On the symmetry breaking instability leading to vortex shedding

It is well known that at early stages an impulsively started flow past a circular cylinder consists of twin vortices which are images of one another by reflection through the mid-plane. While the twin vortices are stable for Reynolds numbers below the critical Reynolds number value (Rec≃48), they become unstable above the critical Reynolds number. At Re>Rec, the flow keeps its symmetric recirculating bubble structure for a short time, undergoes a symmetry-breaking instability, and develops into a Karman vortex street. Foppl’s vortex model is studied here as a low-dimensional model for the symmetric bubble. The stability analysis of a fixed bubble in the model shows that there are two asymmetric eigenmodes, a stable mode and an unstable one. In this paper, we show by two-dimensional direct numerical simulations (DNS) of the impulsively started flow past a circular cylinder how the instability properties of the model qualitatively mimic those of the real flow.