Asymmetries in the wake of a submarine model in pitch

Abstract

Detailed velocity measurements in the wake of a body of revolution are reported for pitch angles up to $12^{\circ }$, over an unprecedented range of Reynolds numbers ($2.4\times 10^{6}\leqslant \mathit{Re}_{L}\leqslant 30\times 10^{6}$). The body of revolution, an idealized submarine shape (DARPA SUBOFF), is mounted using a support that mimics a semi-infinite sail. The wake measurements at all pitch angles and Reynolds numbers reveal the presence of a pair of streamwise vortices of unequal strengths which tend to rotate around each other as they evolve downstream. Various attempts to perturb the upstream conditions on the body had no significant impact on the relative strength of the vortices. In addition, two different models, tested in two different wind tunnels, show similar asymmetries, and we propose that wake asymmetry appears to be a robust feature of this flow, a result previously only seen for sharp-nosed bodies at high angles of attack. It is also shown that the wake behaviour for $x/D>5$, in terms of the streamwise mean velocity and turbulence intensity distributions, appears to become invariant with Reynolds number for $\mathit{Re}_{L}>4.8\times 10^{6}$.