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AUTOMESH-2D USER MANUAL |
2. Mesh Generation After the geometry of the object is defined. Press on the button Mesh Object and Mesh Generator dialog will appear. Now you can mesh the object. In order to achieve better mesh, the concept of the mesh density is introduced first. The mesh density can be defined as the reciprocal of the length of the element side. So the smaller mesh element has higher mesh density and on the contrary the larger mesh element has lower mesh density. Since the object will be represented by the mesh, the smaller elements should be placed in the area where the boundary has high curvature and the larger elements can be used in the area where the boundary is relatively flat. So the mesh density should be specified on the boundary. In order to get more accurate analysis result, more mesh elements should be placed in the area where the field variables (such as temperature, strain, strain rate etc.) have high gradient. So the mesh density also need be specified inside. In AUTOMESH-2D the boundary and internal mesh density can be set and generated automatically by the system. The user can set the mesh density manually. To facilitate the input and generation of mesh density, the density values in the procedure are relative, rather than absolute value. In AUTOMESH-2D, there are two algorithms to generate quarilateral mesh. One is LOOPING algorithm, and the other is PAVING algorithm. Triangular mesh can be generated using Bowyer/Watson algorithm. In AUTOMESH-2D, there are two ways to generate the mesh density: (1) automatic generation by system; (2) user-defined. The mesh and density generation will be introduced in detail as following: |
2.1 Automatic density generation and mesh generation 2.1.1 Initial mesh generation. This situation does not require the old mesh information, solely on the basis of the object geometry and density parameter to generate mesh density, and the mesh. |
The parameters that need to be set are as following: (1) The number of elements to be meshed. Enter the number of elements that you wish the procedure to generate. This number is only approximate. The procedure uses this number to estimate how many elements will be generated. The actual number of the elements may be different, but will be very close to this value. This parameter can be set 0, then the mesh and density will be generated solely on the basis of THICKNESS ELEMENTS. (2) The ratio of maximum and minimum density. The procedure uses this parameter to adjust the density generated on the basis of (a) the boundary curvature and (b) field variable gradient. The ratio of the maximum and minimum density will equal to this value after density adjustment. If this parameter is set 1, the density will be uniform for the above two cases. (3) Number of elements in thickness direction. The procedure uses this parameter to generate a certain number of elements in the area of thin structure. For example, in Fig.2, the mesh and density are generated solely on the basis of this parameter (number = 4). |
Fig. 2 Density and mesh generation on the basis of thickness |
In general, only the above three parameters need to be set. Then click on the button Generate Mesh in the dialog, and the density and mesh will be generated automatically.The generated mesh will also be shown in the graphic window. The user can check the mesh by clicking on the buttons the Toolbar. |
(4) Density window. The procedure does not use the density window by default. If you want to define density in the domain by density window, follow the instruction. Click on the Control Parameters button in the dialog and the dialog Mesh Control Parameters appears. Set the weight value of density window in the dialog. There are totally 5 types of density weight, and the sum of them is 1. They are the weight of the density generated on the basis of (a) boundary curvature; (b) density window; (c) strain distribution; (d) strain rate distribution and (e) temperature distribution. The percentage of the density generated according to the boundary curvature in the final density is determined by its weight. Also the percentage of the density generated according to the density window is determined by its weight. The last three types of weight are used in the remeshing process to determine the percentage of the density generated according to the field variables gradient. If the sum of the weights does not equal to 1, they will be scaled proportionally to make the sum be 1. |
To add a density window, just click on the button Add. To modify the relative density of the window, enter the new value in the edit box. The next step is to define the window polygon. Since the position of the window does not need to be very accurate, just click in the graphic area with the mouse to define the polygon vertex. To modify the position of the vertex, press the mouse button on the vertex and drag it to the proper location. To delete a density window, select the window from the list and click on the button Delete. It should be noted that since the density in the window is a relative value, at least two density windows should be defined and the windows should have intersection with the boundary. When all the windows don't intersect with the boundary, you can define one or more density window in the domain and the relative density value on the boundary will be set 1 by the system. In Fig. 3, three density windows are defined and the relative density value of the each window is 1, 3 and 1 respectively (the weight of the density window is 1 and the other weights are 0). In Fig. 4, two density windows are defined and they don't intersect with the boundary. The relative density value is 2 and 3 repectively (the weight of the density window is 1 and the other weights are 0). |
Fig. 3 Density and mesh generated according to density window only. |
Fig. 4 Density windows don't intersect with the boundary. |