Coherent motions in a turbulent wake of an axisymmetric bluff body

Abstract

The wake flow past an axisymmetric body of revolution at a diameter-based Reynolds number Re = u∞ D/v = 5000 is investigated via a direct numerical simulation. The study is focused on identification of coherent vortical motions and the dominant frequencies in this flow. Three dominant coherent motions are identified in the wake: the vortex shedding motion with the frequency of St = fD/u∞ = 0.27, the bubble pumping motion with St = 0.02, and the very-low-frequency (VLF) motion originated in the very near wake of the body with the frequency St=0.002-0.005. The vortex shedding pattern is demonstrated to follow a reflectional symmetry breaking mode, whereas the vortex loops are shed alternatingly from each side of the vortex shedding plane, but are subsequently twisted and tangled, giving the resulting wake structure a helical spiraling pattern. The bubble pumping motion is confined to the recirculation region and is a result of a Görtler instability. The VLF motion is related to a stochastic destabilisation of a steady symmetric mode in the near wake and manifests itself as a slow, precessional motion of the wake barycentre. The VLF mode with St = 0.005 is also detectable in the intermediate wake and may be associated with a low-frequency radial flapping of the shear layer.