Author(s): Andrew Mccoy; George Constantinescu; Larry Weber
Keywords: Coherent structures; Detached shear layers; Large eddy simulation; Massively separated flows; Mass exchange; Contaminant transport
Abstract: The flow around two spanwise vertical obstructions in an open channel mounted on one of the lateral channel walls is investigated using Large Eddy Simulation. The upstream flow is fully turbulent and the obstructed area represents 17% of the total channel section. The focus of the present paper is to examine the mass transfer between the pollutant situated initially in the groyne area (embayment) and the main channel. The mass transfer is simulated using a passive (conserved) scalar transport equation. The evolution of the concentration field in time illustrates the dispersion process of the pollutant cloud as it is convected downstream of the groyne area. The vortical structures that populate the detached shear layer and their interaction with the recirculation eddies inside the embayment area and with eddies originating at the tip of the downstream obstruction that entrain fluid from the region downstream of the second groyne inside the embayment are shown to play an important role in the pollutant entrainment from the groyne area into the main channel. The LES simulation also allowed a detailed quantification of the non-uniformity of the exchange process over the depth of the embayment. The analysis of the variation of the pollutant fluxes in the top, middle and bottom layers inside the embayment region allowed to better understand how the pollutant exits the groyne area. The decay of pollutant within the embayment was quantified enabling calculation of a global 1D exchange coefficient, k, based on dead-zone theory. It was found that the exchange process is not characterized by a unique value of the exchange coefficient. Rather two distinct phases of the decay process were identified. In the initial phase of decay over which about 68% of the total mass of pollutant leaves the embayment, the exchange coefficient was found to be about twice the value recorded for the final phase.