Hydrodynamic Forces Of Sediment Particles Near Bed And Their Application In Discrete Particle Model

The coupled motion of water and sediment determines the evolution of rivers and coasts,as well as the topographic change around engineering structures.Hence,the research on the coupled motion is of great engineering significance.With the development of the parallel computing technology,using the Discrete Particle Model(DPM)to study the basic mechanisms of the water-sediment coupling motion becomes one of the important approaches for sediment research in recent years.In these DPMs,the calculation of hydrodynamic forces between water and sediment is crucial to the model accuracy.In order to get more reliable result of near bed sediment motion and flow field,theoretical derivation and numerical simulations are used to analyze the hydrodynamic forces acting on sediment particles near bed in both laminar and turbulent flow.Appropriate hydrodynamic force formulae are put forward and the applications to the Discrete-Particle-Model are examined.The specific research contents and main conclusions of this paper are as follows.(1)Based on the finite volume method,a point-particle CFD(Computational Fluid Dynamics)-DPM is established to simulate the coupled motion of water and sediment.The diffusion-based coarse graining method is introduced into the present model to map variables at discrete sediment points to Eulerian fluid elements.The lubrication force calculation is replaced by an adaptive treatment hidden in the collision calculation as an optimization.The simulation results of collision process between sediment particle and a wall in water with the optimized model are compared with the experiment data,this comparison indicates that the present DPM can reasonably describe the coupled motion of water and sediment.(2)The hydrodynamic forces between water and sediment in laminar open channel flow of small Reynolds number are derived analytically.The values of the forces and the convergence analysis are given by the collocation method,and the results are checked by comparing with Direct Numerical Simulation results.The influence of boundary on force and torque is analyzed.The fitted formulae of drag force and moment are presented.(3)The drag force of a particle at various Reynolds number in laminar open channel flow is calculated by particle-resolved direct numerical simulation.The drag force formula applicable to the whole Reynolds number ranges in laminar open channel flow are fitted.The combined effects of particle Reynolds number and boundary conditions on drag force could be reasonably described as the simple multiplication of their independent correction.The simulation results of sediment transport near bed by using the present fitted formula in the laminar open channel flow are compared with that by using two existing drag force formulae,the formula suggested in the present work performs better.(4)The movement of sediment particles near bed in turbulent open channel flow are simulated by the particle-resolved direct numerical simulation.According to the data mapping for force calculation in point-particle CFD-DPM,the results of the particle-resolved simulation are analyzed and processed.Based on the error analysis of the existing formulae in the force prediction,an improved formula for sediment particle near bed hindered by multi-particles in turbulent open channel flow is put forward.(5)The improved force formula is applied to simulate the sediment transport in turbulent open channel flow,and is compared with two commonly used formulae.The results of sediment motion and transport rate simulated by the improved formula show better consistency with that by the particle-resolved model than the other two.