Effects of Waves on the Boundary Layer of a Surface-Piercing Flat Plate: Experiment and Theory

Abstract

Results are presented from a towing-tank experiment conducted in order to document the effects of waves on the boundary layer of a surface-piercing body. A unique, simple model geometry is utilized which makes it possible to identify and isolate the most important features of the wave-induced effects. Measurements were made of wave profiles as well as detailed boundary-layer velocity profiles for three wave-steepness conditions: zero, medium, and large. The effects of the waves for both the medium-and large-steepness conditions are shown to be significant. In particular, the variations of the external-flow piezometric-pressure gradients cause acceleration and deceleration phases of the streamwise velocity component and alternating direction of the crossflow, which result in large oscillations of the displacement thickness and wall-shear stress as compared to the zero-steepness condition. The magnitude of these trends increases with increasing wave steepness. Wave-induced separation, which is present under certain conditions in the experiments, and other relevant phenomena are discussed. The measurements are compared and close agreement is demonstrated with results from first-order boundary-layer calculations with a symmetry-condition approximation for the free-surface boundary conditions.