A new instantaneous mobile bed thickness model is presented for sediment transport in skewed asymmetric oscillatory sheet flows. The proposed model includes a basic bed load part and a suspended load part related to the Shields parameter, and takes into account the effects of mass conservation, phase-lag, and asymmetric boundary layer development, which are important in skewed asymmetric flows but usually absent in classical models. The proposed model is validated by erosion depth and sheet flow layer thickness data in both steady and unsteady flows, and applied to a new instantaneous sediment transport rate formula. With higher accuracy than classical empirical models in steady flows, the new formula can also be used for instantaneous sediment transport rate prediction in skewed asymmetric oscillatory sheet flows.
Current research on pump-turbine units is focused on the unstable operation at off-design conditions, with the characteristic curves in generating mode being S-shaped. Unlike in the traditional water turbines, pump-turbine operation along the S-shaped curve can lead to difficulties during load rejection with unusual increases in the water pressure, which leads to machine vibrations. This paper describes both model tests and numerical simulations. A reduced scale model of a low specific speed pump-turbine was used for the performance tests, with comparisons to computational fluid dynamics(CFD) results. Predictions using the detached eddy simulation(DES) turbulence model, which is a combined Reynolds averaged Naviers-Stokes(RANS) and large eddy simulation(LES) model, are compared with the two-equation turbulence mode results. The external characteristics as well as the internal flow are for various guide vane openings to understand the unsteady flow along the so called S characteristics of a pump-turbine. Comparison of the experimental data with the CFD results for various conditions and times shows that DES model gives better agreement with experimental data than the two-equation turbulence model. For low flow conditions, the centrifugal forces and the large incident angle create large vortices between the guide vanes and the runner inlet in the runner passage, which is the main factor leading to the S-shaped characteristics. The turbulence model used here gives more accurate simulations of the internal flow characteristics of the pump-turbine and a more detailed force analysis which shows the mechanisms controlling of the S characteristics.
SUN HuiXIAO RuofuWANG FujunXIAO YexiangLIU Weichao
Lattice Boltzmannmodel(LBM)in conjunction with an accurate Large Eddy Simulation(LES)technology was proposed to simulate various vortical structures and their evolutions in open pump intakes.The strain rate tensor in the LES model is locally calculated by means of non-equilibrium moments based on Chapman-Enskog expansion,and bounce-back scheme was used for non-slip condition on solid walls and reflection scheme for free surface.The presentedmodel was applied to investigate free-surface and wall-attached vortices for different water levels and flow rate.The vortex position,shapes and vorticities were predicted successfully under three flowing cases(i.e.critical water level(CWL),lower water level,lower flow rate),and the numerical velocity and streamline distribution were analyzed systematically.For CWL based on Froude number considering open channel flows,the shape and the location of various dynamic vortices were captured.Compare to the experimental results of CWL,more vortices were predicted for lower water level,and less vortices were observed for lower flow rate.The predicted velocities and vortex locations are in good agreement with the experimental of a small physical model.The comparisons demonstrated the feasibility and stability of above-mentioned model and numerical method in predicting vortex flows inside open pump intakes.
