| Shear flows in the vicinity of a free surface are a problem with numerous applications, perhaps the most obvious being the wakes of seagoing surface vessels. The flow behind a full-scale ship is extremely complex---so much so that it is frequently more instructive to consider simpler cases highlighting particular elements of the larger problem. To that end, an experimental investigation has been conducted to study the behavior of a turbulent plane mixing layer intersecting a free surface at low Froude number. The local Reynolds number, based on the velocity differential across the layer and the momentum thickness, was approximately 10,000.; The technique of Stereoscopic Digital Particle Image Velocimetry (SDPIV) was implemented to obtain instantaneous three-component velocity measurements within planar slices of the steady-state, spatially developing mixing layer flow. Guided by previous studies of the same flow conditions, specific depths were chosen at a single downstream station for investigation---specifically those in and around counter-rotating streamwise vortices known to exist in the mean flow very near the free surface. 3,000 consecutive SDPIV image pairs were recorded at a rate of 15 per second at each location, giving ample data for Reynolds decomposition and spectral analysis of the velocity fields.; The present study has found that the anisotropy known to exist in some other free surface flows, such as surface-parallel submerged jets, is also present in the case of the mixing layer. Power spectra of all three velocity components are shown to capture part of the inertial subrange; the isotropic energy cascade seen to be present away from the free surface is also seen to disappear near the surface, as surface-normal velocity fluctuations are severely attenuated.; Additionally, a low-frequency spanwise oscillation is deduced from the velocity power spectra and cospectra in the immediate vicinity of the mean streamwise vortices. Not present at all at significant depth, the motions at this frequency are also observed to markedly decrease---in all components---at locations closer to the surface. These observations appear to have both parallels and key differences compared to previously observed meandering of model boat wakes, and the possibility that the oscillation stems from the vortex-pair instability is discussed. |
