Chapter 22 |
PISTON MODELING |
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Meshing the Piston with the Shape Piston Method |
Chapter 22 PISTON MODELING
Meshing the Piston with the Shape Piston Method
The Shape Piston method is a flexible method that can model very complex piston geometries. All meshing for the piston is handled directly by es-ice and it is currently the only method that allows penetration of cylinder head features into the piston area, such as the deep spark plug in our tutorial example.
Most of the techniques used for the Shape Piston method are identical to those used previously to map the cylinder head, intake port and exhaust stub. Thus, cells may be removed to help the template conform better to the geometry, edges and splines are created for control, patches are created and a similar mapping and projecting process is employed. Just like in the previous mapping process, only the surface of the piston will be mapped and its interior vertices will be moved automatically by es-ice during the analysis.
Before we can begin creating edges, splines and patches, we need to create shells that approximately represent the piston surface. These are called “dummy target shells” as they serve as a target for layers of vertices. The layers will consist of a user-specified key grid plane from the original template. Since this grid plane represents the surface of a continuous layer of cells, it must be relatively smooth. Therefore, the dummy target shells must also be relatively smooth. As a result, piston features such as bowls and crowns should not be included when creating the dummy target shells.
We will first create splines and then create the dummy target shells between these splines:
•Isolate the piston shells in the currently active cell set and make sure that it is in the correct TDC position
•Using Vertex for the Pick mode, create a spline starting from one corner of the intersection between the cylinder wall and symmetry plane to the other corner.
•Choosing as few vertices as possible, click only on vertices where there is a change in angle between the flat surfaces and stay along the symmetry plane line of y=0 whenever possible, as shown in Figure 22-1.
•The dashed line shown in the figure indicates the assumed shape of the dummy target shells.
The curved shape of the spline should also be ignored since flat shells will be created between the knots at a later stage.
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Selected vertices
Figure 22-1 Geometry window: Piston and created spline
Next, create two other splines (copies of the spline we have just created) in both the +y and -y direction so that they extend beyond the piston shells. To do this:
•Create new splines by a translation of their knots in the global cartesian y-direction, as shown in Figure 22-2, with the following commands (assuming the created spline ID is 47):
spline,48,from,47,0,-50,0,1 spline,49,from,47,0,50,0,1
The new splines should extend beyond the piston geometry shells, especially when viewed from the global +z-axis direction (view,0,0,1).
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Figure 22-2 Geometry window: Two additional projected splines created
•The dummy target shells can now be created between the two outer splines by typing the following command:
sshell,cursor,1
•Since the cursor is now in pick mode, left-click the two outer splines (splines 48 and 49) with the mouse. Flat shells, our dummy target shells, will then be created between the picked splines and assigned to the next available cell type.
•Clear the three created splines as they are no longer needed
•Isolate the newly created dummy target shells in the currently active cell set, shown in Figure 22-3, and save them to Geometry Cset 2 by clicking the G: piston shells 2 button of the training panel. For the Shape Piston method, es-ice will assume that the shells in Geometry Cset 2 are to be used as the dummy target shells.
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Figure 22-3 Geometry window: Dummy target shells saved to Geometry Cset 2
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Now click the Piston button in the Select |
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panel to open the Piston panel |
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From the pop-up menu at the top of the |
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panel, select the Shape piston option. Since |
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it is critical to minimize skew at TDC when |
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the piston is closest to the cylinder head, the |
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default value for Crank angle (deg) can be |
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left at 360. |
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Click the Map piston XYZ button. This |
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instructs es-ice to calculate and store the |
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position of the bottom layer of template |
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vertices at a crank angle of 360 degrees, |
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assuming the piston was shaped like the |
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dummy target shells in Geometry Cset 2. |
After this finishes, this vertex spacing information can be applied to a number of cell layers starting from the bottom template cell layer; the distance between these layers can also be specified.
Judging by our 1 millimetre approximate valve curtain height and the geometric height of the piston bowl, we will use ten cell layers to model the piston.
• Another cell layer must be added to account for the “key grid plane” so enter a value of 12 for the Layers to read parameter
•Click the Layer DZ button and enter a value of 1 in the adjacent text box
•Click the Read piston XYZ button to display Figure 22-4
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Key Grid
Plane
Piston Mesh
Figure 22-4 Template window: Result after “Read Piston XYZ”
The bottom sixteen layers of the template will now be spaced 1 millimetre apart in the global z-direction, have a vertex spacing defined by the Map piston XYZ function, and have the shape of the dummy target shells. This results in a piston mesh that will match most closely to the combustion dome mesh when the piston is close to the dome, thus minimizing mesh distortion.
Isolate the actual piston shells again in the currently active cell set. The Shape Piston method is the only method that requires the piston to be at its BDC position since the mapping process will eventually be done in the BDC position. With only the piston shells in the currently active cell set, we will gather all piston shell vertices and move them down by the piston stroke length in the global Cartesian system.
•Select Sets > Vset > Newset > Cset, 0 from the pull-down menus (equivalent to command vset,newset,cset)
•Type the following commands:
csys,1 vmod,vset,0,0,-68.5,relative
Similarly to what was done in Chapter 20 with the spark plug (see Figure 20-26), the appropriate template cells need to be deleted so that the piston bowl and crescent conform better to the geometry. The double-plotting feature and other similar techniques should be used as before. An additional technique that needs explanation here is the use of the Plaster button in the Edge or Spline Tool panel. This creates non-hexahedral cells in order to improve mapping to curved geometry surfaces (see also Chapter 20).
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To gather the cells involved in the piston bowl mapping, we recommend creating three cell sets:
1.A cell cset that includes the bottom twelve layers (the Layers to read parameter as specified in the Piston panel) (Cset 10)
2.The topmost layer of the above cset labelled as ‘Key grid plane’ in Figure 22-5 (Cset 11)
3.A cell set formed by subtracting Cset 11 from Cset 10 (Cset 12). This is the ‘Piston mesh’ shown in Figure 22-5
•For Cset 10, view the full model (Cset 1) from the +y direction (view,0,1,0,1) and use command Cset,subs,zone to collect the bottom 11 layers. Then use command Cset save 10 to save the set to Cset 10.
•For Cset 11, execute the following commands:
Cset recall 10 Cset invert
Cset subset cset 1 vset newset cset cset recall 10
cset subset vset any vset none
cplot
cset save 11
•For Cset 12, execute the following commands:
Cset reca 10 Cset dele cset 11 cplot
Cset save 12
The above sets are shown in Figure 22-5
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Cset 11 (Key grid plane)
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Cset 10 |
Z
Figure 22-5 Template cell sets 10, 11, and 12
After collecting Cset 12, perform the following operations:
•Change to the Geometry window
•Collect the piston shells using command Cset newset type cursor
•Turn Off the Mesh plotting and the Fill options
•Create a spline on the piston crown to capture its features using command spline angle 30
•Change to the Template window
•Change the viewpoint to view,0,0,1,1
•Turn On the Mesh plotting and turn Off the Fill option
•Select Dplot
The resulting view is shown in Figure 22-6
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cset,delete,zone |
Feature
Figure 22-6 Double plot of Cset 12 together with the piston geometry
•Using command Cset,delete,zone, remove the cells outside the bowl as shown in Figure 22-6. This process could be repeated several times until you get the cells that represent the bowl. Unfortunately, selecting the cells near the bowl feature can be tricky. Three possible cases are illustrated in Figure 22-7. Figure 22-7a is not desirable because the resulting mesh has distorted cells. Generally, the cell configuration in Figure 22-7b and Figure 22-7c should give you good mesh quality. Ideally, you want to get a cell configuration as similar to Figure 22-7c as possible.
•Save this set as Cset 12 again (Cset,save,12). Note that the selected cells to capture the bowl feature could affect the mesh quality near the bowl region.
•Add Cset 11 to Cset 12, and save this as Cset 13 using the following commands:
Cset reca 12 Cset add cset 11 Cset save 13
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c) Ideal
Figure 22-7 Removing cells outside the bowl feature
Cset 13 is shown in Figure 22-8.
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Figure 22-8 Cset 13 obtained by adding Cset 12 (modified) and Cset 11
After gathering all cells to be mapped to the piston bowl, we can improve the ‘stair-step’ mesh resulting from the cell deletion by filling in columns of prismatic cells. This is accomplished by bounding each set of stair-steps by an edge that runs diagonally across some cell faces.
•Click the Diagonal button in the Edge or Spline Tool to allow edges to run diagonally across a cell face
Rectangular cell faces will eventually be cut into two triangular faces, resulting in trimmed cells of type 1, as shown in Appendix A of the STAR-CD V4.10 Meshing User Guide. Note that a single edge can be used for multiple, adjacent stair-steps.
•Click the Plaster button and then left-click on an edge knot to create the necessary cells, in this case prisms, shown in Figure 22-9
•As with most other cursor-pick functions, type q or click on an empty part of the screen to quit this function
•After the plastered cells are created, the edges are no longer useful so they should be cleared with the Clear edge button
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Figure 22-9 Template window: Piston bowl cells before (left) and after (right) plastering columns
We also wish to put a crown of plastered cells around the piston bowl so that we do not have to map adjoining side and bottom faces to the smoothly-contoured piston bowl (this could result in cells with excessively large interior angles). The task is accomplished by first deleting some cells to obtain a stair-step feature and then plastering it. The plastering function will automatically create prisms, pyramids, tetrahedrals and trimmed cells to fill the stair-step correctly, as shown in Figure 22-10.
Note that one can also create new cells using the Cdx Tool, as described in Chapter 4, “Improving cell connectivity” of this volume. However, new vertices should not be created or they will be fixed into the grid. Cells created with Cdx should use vertices from the original template created by es-ice.
Figure 22-10 Template window: Piston bowl cells before (left) and after (right) plastering crown cells
Similar things can be done for the crescent cells at the other end of the piston. Edges, splines and patches can then be created and the Edge and Surface mapping processes performed previously can be repeated to map the piston surface, as shown in Figure 22-11, Figure 22-12 and Figure 22-13.
