Wake structure of a transversely rotating sphere at moderate Reynolds numbers

The uniform flow past a sphere undergoing steady rotation about an axis transverse to the free stream flow was investigated numerically. The objective was to reveal the effect of sphere rotation on the characteristics of the vortical wake structure and on the forces exerted on the sphere. This was achieved by solving the time-dependent, incompressible Navier-Stokes equations, using an accurate Fourier-Chebyshev spectral collocation method. Reynolds numbers Re of 100, 250 and 300 were considered, which for a stationary sphere cover the axisymmetric steady, non-axisymmetric steady and vortex shedding regimes. The study identified wake transitions that occur over the range of non-dimensional rotational speeds Omega*=0 to 1.00, where Omega* is the maximum velocity on the sphere surface normalized by the free stream velocity. At Re=100, sphere rotation triggers a transition to a steady double-threaded structure. At Re=250, the wake undergoes a transition to vortex shedding for Omega*>= 0.08. With an increasing rotation rate, the recirculating region is progressively reduced until a further transition to a steady double-threaded wake structure for Omega*>= 0.30. At Re=300, wake shedding is suppressed for Omega*>= 0.50 via the same mechanism found at Re=250. For Omega*>= 0.80, the wake undergoes a further transition to vortex shedding, through what appears to be a shear layer instability of the Kelvin-Helmholtz type.