The flow structures of a transversely rotating sphere at high rotation rates

This paper studies the flow structures surrounding a transversely rotating sphere for the previously unreported conditions of flow Reynolds numbers of Re=100-300 and a rotation rate in the range Ω∗=1.25-3.00, where Reynolds number is based on sphere diameter and the free stream velocity and rotation rate Ω∗ is defined as the maximum sphere surface velocity normalised by free stream velocity. The flow remains steady at Re=100. At Re=250 and 300, the flow can be classified into 2 flow regimes at these rotation rates. Flow at lower rotation rates is dominated by the well known Kelvin-Helmholtz type instability vortex formation mechanism which produces regular, coherent vortices. As rotation is increased above Ω∗=1.50, the frequency of vortex formation decreases by an approximate factor of two. Linear stability analysis shows that this change in frequency can be attributed to significant changes in the geometry of free shear layers near the surface of the sphere. As rotation rate is increased further Ω∗>2.00, the flow enters the 'separatrix' regime. The separatrix divides the free stream flow and the surface-driven boundary layer that engulfs the sphere surface as a result of the rapid sphere rotation. The surface-driven boundary layer leads to better surface pressure recovery and lowers the time-averaged CD. On the other hand, time-averaged CL increases with Ω∗ in the 'separatrix' regime.