Numerical Study of Three-dimensional Flow Fields around the Base of A Vertical Cylinder in Oscillatory plus Mean Flow

This paper presents the results of a numerical investigation on the flow fields around a cylinder exposed to oscillatory plus mean flow. The flow fields were simulated using the Navier-Stokes equations expressed in a generalized curvilinear coordinate system. Descriptions are given of the three-dimensional flow structures around the base of the cylinder and examinations are made on the effects of the coexisting current ratio, Vr, and the Keulegan-Carpenter number, KC, on the vortex motion. It is shown that the horseshoe vortex increases in both size and lifespan with Vr and KC. The results indicate that the streamwise extent of the horseshoe vortex can be correlated well with a single dimensionless parameter. The bed shear stress under the vortices is also investigated in detail. The amplification of the bed shear stress is heavily influenced by Vr and KC. The results show that the maximum shear stress beneath the horseshoe vortex is sufficiently larger than that of the pure oscillatory flow and the value is less than that of the pure current case.