The turbulent wake of submarine model in pitch and yaw

Abstract

This thesis aims to further our understanding of high Reynolds number wakes of submarine-like bodies of revolution. These wake measurements are preceded by an investigation of the limitations of the hot wire anemometry technique that is extensively used in this thesis to measure the velocity field.

To study spatial filtering, measurements of the turbulence statistics and spectra downstream of a grid were performed using hot-wires of varying length and compared to the results from a new nano-scale thermal anemometry probe. The effect of spatial filtering on the streamwise spectrum function is observed to extend to almost all wavenumbers, even those significantly lower than the length of the hot wire itself. A criterion is found that must be satisfied in order to avoid the effects of spatial filtering on the turbulence statistics as well as on the spectrum. End conduction effects are investigated numerically and validated experimentally using grid turbulence. A new end-conduction parameter takes into account the effects due to hot wire material, overheat ratio and Reynolds number. We suggest a numerical criterion for the new end-conduction parameter that is necessary to avoid any attenuation in the turbulent fluctuations.

Detailed velocity measurements were then performed using hot wire anemometry and stereoscopic particle image velocimetry in the wake of a body of revolution shape (DARPA SUBOFF) at a Reynolds number of $Re_{L} = 2.4 \times 10^{6}$, for pitch and yaw angles up to 12$^\circ$. These measurements reveal the formation of a pair of streamwise vortices in pitch and yaw that are asymmetric in strength, an unexpected result. In pitch the vortices rotate around each other as they evolve downstream, and they lose their strength by diffusion relatively quickly. In yaw the asymmetry is even more pronounced due to the influence of the sail. The weaker vortex quickly diffuses, and in the absence of further diffusion the stronger vortex maintains its strength even at the furthest downstream location. It appears that small asymmetries in the flow near the nose can result in strong asymmetries in the wake, a result previously only seen for sharp-nosed bodies at high angles of attack.