Oblique and parallel modes of the bistable bluff body wake

Turbulent bluff body flows exhibit large-scale, low frequency dynamics associated with symmetry breaking in the wake which compromise stability and increase surface pressure drag. Previous tomographic PIV had detailed that these instabilities were associated with the stochastic reorientation of a hairpin featuring a near-base “head” connected to counter-rotating streamwise “tails” extending downstream. To better understand the associated flow physics, a half-axisymmetric bluff body is assessed by means of particle image velocimetry and pressure measurements. By application of proper orthogonal decomposition (POD) and an assessment of spanwise phase difference using a Morlet wavelet analysis, we show that the asymmetry of the wake region is intrinsically linked to a large scale circulation reversal responsible for the switch to the opposing symmetry breaking state. The antiphase relationship across the base is established by propagation of an oblique wave in the cross-stream direction at separation and as a result, the wake becomes twisted and skewed to one side inducing an asymmetric base pressure gradient. The two most energetic modes in the near wake are halved with respect to rectilinear bistable bodies; the first shows a helical topology with lateral asymmetry and the second shows a symmetry preserving condition with two counter rotating vortices at each side. Across the switch, the unsteady second mode briefly characterizes the wake. The associated parallel shedding regime with reduced asymmetry promotes axial motion (bubble pumping) which is coupled to an optimal, low drag condition.