Numerical Simulation Study Of DNS-DEM On The Initiation Of Sediment Particles On Rough Bed Surface

The transport of two-phase flow of deposited particles is widespread in nature,and the understanding of the threshold and mechanism of particle initiation is still not perfect.In this paper,the four-way coupling numerical method of turbulent direct numerical simulation(DNS)combined with particle discrete element model(DEM)is used to reproduce the starting and moving processes of deposited particles with different density ratios.Based on the fluid resultant force acting on the particles and the resultant impulse,a new calculation method of fluid starting threshold is proposed.The rationality of the proposed starting threshold is verified by numerical simulation results,the starting mechanism is analyzed,and the influence of boundary layer thickness on particle starting and motion process is discussed.The results show that:Under the condition of low Reynolds number(=180)simulated in this paper,the fluid starting modes of deposited particles with different density ratios are significantly different.The particles with low density ratio(～O(10~1))rise directly under the action of fluid,and the starting process is dominated by the Suffman force acting on the particles.The threshold in the statistical sense can be used to predict whether they can be started by the fluid(Shields number is greater than the critical value,Sh/Shcr=20.9>>1).When the density ratio increases to 370 and 1025,the particles will roll along the bed surface,and the starting process is dominated by drag force and Suffman force.In this case,the threshold based on statistical significance cannot accurately predict the particle starting(Sh/Shcr is 0.6 and 0.2,respectively),and the instantaneous fluid force caused by turbulent fluctuation becomes important in the particle starting process.Further increase the density ratio to 2650,Sh/Shcr=0.086<<1,the particles do not leave the bed surface,only a small swing.For rolling(rocking)starting particles,a threshold impulse expression is proposed based on the impulse concept to predict their starting.Specifically,the fluid force impulse during this period is calculated by integrating a time period when the instantaneous tangential fluid force is greater than the instantaneous tangential resistance.Then,the threshold impulse required for particle starting during this period is calculated by using the expression proposed by us,and the two are compared to determine whether the particles can be started by the fluid.Then,by testing the starting of particles with density ratios of 370 and 1025,it is shown that this method can accurately predict the initial motion of rolling(swing)starting particles.The quadrant analysis of the starting process of particles with a density ratio of 1025 shows that the impulse events mainly occur in the first and fourth quadrants,indicating that the local normal flow velocity pulsation plays a leading role in the starting process of particles.The correlation analysis shows that the streamwise components of force and velocity are more correlated under the condition of larger lag.At the same time,the drag force shows a strong correlation with the streamwise velocity pulsation,and also has a strong correlation with the lift force.The analysis of the flow field structure in the particle starting stage shows that the duration of the particle starting will have a conditional average large-scale fluid structure,which is characterized by a strong down sweep event.The down sweep of the high-speed fluid moves the particles out of the pocket by scouring the bed.Therefore,the down-sweep event in the turbulent near-wall region is the main cause of triggering particle initiation.The boundary layer thickness(high Reynolds number)significantly affects the particle starting process.Through the analysis of the starting process of particles with a density ratio of1025,it is found that under the large boundary layer thickness,the particles first sway and collide frequently with the bed surface.Compared with the lower boundary layer thickness,the Magnus force and streamwise drag force increase,and the instantaneous force of the starting threshold decreases,which makes the starting of particles easier under the large boundary layer thickness.However,by comparing the threshold impulses of particle starting under two boundary layer thicknesses,it is found that they are almost unchanged,which indicates that the standard considering the time of turbulent fluctuation can better reflect the mechanism of particle starting.Finally,we found that when the boundary layer thickness increases,the flow field structure at the particle starting position will also change,and the upward throwing event also plays an important role in the particle starting at this time.