Abstract
Coherent structure of large scale eddies and turbulence within the upper surface shear layer of a wind-driven current has been investigated by utilizing several flow visualization techniques. In the region of transition from laminar to turbulent flow, the visualization demonstrates the existence of a streak structure which can be shown in a plan view at the level close to the water surface and reveals that longitudinal vortices existing near the surface are responsible for the appearance of the streak structure. As the stream goes down into the turbulent region, the flow field related to these vortices develops into that of three-dimensional ‘hairpin’ vortices. A tentative model is presented to describe the evolution of these vortical flow field from a viewpoint of local instability due to the longitudinal vortices themselves. The flow structure of turbulent boundary layer near the surface has been shown to resemble that of usual wall turbulence.
