Installing groynes in a river has important implications for controlling thescouring process along river banks, maintaining channels for navigation, restoringfish habitat and on the mass transport processes. In many rivers flooding occursseveral times a year causing most groynes to become submerged. The momentumconvected into the embayment between successive groynes by the flowovertopping the groyne crest can significantly change the whole circulationpattern inside and around the embayments. In the submerged case the embaymentcan exchange momentum and mass both horizontally via the lateral channelembaymentinterface and vertically via the roof interface. Thus, studying theeffect of submergence on the momentum and mass transfer processes at groynefields is important. Large Eddy Simulation is used to investigate the dynamics ofthe main coherent structures present in the flow around two vertical submergedgroynes situated in a long flat-bed open channel. The Reynolds number definedwith the height of the groyne D and the bulk channel velocity, U, is Re=13,600.The total channel depth is h=1.4D corresponding to flood conditions and theincoming channel flow is fully turbulent. The statistically averaged threedimensional (3D) vortical structures located in the upstream recirculation zone,embayment, detached shear layers, and downstream of the second groyne arevisualized and the flow dynamics in these regions is described. The differencesbetween the instantaneous coherent structures in the turbulent flow and thevortical structures present in the mean flow field are also highlighted andexplained. The role of these large scale structures in the mass exchange betweenthe embayment region and the main channel is investigated.
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