Centrifugal instability of Stokes layers in crossflow

A circular cylinder oscillating in a viscous fluid produces an axisymmetric Stokes layer, a fundamental flow susceptible to centrifugal instabilities [see e.g. Seminara & Hall, Proc. Roy. Soc. London A 350, 299 (1976)]. In the present work we study such problem in the wake flow around a circular cylinder at Re = 100 performing rotary oscillations. For a forcing frequency ff and amplitude Ut, the non-dimensional control parameters f+ = ff /fn and A = Ut/U∞ are related to the flow properties, the inflow velocity U∞ and the natural vortex shedding frequency fn. In a previous work [D’Adamo et al., Phys. Rev. E. 84, 056308 (2011)], we identified experimentally a zone in the parameter space with forcing at frequencies lower than the natural vortex shedding frequency, where the flow exhibited some turbulence features such as a continuous spectrum for the velocity components, and multiple vortex splitting interactions in the wake. We show here using numerical simulations that these observations result from a 3D centrifugal instability.