Pressure-gradient Mechanism for Vortex Shedding in External Flows

In our previous research, a pressure-gradient mechanism is identified as a likely cause of vortex splitting and shedding in constricted two-dimensional channel flows. We now find this mechanism present in the canonical two-dimensional external flows of (a) the cylinder in crossflow and (b) the flow over a step via numerical simulations of the unsteady, two-dimensional, incompressible Navier--Stokes equations. The details of the pressure-gradient mechanism are presented for Reynolds numbers typically found in the literature.