A new phenomenon of bi-stability: experimental and numerical evidence

Bistable flows around circular cylinders typically occur in the critical range of the Reynolds number (Re), due to the formation of a laminar separation bubble and its subsequent collapse on one side of the cylinder only. In turbulent flows, as well as on a rough cylinder at slightly higher Re, bistable flows tend to disappear. However, this paper gives experimental evidence of a new type of bistable flow, which is induced around a circular cylinder of finite length even at moderately high Re. Stiffening rings, applied along the height of a circular cylinder at a distance smaller than the diameter (d = 2/3D in the case of study), were found to be responsible for the phenomenon. This was demonstrated and then cross-checked by wind tunnel experiments on a H/D = 6.7 circular cylinder. The experiments were performed in two different wind tunnels, at WiSt (Ruhr University Bochum) and at CRIACIV (University of Florence). A numerical study of the latter experimental test case is also presented in this paper. A finite volume approach based on the iterative solution of the unsteady Reynolds-averaged Navier-Stokes equations has been used. Turbulence has been modelled using the SST model by Menter. The numerical results confirm the physical fundament of the phenomenon, which likely finds its explanation in the key role of the flow over the cylinder tip.