Author(s): Pablo Ouro; Adrian Navas-Montilla; Mario J. Franca; Carmelo Juez
Keywords: Cavities; Large-eddy simulation; Shallow-water model; Turbulence; Free-surface
Abstract: The hydrodynamic processes developed at the interference of rectangular lateral cavities and the main channel in rivers or canals drive the mass and momentum exchange, which are responsible for the sediment deposition or nutrient transport. The geometric characteristics of the cavities, namely their relative longitudinal and transverse extensions to the river width, determine how the mixing between cavities and main channel occurs since these condition the flow phenomena developed at the shear layer. Longer cavities experience larger entrainment from the main stream flow; furthermore, large-scale coherent structures are developed over the mouth of the cavity. In square-shaped cavities, a single large-scale recirculating eddy develops within the cavities with Kelvin-Helmholtz vortices formed at the interface. We investigate the performance of two state-of-the-art numerical approaches to predict the hydrodynamics developed within lateral river cavities and governing turbulent structures. For three cavity configurations of increasing streamwise length, we compare results from large-eddy simulations (LES) and a two-dimensional depth-averaged shallow water model based on a high-order WENO scheme. The computed mean velocities and turbulence statistics are compared and validated with experimental measurements based on Particle Image Velocimetry (PIV). Despite the relatively shallow flow conditions, the developed flow is highly three-dimensional, thus we quantify the ability of the LES and high-order WENO models to represent the governing flow mechanisms responsible for the mass and momentum exchange. We investigate the characteristics of the shear layers, e.g. peak value of Reynolds shear stress and lateral growth, and developed coherent turbulent structures for the different geometries, e.g. frequency of Kelvin-Helmholtz vortices. Our results will provide a better understanding of the complex exchange mechanisms at the cavity interface, with an analysis of the trade-off between model complexity and hydrodynamic results.