Wakes behind a prolate spheroid in crossflow

Viscous laminar flow past a prolate $(L/ d= 6)$ spheroid has been investigated numerically at seven different Reynolds numbers; $\mathit{Re}= 50, 75, 100, 150, 200, 250$ and $300$. In contrast to all earlier investigations, the major axis of the spheroid was oriented perpendicular to the free stream flow. As expected, the flow field in the wake showed a strong resemblance of that observed behind a finite-length circular cylinder, yet had features observed in the axisymmetric wake behind a sphere. The following different flow regimes were observed in the present computational study: (i) steady laminar flow with massive flow separation and symmetry about the equatorial and the meridional planes at $\mathit{Re}= 50$; (ii) steady laminar flow with massive flow separation and symmetry about the equatorial and the meridional plane at $\mathit{Re}= 75$, but the flow in the equatorial plane did no longer resemble the steady wake behind a circular cylinder; (iii) unsteady laminar flow with Strouhal number $0. 109$ and symmetry about the equatorial plane at $\mathit{Re}= 100$; (iv) unsteady laminar flow with two distinct frequencies and without any planar symmetries at $\mathit{Re}= 200$; (v) transitional flow with a dominant shedding frequency $\mathit{St}= 0. 151$ and without any spatial symmetries at $\mathit{Re}= 300$. For all but the two lowest $\mathit{Re}$ hairpin vortices were alternately shed from the two sides of the spheroid and resulted in a ladder-like pattern of oppositely oriented vortex structures, in contrast with the single-sided shedding in the wake of a sphere. The contour of the very-near-wake mimicked the shape of the prolate spheroid. However, $15d$ downstream the major axis of the wake became aligned with the minor axis of the spheroid. This implies that an axis switching occurred some $10d$ downstream, i.e. the cross-section of the wake evolved such that the major and minor axes interchanged at a certain downstream location. This peculiar phenomenon has frequently been reported to arise for elliptical and rectangular jets, whereas observations of axis switching for asymmetric wakes are scarce.