The Research On The Simulation Evaluation Method For Off-road Vehicle Driving Behavior On Sand Based On The DEM/FEM

Off-road vehicles(OTV) usually work on granular sand terrain. The trafficability, mobility and ride comfort are the main criteria to evaluate the cross-country performance of such vehicles. Wheels are the working parts which in direct contact with the sand, and the wheel-sand interactions which usually lead to wheel slip, wheel sinkage and sand flow, have large impact on the driving behavior of the vehicles. Thus, the in-depth study of this field is significant in accelerating the development of high performance OTV for our country.Numerical method is a promising tool to analyze the wheel-sand interactions from the microscopic view. However, the simulation of this topic is very complicated because of the continuous character of the wheel and the granular feature of the sand. Common numerical methods, including finite element method(FEM) and discrete element method(DEM), are unable to simulate the continuum media and granular media simultaneously. This dissertation starts with the discrete element(DE) road model with regular element array, and then develops the generation method of the DE sandy road, followed with the improved contact algorithm between the DE and finite element(FE) and finally the alternative moving road method. The combined finite element and discrete element methods(DEM/FEM) in the context of wheel-sand interactions are studied systematically. And corresponding software named ORV-SAND(Dadicated Simulation Tool for Off-road Vehicle Driving on Sand) is developed.Firstly, considering the particularity of the wheel-sand interaction problem and the internal property of the FEM and the DEM, the dissertation extends the function of the DEM based on the in house soft ware named CDFP(An Explicit Combined Discrete-Finite Element Program) which is applied to simulate the impact fracture problem of the brittle material. And the extended program applied the DEM, the FEM and the DEM/FEM to the simulation of wheel driving behavior on sand. 3D numerical models of FE wheel and DE sand with regular element array are established, and driving behaviors of the wheel are simulated. The simulation results preliminarily prove the feasibility of the DEM/FEM in the research of this field.Secondly, considering the arrangement of the elements has large impact on the macro-mechanical properties of the DE sand. To further analyze the sand property accurately, the efficient hierarchical generation method is proposed. On this basis, the spatial grid search method(C-Grid) is applied to the contact detection among the DEs, and the Hertz-Mindlin contact theory is applied to calculate the contact forces of the elements, the gravitational field is added and the initial DEs are rearranged to a steady state under self-weight to simulate the real sand. The process is used for the pre-processing for the DE sand model.Thirdly, an improved contact detection method is introduced, it can avoid the the misconvergence problems during the contact detection between complicated wheel structure and sand when using the DEM/FEM to simulate the wheel driving behavior on sand. The contact point between the DE and FE surface is determined by the area coordinate method. The accuracy, efficiency and validity of the algorithm are proved by corresponding simulation models respectively. Considering that only the sand around the wheel are possible to be in contact with the wheel and the wheel is moving during the calculation process, the adaptive slave region method for the contact detection between the DE and FE is proposed. It can reduce the unnecessary contact detection and thus improve the calculation efficiency. Then, 3D numerical models of smooth wheel, chevron groove wheel and sand terrain are established, and the wheel running behaviors are simulated. The comparison of the simulation results between the smooth and chevron wheel and the comparison between the simulation results and the experimental results of smooth wheel, together validate the validity of the simulation method.Finally, considering that vehicles usually need to travel on longish roads in actual performance test which undoubtedly leads to large scale of simulation model. To solve this problem, the Alternately Moving Road Method(AMRM) is proposed. The basic principle of the AMRM is that the sand which has been rolling over by the wheel has less impact on the wheel running behavior, thus such sand can be deleted and new sand is laid in front of the wheel traveling direction. The length of the road keeps constant and the number of the DEs is acceptable during the simulation process, and thus saving computing resources. Moreover, the AMRM also possesses the ability of simulating wheel driving behavior on different types of roads. One identical sand road sample is established, and the running behavior of a smooth tire on longish sand road is simulated. Corresponding simulation results such as the net drawbar pull, the road reaction force and the wheel sinkage are obtained. The results show that the alternation process does not have unacceptable impact on the wheel running parameters for the alternation of identical road sample. In other words, road samples can be smoothly connected during the alternation process. Furthermore, the running behavior of alternating pattern wheel on multi-road is simulated by established fine and coarse sand road samples.According to the above, a numerical tool which is applied to the simulation of wheel-sand interactions is studied systematically based on the DEM/FEM, this laid good foundation for the further in-depth study of pneumatic tire, OTV running behavior on sand.