Sign-up for free online course on ANSYS simulations!Step 5: Mesh geometry
Bring up the MeshTool:
Main Menu > Preprocessor > Meshing > MeshTool
We'll first specify the element type, real constant set and material property set to be used in the meshingmesh the two front surfaces using MESH200. Click Set next to Global under Element Attributes. Make sure that Set the TYPE is to MESH200 and click OK.
According to the ANSYS manual, "Smart element sizing (SmartSizing) is a meshing feature that creates initial element sizes for free meshing operations. SmartSizing gives the mesher a better chance of creating reasonably shaped elements during automatic mesh generation ... The SmartSizing algorithm first computes estimated element edge lengths for all lines in the areas or volumes being meshed. The edge lengths on these lines are then refined for curvature and proximity of features in the geometry." To turn on SmartSizing, check the box next to Smart Size. Drag the slider to a size of 4 to get a finer mesh than the default.
In order to have a little more control over what mesh ANSYS creates for us, we will set the starting element size for SmartSizing rather than use the default. Smartsizing will take this starting element size and modify/vary it over the geometry to account for curvature and corners. Under Size Controls, click the Set button next to Global. Enter an element edge length of 0.12 and click OK. The specified smart size of 4 and edge length of 0.12 are the result of an iterative process. You should experiment with different settings for these parameters to study the effect of the mesh on your solution, as discussed in Step 9. The goal is to obtain a solution that doesn't change as you refine the mesh.
Select Areas to be meshed with a Quad shape using the Free mesher. Click Mesh. Pick the front face of the crank and the pedal shaft.
Click OK. You will now see:
You'll get the following warning:
The specified smart size 4 and edge length of 0.12 are the result of a iterative process. ANSYS uses a sizing algorithm that creates smaller mesh elements in areas of potentially high stress concentration (e. Elements that exceed shape warning limits can lead to degraded accuracy. Here it is a minor concern since only 1 element out of 682 is causing the warning. So it is reasonable to press on. In general, it is always a good idea to pay close attention to the warnings and understand their effect on your solution. As a veteran in these things, I can attest that ignoring warnings can come back to bite you in incovenient parts of the anatomy. Close the warning window.
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In the above, we chose the front faces of the crank arm and pedal shaft as the surface meshes for sweeping. However, we have found that for other crank geometries, when meshing using the MESH200 elements, it is a good idea to choose the two back faces of the crank arm and pedal shaft that are flush with each other (i.e. the negative-z faces). This ensures that the nodes around the circumference of the circle on the two parts will match up and may prevent problems in sweeping the volume elements. |
Bring up the MeshTool again. Click Set next to Global under Element Attributes. Set the TYPE to SOLID45 and click OKg. curved regions) to improve solution accuracy. Selecting a smaller (finer) smart size will decrease element size close to the cutout curves. Specifying an edge length provides a uniform size for the rest of the area being meshed. In this case, since the material above and below the cutout act as beams, we want at least two mesh elements in the vertical direction. Feel free to experiment with the mesh tool parameters.Bring up the MeshTool again, and now Set the mesh type to SOLID45. Under Global Size Controls, click Set. We want four layers of mesh elements to span the thickness of the volumecrank, so we will enter a SIZE Element edge length of the desired element edge length in the sweep direction is (0.5 /4) = 0.125 . Click OK. Since we had created a MESH200 on the face of the crank, we can now simply sweep that mesh across the volume. Choose Volumes with a Shape Hex/Wedge to be meshedin. Under Size Controls, click the Set button next to Global. Enter an element edge length of 0.125 and click OK. We will now sweep, i.e. extrude, the surface meshes created above across the corresponding volumes. Select Volumes to be meshed with a Hex shape along with the Sweep option as shown below. Make sure Auto Src/Trg is selected; this will automatically pick a source (Src) surface mesh and sweep/extrude it to a target (Trg) surface.
Click Sweep and Pick All to sweep-mesh both volumes. and click Sweep. Now all we have to do is pick the crank volume, and ANSYS will extend our previous mesh surface meshes across the volumecorresponding volumes.
You can always see the rest of your model by selecting Utility Menu > Plot > Volumes
Now repeat the whole procedure with the crank shaft (starting from the top of step 5). Remember that the shaft element size must be the same as the crank so that the shaft elements and crank elements in the hole line up. (You can ignore any errors that may pop up regarding element size. ANSYS is complaining because the shape of some of the quadrilateral elements in high stress areas have a non-ideal element shape) Your final meshed model should look like the following. We're almost ready to solve the problem.
ANSYS issues a warning that 5 out of 3986 elements violate shape warning limits. Since the number of "bad" elements is small, this is a minor concern and we'll press on. But keep in mind that what we'll obtain is a reasonable first-cut solution but it will not be the final word. For that, you'll have to show that the solution is independent of the mesh. Close the warning window and the Meshtool.
Save Your Work
Toolbar > SAVE_DB
Go to Step 6: Specify boundary conditions