortex shedding, which dissipate rapidly in the flow. The velocity profile shows a continuous periodic oscillation with a small amplitude after stabilization, where the oscillation frequency decreases with the increment of Kn until the flow transitions to the steady state, while it increases slightly as increasing Ma. Additionally, we conduct a quantitative analysis of typical parameters that characterize flow patterns. It is found that the rarefaction effect promotes the disappearance of vortices across the entire Ma range. The local flow characteristics around the cylinder are analyzed, and it is shown that the slip velocity exhibits complex distributions influenced by Kn and Ma. In addition, the local Kn and pressure coefficient deviate significantly from the free-stream value under the non-equilibrium effect. The mean drag coefficient increases with the increment of Kn at constant Ma, which is more pronounced at smaller Ma.