Multiple solutions of the Navier-Stokes equations computing water flow in sand traps

Two cases are presented wherein the main flow pattern in sand traps changes considerably as a function of discretization scheme, grid resolution and turbulence model. Both cases involve channel flows directed into a desilting basin, where the main current changes from one part of the geometry to the other. The CFD computations are validated with field or laboratory measurements. The first case presented is one of the sand traps of Khimti hydro power plant in Nepal. According to the laboratory measurements, the recirculation zone for this case is close to the bed, with the main current following the water surface. This is reproduced by the numerical model when using a first-order upwind scheme. Using a second-order upwind scheme, the main current is close to the bed, and the recirculation is formed at the surface. The second case is one of the sand traps of Tonstad hydro power plant in Norway. CFD computations predict the main flow field to follow the right or the left sides or the centre of the expansion region, depending on the discretization scheme, grid resolution and turbulence model. Field measurements show that the main current follows the centre of the expansion zone.