ABSTRACT
The flow of a river past a shallow embayment (similar to that past a pair of groynes) was physically modeled in a laboratory channel that was 2 m long and 0.4 m wide with a 0.24 by 0.24 m2 embayment at mid-channel. Measurements were obtained using three-dimensional particle tracking velocimetry. Three cameras were mounted above the channel to track neutrally buoyant particles seeded into the flow. Based on these measurements, the entrainment mechanisms between the embayment and the main flow were studied. Analysis has shown that particles tend to penetrate the embayment at the downstream end of the interface, closer to the bottom, and leave the embayment through the upstream end of the interface, closer to the surface. Two theories are proposed to explain this behaviour. One is the secondary circulation in the embayment, that occurs as a result of cyclostrophic imbalance near the bottom solid boundary. A pressure gradient, balanced by the centrifugal force in the embayment vortex, dominates near the bottom, where the flow is decelerated by the bottom friction. As a result, particles at the bottom boundary start to move radially towards the centre of the embayment, where, by continuity, they subsequently start to rise slowly, in a spiral-like motion, with increasing radius of rotation. Once the particles reach the surface, they continue spiraling outwards towards the edges of the embayment, where re-entrainment into the main flow can occur. Along the edges of the embayment the particles are then entrained into a downward flow back to the bottom boundary, closing the flow path. This circulation can be partially responsible for the previous observations of particles entering an embayment along the bottom boundary and exiting it near the surface. The second mechanism affecting the entrainment mechanism between the embayment and the main flow pertains to the importance of the Kelvin-Helmholtz vortices in the shear layer between the slow-moving embayment flow and relatively fast channel flow. The bottom shear affects the KelvinHelmholtz structures, distorting the vortices that initially have a vertical axis of rotation. Deceleration of the flow near the bottom can tilt the axis of these vortices forward, resulting in the amplification of the effect, where the flow is principally oriented into the embayment along the bottom boundary and out of the embayment near the surface.
