Direct numerical simulation of oblique vortex shedding from a cylinder in shear flow

The vortex dynamics of a shear flow over a circular cylinder is studied by means of Direct Numerical Simulation. Four flow configurations are selected in order to consider the influence of three physical parameters: the vertical extension of the shear zone, the vertical domain size and the shear intensity. Despite the moderate value of the median Reynolds number considered (Re=200), the non-uniform character of the upstream flow leads to the formation of complex Karman streets behind the cylinder for each case. The analysis of the animations shows the occurrence of oblique vortex shedding driven through complex synchronization processes. The shear imposes strong distortions on the Karman vortices and dislocations are regularly observed. The simple observation of the vortical animations does not allow an unambiguous identification of the cellular pattern of vortex shedding. This phenomenon can be more rigorously described by a frequency analysis presented in this paper. It is shown that the main frequency selection is mainly conditioned by a local adjustment of oblique vortex shedding to the upstream velocity. However, due to the preservation of the spatial coherence of the flow, the variation of the main flow frequency occurs by jump along the cylinder axis direction, the distance between two jumps corresponding to the size of a cell. The Karman vortex formation is triggered in the high-speed region of the flow, but paradoxically, the local vortex shedding frequency found in this zone seems to be strongly influenced by the dynamics of the slow part of the flow.