A Numerical Study of Bluff Ring Wake Stability

A numerical study of the wake dynamics and stability of the flow around bluff rings placed normal to the flow direction at low Reynolds number is presented. An attractive feature of this bluff body geometry is that it effectively behaves locally like a circular cylinder at high aspect ratio while, as the aspect ratio is reduced to zero, the body is transformed to a sphere. Thus, it covers a rich diversity of wake topologies and transitions. A spectral-element method is used to solve the unsteady axisymmetric Navier-Stokes equations governing the fluid flow. Wake stability is determined using a linear Floquet-type stability analysis. Strouhal-Reynolds number profiles are provided for a range of ring aspect ratios, as are critical Reynolds numbers for the onset of flow separation and periodic flow. A decrease in shedding frequency and an increase in the critical Reynolds numbers for separation and unsteady flow with decreasing ring aspect ratio is shown. Stability analysis has enabled interesting flow features with respect to the three-dimensionality of the flow to be obtained for different ring aspect ratios.