Spanwise correlation and coherent structures of separated flow around rectangular 5:1 cylinder

Abstract

Spanwise correlation and coherent structure of separated flow around a rectangular cylinder with a side ratio of B/D = 5 (B: breadth; D: depth of cylinder) are numerically investigated by Large Eddy Simulation (LES) at Reynolds number Re = 22,000. The investigation utilizes one series of three-dimensional simulations with various spanwise lengths up to 10D at a grid resolution of D/16 and another series of simulations with various spanwise grid resolutions as small as D/40 at a spanwise length of 5D, in addition to a two-dimensional simulation. The results show that a spanwise coherent flow structure with a wavelength of roughly 1.4D, associated with longitudinal vortices, exists in the separated flow. A spanwise length of 3D seems long enough to eliminate the influence of spanwise coherent flow structure on the global aerodynamic parameters and leads to convergent simulation results. The spanwise correlations of sectional forces and surface pressures decay with the spanwise distance. A finer spanwise resolution results in decreased spanwise correlations in the investigated spanwise resolution region. The results also imply that it is necessary to consider the combined effects of multiple interacting factors such as numerical method, three-dimensional grid discretization and turbulence model when selecting the spanwise resolution.