Exchange Processes in a Channel with Two Vertical Emerged Obstructions

Abstract

The flow around two vertical obstructions situated in a long open channel with lateral walls is simulated using Large Eddy Simulation (LES). The emerged obstructions (groynes) are oriented perpendicular to the channel side walls. The incoming channel flow is fully turbulent. The focus of the present paper is to examine the mass transfer between a contaminant situated initially in the (embayment) region between the obstructions and the main channel. The mass transfer is simulated using a passive (conserved) scalar transport equation. The scalar is introduced instantaneously inside the embayment. The eddy structures that populate the detached shear layer starting at the tip of the upstream obstruction and their interaction with the coherent structures inside the embayment area are shown to play an important role in the contaminant entrainment from the embayment area into the main channel. It is also found that the flow convected into the embayment from the region downstream of the second obstruction can substantially accelerate the removal of the contaminant from the embayment region. A detailed analysis of the contaminant flux variation in the top, middle and bottom layers inside the embayment region is carried out to better understand how the contaminant exits the embayment area, the role played by vertical motions within the embayment, and the effects induced by the presence of the bottom surface which delays the contaminant purging from the bottom layer. It is found that one half of the contaminant mass situated initially in the bottom third of the embayment volume is not leaving the embayment through the corresponding embayment–channel interface. The opposite is observed for the top third layer where the mass of contaminant leaving through the top interface area is 50% higher than the corresponding mass of contaminant initially situated in the top layer. This shows that the mass exchange is highly non-uniform over the depth and there is an overall contaminant flux within the embayment toward the free surface. The decay of contaminant mass within the embayment is calculated enabling estimation of a global 1D exchange coefficient based on dead-zone theory. It is found that though dead-zone models can relatively accurately describe the contaminant mass decay in time within the embayment, the mass exchange is not characterized by a unique value of the exchange coefficient over the whole length of the ejection process for the present geometry and flow conditions. Rather, two distinct phases of the decay process are identified. In the initial phase of decay over which about 68% of the total mass of contaminant leaves the embayment, the exchange coefficient is found to be about twice the value estimated for the final phase.