Numerical simulation of oscillating flows over a bed-mounted circular cylinder

An immersed boundary (IB) method has been used to simulate oscillating flows over a bed-mounted circular cylinder at Reynolds number Re=1100. Two Keulegan–Carpenter (KC) numbers are considered in the study. For the smaller number of 0.99, there is no boundary layer separation and hence the flow is relatively simple. The acting forces on the cylinder can be evaluated by the potential theory. For the larger KC number of 6.6, the flow is very complicated due to the interactions between the shed vortices and the cylinder. The mechanisms of the boundary layer separation, vortex shedding and motion are explored by carefully calculating the transient vorticity field near the cylinder. In particular, the predicted vortex pair movement towards the bed is consistent with other researchers’ experimental flow visualization and is well explained by this numerical investigation. In addition, possible local scouring near the cylinder is suggested by the calculation of the bed shear stress and is proved by laboratory experiments. The acting force on the cylinder is also explained in relation to the patterns of the flow field.