Analysis of Mechanisms of Contaminant Removal from a Bottom River Cavity

The mechanisms responsible for contaminant removal from a bottom rectangular cavity in a channel are investigated using fully three-dimensional (3D) Large Eddy Simulation (LES) simulations. The aspect ratio (length/depth) of the 2D cavity is L/D=2 and the neutrally buoyant contaminant (passive scalar) is introduced instantaneously inside the cavity once the flow has developed. The flow upstream the cavity is fully turbulent. The large scale coherent structures in the shear layer region induce large scale pressure fluctuations at the trailing cavity edge and convection of patches of vorticity inside the cavity, parallel to the trailing edge. These patches modulate the intensity of the jet like flow that develops along the trailing edge and bottom cavity walls. It is shown that the eddies convected from upstream of the cavity can play an important role in accelerating the extraction of contaminant from inside the cavity. It is found that the contaminant ejection can be described using simple 1D dead-zone theory models in which a single-valued global mass exchange coefficient can be used to describe the contaminant mass decay inside the cavity over the whole extent of the ejection process.