Hexa Meshing. Defining Surface Parameters for the Mesh Defining Edge Parameters to Adjust the Mesh Checking mesh quality for determinants and angle

Transcription

1 4.2.6: Pipe Blade Overview This tutorial example uses the Collapse function to create a degenerate topology in a Conjugate Heat transfer problem around a blade located in the center of a cylindrical pipe. a) Summary of Steps The Blocking Strategy Starting the Project Creating Parts in the Mesh Editor Starting Blocking Using Prescribed Points to Fit the Blocking Splitting the Topology Using Prescribed Points Collapsing Blocks to Represent the Blade Material Edge to Curve Association on the Blade Moving the Vertices Generating the O-grid Defining Surface Parameters for the Mesh Defining Edge Parameters to Adjust the Mesh Checking mesh quality for determinants and angle 189

2 Saving before Quitting b) The Blocking Strategy In this lesson, the blade is regarded as a Solid region, while the region surrounding the blade is regarded as the Fluid region. Using Block Splitting at Prescribed point, the user will generate a Hexahedral Mesh for both of the regions, so that the topology of the solid region is a degenerate Hexahedral mesh. Before the user employs the Collapse function for his/her own applications, confirm that the solver accepts degenerated hexas (for a structured solver) or penta_6 elements (prism) for an unstructured solver. Note: Settings >Selection>Auto pick mode should be turned OFF for ANSYS ICEM CFD to behave exactly as this tutorial describes. c) Starting the Project The input files for this tutorial can be found in the Ansys installation directory, under../v110/docu/tutorials/cfd_tutorial_files>pipeblade. Copy and open the geometry.tin file in your working directory. d) Creating Parts in the Mesh Editor Right click in the Display Tree on Parts > Create Part to create different Parts and assign the different surface of the geometry to the appropriate part. Refer to the figure below for the Surface part assignments. 190

3 Figure The Pipe Blade configu ration e) Surface Parts After the Pipe Blade project is open, activate the Points and Surfaces from the Display Tree. Switch on Points > Show Points Names. Begin the Surface part reassignment by changing the region enclosed by GEOM/4 - GEOM/7 to the part OUTLET. The region that is denoted by GEOM/0 - GEOM/3 should be reassigned to the part INLET. The Surface defining the Cylinder pipe will be placed in the Surface part, CYL. The surfaces belonging to the solid blade in the middle of the cylinder should be classified as BLADE. When all of the Surface parts have been assigned (INLET, OUTLET, CYL, BLADE), press the middle mouse button to exit from continuous mode. f) Curve Parts and Point Parts For this tutorial, we will leave the curves and points assigned to the initial part GEOM. 191

4 g) Creating the Material Points Select G eometry > Create Body > Material Point Enter FLUID in the Create Body window that appears. The material point that will be created will help us to keep the FLUID region separate from the SOLID region, but is not necessary since blocks can simply be created in the FLUID part rather than creating a material point. With the left mouse button, select two locations on the opposite sides of the cylinder. Note that the FLUID material point should not be within the BLADE. If tetra meshing, this location would be important. With Hexa meshing, it is not. Press the middle mouse button to accept the selection, and press Apply and the Body name FLUID should appear within the geometry (midway between the selected locations). Rotate the model to confirm that FLUID is in an appropriate location. Now enter SOLID as the new Part Name in the Create Body window. P ress the location selection icon and select two locations on the blade surfaces so that the midpoint will be inside of the blade. Press the middle mouse button to accept, and press Apply. After accepting this Parts assignment, dynamically rotate the model to confirm that SOLID is inside the blade. When this is complete, all components of the Geometry should now have part name assignments. Delete any Empty Parts: From the Display Tree, right mouse select on Parts > Delete empty Parts. File > Save Project As to save the updated model before continuing on in this tutorial. Give the project any name you want. h) Blocking Initialize blocking, which will create the first block, by going to Blocking > Create Block > Initialize Block. The Create Block window will open. 192

5 Figure Create block window Select the block Type as 3D Bounding Box (default) from the pull down arrow. Name the Part as Fluid. Press Apply without selecting anything, and the initial block will be created around the whole model. i) Association of vertex to point To fit the Initialized Blocking more closely to the geometry, the user will associate vertices to points. Select Blocking > Associate > Associate Vertex and the window shown will open. Toggle ON Blocking > Vertices and right mouse click on Vertices > Numbers under Blocking in Display Tree. 193

6 Figure Associate vertex window Select Point under Entity. Press the vertex selection icon and select Vertex 42. Press the point selection icon and select Point GEOM/7 and press Apply to associate them as shown below. Similarly, associate the other vertices and points for the inlet and outlet so that after completion the geometry should look like below. 194

7 Figure Moving the vertices Figure Geometry after associating all vertices to corresponding points 195

8 Note: When possible, the Block vertices on any circular geometry should be placed so that edges are equal in length and the angles between edges are 90 degrees. This amounts to vertices being placed at 45, 135, 225, and 315 degrees around the circle. This results in the best mesh quality. j) Associating edges to curves Select Associate >Associate Edge to Curve. Press the edge selection icon then select the four edges shown in the figure below and press the middle mouse button. Then press the curve selection icon and select the four curves shown in the figure below and press the middle mouse button. Notice that the block edges then transform from white to green, confirming their association with the curve. Also notice that the four curves become one color, indicating that they have been grouped into one curve. Figure Association window 196

9 Figure Edges and Curve selection for association Similarly, associate the four edges on the other circle to the corresponding four curves. To see a confirmation of these associations, right mouse click on Blocking > Edges > Show Association in the Display Tree. 197

10 Figure The Edge Projection Note: If the edges lie on the geometry, as is the case with longitudinal edges, the projection arrows are not shown. By default, all external edges are surfaceassociated to the nearest active surface and appear as white. The association can be set to this default using Associate > Associate Edge to Surface. This operation is useful to correct any Edge to Curve Association mistakes. All internal edges, by default, have no association, and appear as blue. You can set this association, which is really deleting an association, by pressing. k) Grouping curves Note: This section does not need to be performed on the model, but it shows the user how to manually group curves. Select Blocking > Associate > Group curves. 198

11 Figure Group curve window Select the four curves corresponding to OUTLET as shown in the figure and press Apply to group them. l) Splitting the Topology Using Prescribed Points and Screen Select The following steps instruct the user to split the block in the k and j directions around the blade, thus creating further blocking topology for the blade. The k-direction splits will be created through the prescribed point method, while the j-direction splits will be made by visual judgment. Press View > Top, then Fit Window Turn off Vertices at this stage. Choose Blocking > Split Block >Split Block and it will open the window as shown in the figure below. Choose All visible and Split method as Prescribed Point. Select the edge selection icon then select 199

12 one of the edge which is along z-direction. After selecting the edge it will prompt you to select the point Select the Prescribe point, GEOM/9 and press middle click to accept the selection. Similarly, make another split using the same edge but through the Prescribed Point, GEOM/8. Similarly, make another horizontal split through the prescribed point GEOM/12. The final result will have three horizontal splits as shown in below. Note: Make sure that the Edge that is selected lies within the range of the Prescribed Point that will be selected. Figure Split block window 200

13 Figure Make the horizontal splits in the block These are the splits in the k -direction. The next set of splits will be in the i direction. Now select the Split method as Screen select. Press the edge selection icon and select any of the horizontal edges (which is along x- 201

14 direction) to create a vertical split. If Settings>Selection>Auto pick mode is OFF, press Apply, and it will ask for a location on the scre en to split through. Select on a curve or edge on any location that is vertically in line with the right side of the blade. If Auto pick mode is ON, you should left mouse click on the edge and hold the button while dragging the split to where you want it. Press the middle mouse button to complete the split operation. Then use the same method to create another vertical split on the left side of the blade. 202

15 Figure Horizontal splits on blade sides 203

16 Note: Every time a block Split is performed, the Index control is updated. After the splits are complete, the new range of the K index will be from 0-6. m) Collapsing Blocks to Display the Blade In this section, the Collapse feature is introduced to create degenerate blocks for the blade. For clarity in these operations, right mouse click in the Display tree on Blocking>Index Control. Change the Index control for the I dimension so that the Min is 2 and the Max is 3. Turn OFF the Points from the Display window. The restricted topology consists of four blocks, where the two center blocks belong to the blade. Before collapsing the blocks, change the Part family of the two center blocks to SOLID, the material representing the blade. Right mouse click on SOLID>Add to part underneath Parts in the Display Tree, and it will open the Add to Part window. Select Blocking Material, Add Blocks to Part, and select the blocks of the blade as shown below, then press the middle mouse button to complete the operation. Figure Assigning the blade blocks 204

17 Now select Blocking > Merge Vertices > Collapse Block. Choose the edge that should be collapsed. In this case it is the shortest edge of the selected blocks. Select the two blocks shown in the figure below. Press Apply to Collapse the blocks. Figure Collapsing the blade Blocks After collapsing we get the model as shown below. 205

18 Figure The Collapsed Blocking n) Edge to Curve Association on the Blade Choose Blocking> Associate >Associate Edge to Curve.The Associate edge to curve window will open as shown below. Note: Make sure Project Vertices is disabled. 206

19 Figure Association Edge to Curve Window You should associate the Edges and corresponding blade curves as shown below.do this to the top and bottom of the blade, on both sides. After associating, Switch on Blocking > Edge > Show Association from the Display Tree. The geometry should look as shown. 207

20 Figure Blade edges to be association to curves 208

21 Figure Blade edges Associated to curves o) Moving the Vertices This section shows the user how to move all the associated vertices onto the geometry in one step. Snap the appropriate block vertices onto the geometry by selecting Associate > Snap Project Vertices.All Visible should be toggled ON. Then Press Apply. Switch off Edges > Show Association. All the vertices belonging to blade, inlet and outlet are moved to the locations as shown below. 209

22 Figure The final positions of the vertices before the O grid p) Vertex Color Distinction Notice from this lesson and from previous lessons, that the movement of the vertices is restricted to the associated Curve. The colors of the vertices indicate their associations and degrees of freedom. Vertices associated with Prescribed Points are red and are fixed at a point. Vertices associated to a curve are green and can be moved on the associated curve. By default, all the vertices lying on the block material boundary are white and are free to move on any surface. Additionally, internal surfaces are blue and can be moved along the blue block edges to which they are connected. q) Generating the O-Grid If the pre-mesh is generated at this point, the existing blocking would result in skewed cells on the four corners of the pipe. Converting the existing H-Grid type topology to an O-grid type topology inside the pipe will produce a mesh that is low in skewness, with orthogonal grid on the pipe walls. The following steps will improve the overall mesh quality. 210

23 Press Blocking >Split Block > O grid Block Press and select all the Blocks of both the FLUID and SOLID regions since the O-grid will be added in the entire pipe as shown in below. Press the middle mouse button to accept. Similarly, press and select the two INLET faces and two OUTLET faces as shown. Press the middle mouse button to accept, and Press Apply to create the O-grid. Figure Add the faces of the outlet and inlet to O-grid After creating the O-Grid, the blocking will appear as shown. 211

24 Figure The O-grid r) Defining Surface Parameters for the Mesh In this step, the user will define node distributions on the blocking using surface parameters. Surfaces should be turned ON in the Display Tree so they can be selected from the screen. Select Mesh > Surface Mesh Setup and select the surface selection icon. Then select all the surfaces by box selecting the entire model or pressing a. Enter the Maximum Element size as 0.3, Height as 0.03 and Ratio as 1.25, as shown. 212

25 Figure Surface mesh size window Press Apply to assign the surface parameters. Display the surface parameters by right mouse clicking in the Display Tree on Geometry> Surface > Hexa Sizes. The surfaces will show hexa icons as shown. 213

26 Figure The surface parameters Switch OFF Surface > Hexa Sizes. s) Defining Edge Parameters to Adjust the Mesh Although it may be enough to define the meshing with surface parameters, the mesh quality of more complex models can be improved by defining additional edge parameters. Perform these next steps to redistribute points along the diagonal (radial) edge of the O-grid. For the convenience of selecting the edges, right mouse click in the Display Tree to turn ON Vertices > Numbers and Edges > Bunching. Then make sure Vertices in ON. Zoom-in on the OUTLET area of the blocking. Select Blocking >Pre-mesh Params >Update Sizes.Make sure Update All is toggled on (default), and Press Apply. This will compute the node distributions on the blocking edges from the surface parameters. Turn ON Blocking > Pre-Mesh from the Display Tree. Press Yes, when it says, Mesh is currently out of date recompute? Right click on Blocking > Pre-Mesh > Solid and Wire in the Display Tree to display the mesh in Solid/Wire for better Visualization. The mesh will look like as shown below when viewing the OUTLET. 214

27 Figure Mesh before changing mesh parameters The mesh is denser at the walls. The near wall elements will have the same initial height that was set on the surface parameters, which was It may be desirable to have denser near-wall spacing. Select Blocking >Pre-mesh Params >Edge Params. Turn OFF Blocking > Pre-Mesh so the edges can be easily seen and selected. Select any of the radial edges. These are the edges created by the O-grid that are oriented radially in relation to the grid lines that run circumferentially around the tube. Or you can select the same edge shown in the figure below, which is the blocking Edge Set Spacing1 to 0.015, which is half the previous value. Set Spacing2 to 0, which will allow it to go as large as possible. Increase the number of nodes to 13 so the Ratio1 (1.25) can be met. Enable Copy Parameters and select Method Copy to Parallel edges to duplicate these settings on parallel edges in the blocking. Then press Apply. 215

28 Figure Setting edge meshing parameters 216

29 Figure Selection of edge for changing Parameters Note: Spacing1 is the first element size at vertex 118 while spacing2 is the first element size at vertex 196. Side 1 and Side 2 are indicated by the direction arrow that displays on the edge after it is selected. Switch OFF Edges > Bunching in the Display Tree. Switch ON Blocking > Pre-Mesh in the Display Tree. If you right click on Blocking > Pre-mesh, you should see Project faces checked ON by default. Choose Yes when asked to recompute the mesh. Switch OFF Geometry, Vertices and Edges in the Display Tree. Turn off the SOLID volume part name from the Display Tree and right click in the Display Tree to turn on Blocking > Pre-mesh > Solid and Wire if it is not already on. 217

30 Figure The final mesh displayed in Solid and Wire t) Checking mesh quality for determinants and angle To check the mesh quality, select Blocking >Pre-mesh Quality Histogram. Select the criterion as Determinant (2x2x2) and enter the Min-X value 0, Max-X value 1, Max-y height 12 and Num of bars 20. Press Apply. The histogram will be displayed in the lower right. A value of determinant greater than 0.2 is acceptable for most commercial solvers. 218

31 Figure Pre-mesh quality window while selecting Determinant 2x2x2 Figure Histogram showing Determinant 2x2x2 Then, in the Pre-Mesh Quality window at the upper left, select Angle from the Criterion pull down. Enter the values as shown below and press Apply. A new histogram will appear for the internal angles of elements as shown. 219

32 Figure Pre-mesh quality Window while selecting Angle An angle greater than 18 degrees solvers. is acceptable for most commercial Figure Histogram showing Angle Note: As taught in the 3DPipeJunct example, to display cells of a particular determinant or angle value, select a histogram bar and then select Show. Cells within that range will be highlighted. The user should then inspect the elements and decide on a solution. In most of the cases, block vertices can be moved or edge parameters can be changed to improve the area. 220

33 u) Running Pre-mesh smoother Before converting the Pre-mesh to an unstructured or structured mesh, the user may choose first to smooth the mesh. Select Blocking > Pre-mesh Smooth. The Pre-mesh smooth window will then appear. Select the Method as Quality. Select the Criterion as Angle and enter Smoothing iterations 3 and Up to quality 0.5 as shown. Figure Pre-mesh smooth window Press Apply to smooth mesh. Changes in the minimum angle of the mesh can be seen in the histogram as shown. The node position changes made by the pre-mesh smoother will not be saved to the blocking. So reloading the blocking and computing the mesh will always produced the mesh before smoothing. So at this point, you should not recompute the mesh. 221

34 Figure Histogram after running smoother v) Saving Select File > Blocking > Save blocking As and enter a name, such as b1.blk. Saving the blocking will allow the user to change any meshing parameters in the future by reloading the blocking onto the geometry. To write the mesh in an unstructured format, right mouse click in the Display Tree on Blocking > Pre-mesh > Convert to Unstruct Mesh. This will write the default name hex.uns to the working directory, and immediately load the mesh. To save the mesh to a different nam e, the user can then select File>Mesh>Save Mesh As. To write the mesh in a structured format, right mouse click in the Display Tree on Blocking > Pre-mesh > Convert to MultiBlock Mesh. Finally, save the project. 222