Focus on 3D - focus3d.htm

Building Geometry for CAE Applications - Focus on 3D

Introduction

Up till now, we've discussed generation of 2D finite element meshes and 3D Surface only finite element meshes (see Building Geometry for CAE Applications - Focus on 2D) and have not mentioned generation of 3D Volume finite element meshes. The only ways presently available to you in FEWaves to create 3D volume finite element meshes is by using the Generate|Extrude 2D Mesh or Generate|Revolve 2D Mesh pull down menu or by importing a mesh from a mesh generation package (SDRC IDEAS only). Using the Generate|Extrude 2D Mesh pull down, the model is first generated in 2D (X,Y) space and then extended along the Z axis to form a three dimensional object. Using the Generate|Revolve 2D Mesh pull down, the model is rotated about the vertical (Y) axis in angular increments. Material regions are changed as the model is extended or rotated layer by layer. FEWaves has a completely automatic 3D volume mesh generation algorithm with an intuitive user interface. After trying a few examples, you should begin to appreciate the power and ease using this mesh generation method. Two simple examples will illustrate how we create 3D volume finite element meshes. Creating a rectangle into a block of two different material regions will exemplify the extrusion method and generating a cylinder by revolving about the vertical axis will exemplify the revolving method.

Extrusion Reserved file extension:

JNL - the extension of the journal file which saves the extrusion process.

Linear Extrusion

To extrude an object, you must first read in the 2D geometry file and either read in a 2D mesh file or create a 2D mesh using our automatic mesh generator. You must also read in the material file, the format depends on your analysis package. We have read in the following files, called demo.geo, demo.pts, and demo.edg:

The first figure is the GEO file showing various constant defined for the problem. The second is the PTS file with the location of the geometric vertices. The last file is the EDG file showing the geometry 2D topology. We have defined here a rectangle in 2D of width W = 2 and height H = 2. Once reading these in, we have created a 3 x 3 2D first order mesh. This results in the following screen grab:

Now open the Generate|Extrude 2D Mesh window. Notice how the Generate|Revolve 2D Mesh is not available. This is because the model contains X values < 0 and since the revolve axis is the Y axis, this is not physically possible. The 2D surfaces are numbered on the model. The first thing we want to do is extrude this rectangle along the Z axis a distance of 2 units with 8 layers. Thus type in a starting Z location of 0.0 an ending Z location of 2.0 and the number of layers to be 8. This will be our first region, so assign surface 1 to region 1. The window should look like:

Now click on BUILD LAYERS. This clears the ending Z, puts the last ending Z in the starting Z location and clears the number of layers. You are now ready to make the next region. Enter an ending Z of 6.0 and the number of layer of 16. Change surface 1 to region 2 signifying a new material region. Click BUILD LAYERS. Now add the last layer by putting in an ending location of 8.0 with the number of layers equal to 8. Assign surface 1 to region 1, the same as the one from 0 to 2 units. Now click BUILD LAYERS and the DO EXTRUDE. The window should appear like:

You have created a 3D model with 528 nodes and 288 3D elements. In this process a journal file, or a file that remembers how you extruded, was created. Let’s look at this file and save it for our records. Click on VIEW JOURNAL:

You can see we’ve created two regions, the first extending from Z = 0 to Z = 2 with 8 layers, the next from Z = 2 to Z = 6 with 16 layers, and the last from Z = 6 to Z = 8 with 8 layers. The first and last extrusion have the same region number which forces them to be the same material. We could have called the last extrusion region 3 enabling it to be assigned a different material. You make changes to this journal by clicking in the cell, making the changes, and clicking UPDATE JOURNAL. Click CLOSE SCREEN to exit this view window. Save the journal by clicking on SAVE JOURNAL and giving it a file name with extension JNL. To use this journal file again click OPEN JOURNAL. This should be done immediately upon entering the window. If you read in a journal, simply click on BUILD LAYERS and then DO EXTRUDE to create the 3D model and you would be back to this same point.

Now click on ACCEPT 3D MESH to exit the window and change the model space to 3D. Take note that you must save the new 3D geometry and 3D mesh. You should do this immediately and save these files under different names than the 2D model. Now your 3D model looks like:

Now lets look at the regions you’ve created by turning on View:Display Options:Color Region. Your screen should now look like:

To look at region 1 only, go into View:Hide/View Surfaces... and turn off region 2. This will turn the yellow region off and identify region 1. Turn off region 1 and only the center region will be visible.

Congratulations, you’ve generated a 3D Mesh! If you think you have a handle on this, try extruding the model island3a.geo provided with your installation package.

It’s easy to imagine generating a more complex shape from a collapsed 2D model. Regions can be removed by entering region 0 when extruding. A region 0 is bypassed during the extrusion process - i.e. no elements are created. This effectively creates a hole in the model. This idea of adding and removing regions combined with the Materials:Assign to Regions pull down can create many types of models. For anisotropic materials, use the Generate:Rotate Coordinates pull down to orient the model along the axis of the material constants. Here you input the angular rotation in degrees for each axis. The entire model will be rotated. For instance, entering 90.0 after choosing Generate:Rotate Coordinates->X Axis rotates the entire model (geometry and mesh) about the X - Axis to align with the Z - Axis.

Revolving About an Axis

Another way to create a 3D finite element mesh is to revolve a 2D mesh about the Y axis in angular increments. Lets start with a simple case of creating a 3D cylinder using FEWaves. We first start with a rectangular geometry in 2D.

The first screen grab is the geometry file for the 2D rectangle, the next two are the points and edge files, respectively. We have defined a cylinder section in 2D with a height of 6 and radius of 2. Lets mesh this with a a basic division of 6, so that we end up with a 2 x 6 mesh, as shown:

Now we are ready to bring up the Generate:Revolve 2D Mesh window. Notice the similarities between this window and the Generate:Extrude 2D Mesh window. If you have mastered linear extrusion, the angular extrusion should be a breeze! One requirement that must be followed in the angular extrusion is that you can only revolve in angular steps of <= 90 degrees. Hence, it will take four steps to complete this cylinder. Lets make the first quarter have 4 layers. Type in a ending angle of 90 and 4 layers and call this region 1. Click on Build Layers. Now type in 180 for the ending angle and 6 layers. Call this region 2. Click on BUILD LAYERS Now enter 270 and 4 layers with region 3 and 360 and 6 layers with region 4, clicking BUILD LAYERS at each region change. The final screen should look like this, where 240 3D elements have been created:

Now click on Accept 3D Mesh and view the final 3D mesh. Again, make sure you save both the 3D geometry and 3D mesh to new files! The final mesh should like this, where the View:Display Options:Color Region has been turned on:

Notice how all the elements of the 2D model have been simply rotated about the Y axis. You can use the Generate:Rotate Coordinates->X Axis pull down and type in 90.0 degrees to have the cylinder orientated along the Z-Axis, as shown below:

For a challenge, consider making the third volume a void during the rotation. The only difference would be that you would assign the region to 0 when doing the rotation from 180 to 270 degrees. This would result in the following 3/4 cylinder which has rotated about the X axis 90 degrees:

Hence, as these past few examples have shown, it is easy to generate 3D meshes this way using the GUI provided in FEWaves. It has been the experience of MSTR Technology, that these simple tools will enable most of your required 3D volume meshes to be generated. For the few cases where you need more complex meshes, we provide a limited Import capabilities.

Importing 3D Volume Meshes

Currently, it is only possible to import 3D volume meshes from SDRC Master Series files. These files must be exported from SDRC IDEAS using the Universal File format. We support Universal Dataset Numbers 2411 and 2412 for geometry information. We do not support attachment of boundary condition information for this release. If you have a need for more importing or importing a "home grown" data structure, call us for details. We may be able to provide you custom modules at standard contract rates.