Large Eddy Simulation of a Submerged Vortex in a Simplified Computational Model

Abstract

The flow structures of a submerged vortex that appears in a model pump sump were numerically investigated by performing large eddy simulation (LES) of a model vortex in a simplified computational model with a sufficiently fine grid that could resolve the vortex core. The simplified model is designed to simulate the flow under the bellmouth in a model pump sump. The model pump sump is composed of a 2,500 mm-long water channel of rectangular cross section with a width of 300 mm, a water height of 150 mm and a vertical suction pipe with a diameter of 100 mm installed at its downstream end. Our previous large eddy simulations, which used approximately 2 billion grids and were applied to the model pump sump, have fully clarified the origin and formation mechanism of a submerged vortex. In these computations, however, the static pressure in the vortex core decreased only by as much as 4 kPa at a channel velocity of 0.37 m/s. The decrease in the static pressure was far smaller than the one for which one can expect initiation of cavitation in the vortex core. The static pressure drop was most likely to be underpredicted in our previous LES. Insufficient grid resolution was assumed to be one of the reasons for this underprediction. In the present study, LES with a sufficiently fine grid was applied to the simplified computational model that represents the stretch of a submerged vortex under a constant acceleration of the vertical velocity. Case studies for which the grid resolution was varied between 3.25 and 150 micrometres were performed while the size of the vortex core appeared in the simplified model was 500 micrometres. As a result, we confirmed the grid resolution finer than 15 micrometres is needed to resolve the vortex core with a diameter of 500 micrometres. Vertical and tangential velocities obtained by averaging those distributions of a submerged vortex that was computed in our previous LES were prescribed at the bottom wall of the computational domain as the inlet boundary conditions. In the present LES with the grid resolution finer than 15 micrometres, the static pressure decreased by more than 100 kPa. In addition, the parametric studies where the initial swirl numbers were changed have fully clarified the change in the dynamics of a submerged vortex. We found that a strong submerged vortex appears only at a relatively small range of the swirl-number from 1 to 3.