Large Eddy Simulations of Flows Over a Submerged Weir

The weir or small dam is a low-head instream structure constructed in the waterway channel. The major role of the weir is to raise the upstream water level by blocking the flow. This decreases the velocity and facilitates sedimentation upstream of the weir. Albeit of its practical use, local scour downstream of the weir often threatens channel stability and safety of the weir itself. Therefore, prediction of flow disturbance by the weir and local scour downstream of the weir is critical.

Local scour downstream of the weir can be attributed to the force exerted on the bed by highly 3D flows generated by the weir. Specifically, such flows are characterized by the massive separation and unsteady large-scale coherent structures. Experiments are, in general, difficult to obtain accurate information on the mean flow and turbulence statistics specially in the vicinity of the bed. Numerical studies using the conventional RANS approach do not provide information on the coherent structures which may play a key role in the scour process. This study attempts to compute the flow over the weir in an open channel using the large eddy simulation.

This study used the WALE model for the LES computation, and the VOF method for tracking the free surface. The pimpleFOAM (piso+simple algorithm), available in OpenFOAM, was used to solve velocity-pressure coupling. Generalized geometric-algebraic multigrid (GAMG) was used to solve for the pressure at the cell centers.

The numerical model was applied to two different channels with a submerged weir located at the middle of the channel. One is over the scoured bed whose geometry is at the equilibrium phase, and the other is over the flat bed. Time-averaged velocity and turbulence statistics were compared with experimental observations for validating computations. Mean flows and turbulence statistics as well as coherent structures with and without the scour hole were compared. It was found that the scour depth is maximum at a location where the donwflow due to the recirculation is strong and the recirculation zone moves downstream, being weakened, as the scour proceeds.