The Third International Conference on Coasts, ports and Marine structures (ICOPMAS "98")

Study of the hydraulics of mounted structures on the rivers, natural 3-D obstacles such as big rocks used as fish habitat structures or large dunes and especially their wakes in shallow open channel flow can lead to a better understanding of the flow characteristics and proper design of these bodies. Wakes have been studied extensively in the Fluid Mechanics literature for different applications, considering mostly deeply submerged wakes, which are Reynolds control led. This is not the case in shallow streams or where the size of the obstacles is considerable compared with the depth of the flow. In such systems, the flow is modified greatly by the structure, with significant changes of the water surface under a Froude control type of problem. Structures of the flow around simplified obstacles like hemispheres with a range of depths of flow have been studied with a focus on their wakes in this study. The presence of horse-shoe and arch vortices and their extent are observed in the wake of bodies. The difference between vortex system around 3-D bodies and 2-D obstacles such as bridge piers are studied. The effects of the free surface on the wake of the body and the dynamics of the vortex system are also investigated. Four flow regimes are recognized based on the relative depth of the flow, i.e. the ratio of the depth of the water d to the height of the body h. The system of the vortices active in each regime is defined and studied. Significance of these regimes was confirmed by further measurements. Experimental measurements were conducted such as velocity field in longitudinal and spanwise directions. The wake of the body and its geometry under various flow conditions are studied. The effect of horse-shoe vortex and downwash of the flow from the top of the body are investigated. The wall-wake behind the body on the plane of symmetry and the transverse velocity profiles were analyzed based on plane-wake theory. This analysis was satisfactory for a limited distance in the longitudinal and spanwise directions.