VORTEX GENERATION IN WATER-WAVES PROPAGATING OVER A SUBMERGED OBSTACLE

Laboratory experiments have been conducted to investigate flow separation effects induced by time-periodic water waves travelling over a submerged rectangular obstacle. Water surface elevations and fluid velocities were obtained for a range of non-breaking wave conditions. From these measurements, the kinematics and dynamics of eddy motions were studied. The measured velocities were compared to the theoretical predictions of a linear inviscid model. It was found that the formation and growth of separation region respond directly to the wave transformation above the submerged obstacle, leading to a variety of different eddy geometries. The interaction of the separated flow with the wave field significantly modifies the transmission process but has little effect on the reflection process. The most important consequence of flow separation is energy dissipation at the expense of the transmitted waves. The observed data indicate that separation loss is the results of viscous shear in the formation of eddies and the subsequent dissipation of energy as the eddies decay. When the obstacle is slightly submerged the presence of separation modifies the effective boundary of the flow above the obstacle. The complex nature of this problem suggests that it would be very difficult to determine flow separation effects unambiguously without solving the viscous flow equations in the near field.