Numerical Simulation Of Sediment Movement And Ripple Evolution

In the river,estuary and coastal areas,the problems of flow and sediment movement and morphological evolution are generally widespread,which is the result of the mutual interaction,influence and adaptation among the flow,sediment and bed surface.In recent years,with the rapid development of computer technology,the researchers gradually adopt the coupling method of the computational fluid dynamics and the granular discrete element to conduct a research of flow and sediment movement and geomorphic evolution from mesoscopic point of view.Based on CFD-DPM,a coupled numerical model is established to study the characteristics of bed load movement,ripple evolution process and suspended load movement under the turbulent flow.(1)Under flat sediment bed,it is to study the effect of bed load sediment on the flow and characteristics of sediment movement,thickness of bed load layer,velocity of jumping sediment particle and sediment transport rate.Compared with the fixed bed,the interaction of flow and sediment movement results in the increase of flow velocity close to surface bed and velocity profile deviates from the logarithmic distribution;flow velocity in the mainstream area decreases,but still conforms to logarithmic rate.The thickness of the sediment layer increases with the increase of the flow strength,and the velocity of the jumping sediment particles increases with the increase of the flow strength.(2)In the process of ripple evolution,sensitivity analysis about the influence of the scale of fluid computational grid,lift coefficient and elastic recovery and length size of computational domain on ripple evolution process is investigated.It shows that the scale of fluid computational grid,the lift coefficient and elastic recovery coefficient has a certain influence on ripple evolution process,but has no effect on the size of ripple at equilibrium state.The length size of computational domain has not only influenced on ripple evolution process,but also affected on the size of ripple at equilibrium state(3)In the process of ripple evolution,the velocity of sediment movement at different stage is analyzed.The evolution stage of ripple is divided into rapid evolution stage and slow evolution stage.The velocity of sediment movement is quick in the rapid evolution stage,and the velocity of sediment movement is slow in the slow evolution stage.(4)In the process of ripple evolution,on the whole,due to the increase of ripple length and height,the shape resistance of ripple increases,the sediment transport rate reduces.But in the process of ripple mergement,the shape resistance of ripple shows a decrease trend,and sediment transport rate has an increase trend due to decrease of ripple length and height.(5)In the process of ripple evolution,the time of ripple reached equilibrium state and the relation between the sediment transport and the speed of ripple migration under different flow intensity are studied.The time of ripple reached equilibrium state decreases with the increase of flow intensity.For the two-dimensional sand wave,the sediment transport rate can be calculated according to the migration velocity of sand wave.(6)Suspended load movement is investigated under the turbulent flow,it shows that there is a transition zone between the bed load and suspended load.The bed load and suspended load in the transition zone coexist,with the increase of flow intensity,the suspended sediment becomes the main part of the sediment movement.