Discrete Element Simulations Of Intruder-induced Granular Flow

Different with simple granular flow under gravity, intruder-induced granular flow is so complicated that it has been one of the important issues in the granular world. Intruder-induced granular flow has many applications in some fields such as geotechnical mechanics, agricultural engineering, bionics and military. In order to fully analyze the force applied on the intruder, four different types of intruders which include thin vertical wall, inclined wall, thick wall and saw-tooth-shape object were used in this paper. Both indoor physical experiments and Discrete Element Method were utilized to solve this problem. The main objective in this paper is to investigate the drag and lift force of different shapes of intruders. The main focus is to obtain the relation of displacement and drag and lift force. This will provide theoretical guidance for the practical application.The main conclusions can be drawn as follows:(1) In the physical experiment, the intruder with two wings, which can prevent the particles from flowing around the intruder, was designed. This intruder in the experiment can be regarded as the 2D case because almost all the particles flow paralleling with the intruder. Using a new developed increased-gravity method, the DEM granular bed with free surface was created. The advantage of this method is that it can make the whole system reach equilibrium rapidly, and prevent the top particles from rebounding;(2) Based on the concept of relative density, the link of porosity in 2D and 3D was established. This is of great help for 2D simulation and 3D physical particles. The coefficient of restitution was obtained by free fall test of glass beads, based on this the relation of damping ratio and coefficient of restitution has been found. And this relation is independent of initial positions of the glass beads. This damping ratio was used as the contact model in particle level;(3) As for the vertical thin wall, the boundary effect was found in all the simulations except the case of periodic boundary condition. This effect occurs when the intruder is moving near the boundary, because the piling up height in front of the intruder grows dramatically when near the boundary; the simulation results showed that drag force is independent of the microspic parameters such as particle stiffness, particle size and inter-particle friction; the drag force is almost independent of the distance between the intruder and the bottom boundary. However, it has slight effect when the distance is small enough. The drag force is increasing linearly with the immersion depth and nonlinearly with the velocity;(4) As for the inclined thin intruder and saw-tooth intruder, the drag force is linearly increasing with the incline angle and the drag velocity. Three impact parameters were used to describe this linear relation. Specifically, the velocity and depth parameters are quadratic with the inclined angle, and the system parameter is linear with the inclined angle; The ratio of drag and lift force of the inclined intruder is equal to the tangent value of the incline angle. This founding establishes the link of drag and lift force. Therefore, the lift force can be obtained from the drag force;(5) The edge effect was found in the bottom corner of the thick intruder. The DEM results showed that the drag force grew dramatically in the bottom corner. This phenomenon was explained qualitatively by using a collision frequency theory;(6) For different intruders, based on the movement of particles and displacement field, the predictive formula for the piling-up height was established. the wedge model based on the modified Coulomb earth pressure theory for predicting the drag force was developed, and the other model based on the modified Rankin Theory is also established. These two models were verified by the indoor physical experiments. The results showed that these two models are very well for forcasting the drag force. For the modified Rankin theory, the dynamic pressure coefficient is recommended, and for the modified Coulomb theory, the dynamic frictional angle is recommended.(7) By analyzing the displacement field, stress distribution, shear rate distribution and generalized inertial number, it also verified that the predictive model is reasonable. in addition, trying to use the finite element method, the Druck Prager model was utilized. From the displacement field and plastic strain from the FEM simulation, it showed that it was consistent with the DEM model.