Numerical investigations using LES: exploring flow physics and mass exchange processes near groynes

This dissertation presents a detailed numerical study using fully three-dimensional Large Eddy Simulation (LES) simulations of the flow and mass exchange processes in straight channels containing one or multiple embayments with vertical spanwise walls on one side corresponding to the presence of groynes in river reaches. The main focus is on identifying, understanding and quantifying the role played by the coherent structures and large-scale motions in the momentum and mass exchange processes within the embayments and between the embayments and the main channel. Of the three configurations that are studied, the first two contain only one embayment, and the water depth in the embayment area is equal to the one in the main channel. The two groynes are fully emerged in the first configuration and fully submerged in the second. This allows the study of the intensification of the flow three-dimensionality and acceleration of the mass exchange processes between the embayment and the channel due to the top interface and associated detached shear layer that are present in the submerged case. In both cases, the mass exchange across the embayment-channel interface is highly non-uniform across the depth. The third configuration corresponds to a straight channel with multiple groynes and is identical to one of the cases studied experimentally in a previous scaled model study. The mean water depth in the embayment area is about half that in the main channel; the width and length of the embayments are large compared to the depth; and the width over length ratio is relatively large such that a one gyre circulation pattern is observed inside the embayments. The model is validated with the experimental data available at the free surface. Even for shallow emerged embayments, most of the contaminant leaves the embayment through the top layer of the channel-embayment interface. The present study shows that 3D LES simulations can clarify several important aspects of flow past groyne fields that are very difficult or expensive to quantify using experimental techniques.