Comparison Of The Accurate Mesh Methods The Undulating Interface Based On True Grid And The Ground Motion Simulation Results

The hexahedral grid is suitable for the spectral element method,and has significant advantages over other volume grids in terms of calculation accuracy and storage space.When the regular hexahedral element is used to mesh the undulating interface terrain,the interface fluctuation is relatively large.The element cannot fit the interface and often choose to use a stepped grid to approximate the undulating interface,and this kind of stepped grid usually produces scattering during numerical simulation.In order to eliminate the influence of the stepped grid on the ground motion simulation calculation of the undulating interface,this paper aims to find a hexahedral element grid mesh method suitable for the spectral element method,so that the grid is as uniform as possible and the mesh is as close as possible the undulating interface.In this paper,an application program is written in the True Grid language,and three methods for meshing hexahedral elements at the undulating interface are proposed.Through this program,a uniform hexahedral mesh model at the undulating interface can be quickly established and the mesh model can be twisted and orthogonal.Equal quality evaluation,and verify the correctness of its model through the spectral element method.The main work of the thesis is as follows:1.In this paper,the study area with undulating interface is meshed with hexahedral elements.The two-in-one method of hexahedral elements is used in the previous stepped grid to transition and connect different numbers of elements in the depth direction(z-axis).The model eliminates the stepped grid,turns the stepped grid into an inclined grid,and projects the inclined plane to the undulating interface so that the grid fits the undulating interface completely.2.Since the three-dimensional model has stepped grids in the two depth directions of x-z and y-z,the four corners where the two directions meet,the two-in-one processing of the two elements is superimposed,so that the elements at the corners are distorted.Therefore,this paper adopts three meshing methods to compare and solve.Method one deletes four rows of units at the corners,and uses 16 units to merge into 12 units;method two deletes one column of twisted units at the corners,and uses 4 units to merge into 3 units method.The third method is to construct as a separate transition ring.3.The quality of the overall grid generated by the three meshing methods is evaluated.The main indicators are element distortion and orthogonality.The evaluation results show that the quality of the grid is good,and it is basically an orthogonal hexahedral element.4.Numerical simulation calculations of the three models through the spectral element method verify the correctness of the three meshing methods.When the calculation point is located at the top-down corner unit,the comparison result shows that the mesing method will cause waveform scattering.The second and third meshing methods are both regular waveforms;when the calculation point is located in a horizontal surface unit with different distances from the corner,and the distance from the corner is greater than 900 m,the simulated results of the three subdivision methods are relatively consistent;at 300-900 m When it is smaller than 300 m,there is a big difference between the first and second and third methods,and the second and third methods always have a small difference.The conclusion is that both the method of deleting one column of units at the corner and the method of setting a transition ring can be used,and the method of deleting four rows of corner units is not recommended.The three hexahedral element meshing schemes proposed in this paper are suitable for the spectral element method to make the elements uniform and the mesh completely fit the undulating interface,which improves the previous problem of using stepped grids to approximate the undulating interface,and eliminates the scattering and diffraction problem of the stepped grid in the numerical simulation.The element size is as uniform as possible,avoiding the difference in the grid size and causing the divergence in the calculation;the grid fits the undulating interface as much as possible,which improves the accuracy of the numerical simulation.It helps to improve the flexibility of the spectral element method to deal with undulating interface problems.