Interaction of wake turbulence with a free surface

Abstract

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 field 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 flow 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.

Structure of wake of a sharp-edged bluff body in a shallow channel flow

“…The range of the signed swirling strength magnitude decreases mildly from the mid-depth to the near-surface location. This is probably due to the appearance of structures parallel and close to the free surface, as found by Logory et al (1996). The origination of these structures may contribute to the reduction of the swirling strength magnitude close to the free surface.…”

“…Amongst other observations, he found that vortices from the separating shear layer grow in the near-wake region, but starts to dissipate due to the bed friction effect at the intermediate and far-wake regions. The effect of free surface on the wake structure was explored by Logory et al (1996) and Maheo (1999). The free surface was mainly found to be characterized by 'upwelling' and 'downdraft' type of events.…”

Free-surface turbulence and air-water gas exchange

“…Measurements of the free-surface velocity field induced by the oblique interaction of a vortex ring with the surface have been reported by Weigand [138]. A similar approach was used by Logory et al [89] in their work investigating the free-surface wake behind a vertical flat plate. Both of these studies report illuminating laser light sheets within 1 mm of the free surface, the thickness of the sheets also being approximately 1 mm.…”

Large eddy simulation of temporally developing juncture flows

“…In the outer region, the flow is up towards the free surface and out away from the wall which thickens the boundary layer near the free-surface. Apart from these studies, there are two other experimental studies, namely [3,28]. While the measurements of Logory et al [28] are taken in wake of surface-piercing flat-plate, the measurements of Longo et al [3] are for both boundary layer and wake.…”