A new type of bistable flow around circular cylinders with spanwise stiffening rings

The bistable flow condition around a single circular cylinders is a well-known fluid dynamic phenomenon in the critical range of Re. It is sensitive not only to small variations of the Reynolds number, but also to turbulence of the incoming flow and to surface roughness of the cylinder. Bistable flows are also common for side-by-side cylinders, depending on their centre-to-centre transverse pitch ratio. The paper reveals – through wind tunnel tests – the existence of a new type of bistable flow, induced around a single circular cylinder with a free-end by the presence of spanwise rings. There are some analogies with the aforementioned bistable phenomena, but the conditions of occurrence are profoundly different. The peculiarity of this phenomenon is that it does not disappear at moderately high Re. Its existence is confirmed by a cross-check of results in two different wind tunnels. In order to characterize such a bistable pressure field, the pressure is modeled in the paper as the output of a non-linear conservative system with asymmetric potential wells forced by a proper stochastic process. The identification of the system parameters is performed by the comparison between the theoretical distribution of the oscillator response and the histogram of the recorded pressure.