Unsteady vortex structures in the wake of nonaxisymmetric bumps using spiral MRV

Abstract

The temporal characteristics of the wake structure behind a nonaxisymmetric bump are studied using spiral magnetic resonance velocimetry (MRV) to measure time-series of all three velocity components in y–z-planes for bump orientations of $$5^\circ$$ , $$10^\circ$$ , $$15^\circ$$ , and $$20^\circ$$ relative to the freestream flow. The mean wake structure was shown previously to be highly sensitive to bump orientation (Ching et al. Exp Fluids, 2018). The spiral MRV method is validated by comparison to the established MRV techniques in a flow with forced periodicity. Measured velocity fluctuations in the bump wake decrease significantly as the bump angle is increased. A quasi-periodic shedding cycle is reconstructed using spectral proper orthogonal decomposition. The reconstructed cycles of each velocity component are matched through a divergence minimization method to examine the structure of the shed vortices. The vortex shed from the trailing edge side of the bump weakens as the bump angle is increased, whereas the vortex shed from the leading edge side maintains its strength. The alternating vortices move across the centerline as they are advected downstream, explaining the common-down vortex pair observed in the far wake mean fields.