Large Eddy Simulation of Flow and Separation Bubbles Around a Circular Cylinder from Sub-critical to Super-critical Reynolds Numbers

Abstract

A large eddy simulation of wall-adapting local eddy-viscosity model (LES-WALE) is used to simulate the three-dimensional flow around a circular cylinder with a diameter of 0.25 m from sub-critical to super-critical Reynolds numbers at 1 × 10⁵, 2.5 × 10⁵, and 7.2 × 10⁵, respectively. The present results such as drag crisis, surface pressure distribution, and Strouhal number are in good agreement with the classical experimental data. When entering the critical region, a small plateau was found on the pressure distribution curves, corresponding to the appearance of laminar separation bubbles, and the separation point is delayed and the recirculation bubbles become narrowed and shortened. The tangential velocity of the cylinder surface changes from positive to negative at the separation point. The instantaneous vorticity and time-averaging separation bubbles embody an unstable feature. Within the separation bubble, the pressure varies dramatically with time, but not with position. The surface pressure fluctuates greatly after the laminar separation bubble appears, and it is gradually stabilized until the basic pressure is reached. The process of laminar separation, transition from laminar flow to turbulent flow and turbulent reattachment is also shown. The three-dimensional Q criterion of vortex structure and the two-dimensional spanwise vorticity reveal the phenomenon that the wake structure narrows with the increase of the Reynolds number.

Keywords

Large eddy simulation / Critical Reynolds number / Drag crisis / Laminar separation bubble / Vortex shedding