Interaction of wake turbulence with a free surface

The turbulent wake of a flat plate aligned with a uniform water flow and extending through the free surface was investigated experimentally. Laser-Doppler velocimetry (LDV) measurements show good agreement with published data for a two-dimensional wake, except in a shallow layer near the free surface. In this surface layer, the wake width is observed to double while the wake centerline velocity remains essentially unchanged from its value at depth, resulting in a wake momentum deficit that is twice that at depth. Instantaneous, full-field measurements of the velocity were made using digital particle image velocimetry (DPIV) to elucidate the role of vortical structures in the development of the surface layer. DPIV measurements reveal that in the deep wake, vortex structures predominantly of opposite sign exist on opposite sides of the wake centerline and contribute to a velocity and vorticity field that is two-dimensional in the mean. However, near the surface, vortex structures tend to become either surface-normal or surface-parallel and contribute to a velocity and vorticity held that is highly three-dimensional in the mean. The resulting surface layer is characterized by surface-parallel structures interacting with their images above the surface to retard and widen the surface how to a depth comparable to the size of the vortex structures. Histograms taken from many independent DPIV realizations of the flow characterize the distribution of vorticity in the wake and verify a mean flow consistent with. the LDV measurements. The existence of these structures in the surface layer is further confirmed by flow visualization using laser-induced fluorescence. (C) 1996 American Institute of Physics.