MESHING WITH THE MAPPING METHOD	Chapter 20
Creating the 2D Base Template

Creating the 2D Base Template

The first step in creating the 2D base template is to set some parameters in the

General parameters and Events parameters panels.

•Click Create Template in the Select panel to open the Create Template panel

•Select the Mapping option

•Now click General to open the General parameters panel (see Figure 20-7)

•Change the Base style to 2/4 Valve, which is the option for modelling half of a symmetric 4-valve engine

•Check that the Engine type is Gasoline and the Cylinder radius is 45

•Click Ok at the bottom-left of the panel to accept the values and close the panel (Apply will accept the values but keep the panel open)

•Next, click Events in the Create Template panel to open the Events parameters panel (see Figure 20-7)

Since the analysis in this example will only consider the intake and compression strokes, the starting crank angle will be the 0-lift point before the valve begins to move (see vlift01.dat).

•Click Events in the Create Template panel to open the Events parameters panel (see Figure 20-7)

•Set the Crank angle start (deg) to 320 and the Crank angle stop (deg) to 720

•Check that the Engine RPM is set to 3600, the Connecting rod length to 145, the Piston pin offset to 0 and the Valve lift periodicity (deg) to 720

•Click Ok

Figure 20-7 Modified General parameters and Events parameters panels

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•	In the Create Template panel, select
	Section 1 from the Sections pull-down
	menu to open the Section 1 Tool panel.
	This will help you create the 2D mesh in a
	quadrant around Valve 1.
•	First, click on the upper Load button
	located next to the valve01.dat field
	to load the internal valve information for
	Valve 1. Note that Valve01.dat is an
	image file created in Chapter 3,
	“Modelling the Valves” of this volume.
•	Click Create to activate the General
	Workspace window and plot the section, as
	shown in Figure 20-8.

Figure 20-8 General Workspace window: Section 1 after loading valve information

The mesh in this section needs to be modified

next. The cell density in the valve region is a major factor affecting the overall cell count of the model and is controlled by the number of circumferential cells around the valve.

•A coarse model will be built in this tutorial example, so change the Circumferential cells (section) parameter at the top of the panel to a value of

56

•To see the result of this modification, click Create in the Section 1 Tool panel

For most mesh adjustments, it is generally useful and easier to use the cursor in a graphical interactive mode.

•Click Adjust in the Section 1 Tool panel and notice the several red dots appearing on the plotting window, shown in Figure 20-9. Interactive GUI tools can then be used to alter the section until a mesh of reasonable cell size and quality is achieved.

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Adjusts “Bottom ring radial cells”

Adjusts number of circumferential cells around valve

Adjusts “Outer ring radial cells”

Figure 20-9 General Workspace window: Section 1 in ‘Adjust’ mode

The red dot in the centre of the valve grid can also be used to change the number of circumferential cells around the valve. Note the text at the bottom of the General Workspace window when moving the cursor over this dot:

•A left-click or middle-click will decrease or increase, respectively, the value by 2

•A right-click will reset the value to the default of 72

•Typing a number followed by a left-click or right-click will decrease or increase, respectively, the value by that typed number

•Typing u or r will successively undo or redo, respectively, the latest adjustments

•Clicking with any mouse button off the mesh in an empty part of the window or typing q will quit the ‘Adjust’ mode

The valve mesh is known as an O-grid, being made up of a 12x12 Cartesian mesh with a single polar mesh layer surrounding it. This mesh is called the “Bottom ring radial cells” and is shown in the Section 1 Tool panel. The red dot associated with this parameter is located along the mesh line of the core Cartesian grid. We will coarsen the polar mesh around the valve region called the “Outer ring radial cells”.

•Left-click with the cursor over the red dot labelled in Figure 20-9 twice to decrease their numbers from the default of 5 to a value of 3. The “Inner ring radial cells” can be left at the default of 1.

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	The end result of the above adjustments is shown in Figure 20-10.

Figure 20-10 General Workspace window: Section 1 after valve modifications

If possible, matching some areas of the 2D base template with features of the cylinder dome should always be attempted. For this tutorial example, there is a feature between the flat and angled portions of the combustion dome that can be matched with a mesh line in Section 1. This line can be obtained by adding a special triangular region to the section. From the current viewpoint of looking down from the +z axis, this geometric feature appears to the right of Valve 1.

Let us use the ‘double-plotting’ feature by overlaying both the Geometry and General Workspace windows with the suggested plot settings shown in Figure 20-11:

•In the Geometry window, isolate the cylinder dome cells and turn off the Mesh option while keeping the Fill option on in the Plot Tool panel.

•Activate the General Workspace window and turn off the Fill option while keeping the Mesh option on in the Plot Tool panel.

•Click the Dplot button in the Plot Tool panel. The currently active cell set of the Geometry window is plotted first and then the General Workspace window is plotted over it.

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Figure 20-11 Plot Tool panel settings for double-plotting

2. Left-click and drag to feature

1. Left-click

to choose

Figure 20-12 General Workspace-Dplot: Adjusting the right bottom position

From the difference in colour shades on the cylinder dome appearing in Figure 20-12, the feature between the flat and angled portions of the dome can be seen as a vertical line.

•Click Adjust again in the Section 1 Tool panel and left-click the red dot on the lower-right corner to change the bottom position

•Left-click and drag the mouse until the cursor is at the previously mentioned feature to move the vertical mesh line along the bottom edge of the x-axis to a new parallel position, as shown in Figure 20-12

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	The right-hand boundary of the section has now moved to match the feature, as
	shown in Figure 20-13.

Figure 20-13 General Workspace-Dplot: After right bottom adjustment

•Quit the ‘Adjust’ mode

•In the Section 1 Tool panel, change the Right triangle exists option to Yes and then click Create

A new triangular region will be created to the right of the 2D template. Note that, with the Right triangle exists option turned on, further adjustment of the vertical mesh line will also automatically adjust the newly created triangular region.

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Figure 20-14 General Workspace-Dplot: With Right triangle exists option

•With this mesh line in the correct place, return the plot in the General Workspace window back to the previous settings by turning on the Fill option and clicking Cplot in the Plot Tool panel

Other important areas needing modification are the three triangular regions on the corners of the section and the “Right triangle”. There are two issues with these regions:

1.The placement of the corner attachment points

2.The cell density within the regions

•Move the cursor over the upper red dot of the “Right triangle” region and note the text at the bottom of the window, shown in Figure 20-15

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1. Left-click to choose

2. Left-click to choose new attachment point

Figure 20-15 General Workspace window: Adjusting the attachment point of the right triangle

•Left-click to choose this point for adjustment. All other red dots will become clear and the text will then change to the following:

You can now select a vertex along the perimeter of the “Outer ring radial cells” to be the new attachment point.

•Left-click the vertex that is one position away in the clockwise direction, as shown in Figure 20-15. Notice the improvement in the interior angles connected to the new attachment point and the improved mesh orthogonality in the region outside the valve and closest to the cylinder wall.

Similar improvements can be made by repeating the above steps for the other three triangular regions. The triangular region located at the lower-right can have the left attachment point moved counter-clockwise by one position. The triangular region located at the lower-left can have the right attachment point moved clockwise by one position and the top attachment point moved counter-clockwise by one position. The triangular region located at the upper-left can have the bottom attachment point moved clockwise by one position and the right attachment point moved clockwise by two positions.

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The above operations will result in greater cell size uniformity in the “Outer ring radial cells” region, as shown in Figure 20-16.

Figure 20-16 General Workspace window: Section 1 after attachment point adjustments

Since the spark plug is located in the triangular region on the lower-left of section 1, it also desirable to increase the cell density there.

•Move the cursor over the red dot located at the centre of this triangular region and note the text at the bottom of the plotting window. Notice the three red dots in the middle of each edge of the triangular region, shown in Figure 20-17, and the change in the text.

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Before	After

Electrode

2. Middle-click to increase

1. Left-click to choose

Figure 20-17 General Workspace window: Changing the cell count within a triangular region

The cell density and distribution in the triangular region may be altered by increasing or decreasing the number of cell layers from the centre to each of the three edges.

•Middle-click the red dot in the interior of the section, as shown in Figure 20-17, to add another cell layer between the centre and the corresponding edge

•Quit the adjustment of the triangular region by clicking off the mesh or typing q on the keyboard

The increase in cell density for that triangular region can now be seen and you are still in ‘Adjust’ mode.

The cell count in the “Right triangle” region should also be reduced in a similar way.

•Left-click the red dot in the centre of this region to choose it and then middle-click the bottom red dot three times to increase the number of cells from that edge to the centre. This will decrease the cell count in the region.

•Quit the region adjustment by clicking off the mesh or typing q on the keyboard.

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Section 1 now has an acceptable cell size and cell quality, as shown in Figure 20-18.

Figure 20-18 General Workspace window: Final Section 1

•Type q with the cursor in the window or click on an empty part of the window to quit the ‘Adjust’ mode

•The Section 1 Tool panel is no longer needed, so click Close to close it.

Section 2 can now be built in a similar way. Starting with the valve region, click the Load button to load the valve information and reduce the number of circumferential cells to 48. The “Outer ring radial cells” should be decreased to 4. Usually the exhaust valve is smaller than the intake valve. Therefore, the exhaust valve section should have fewer circumferential cells but more “Outer ring radial cells” in order to maintain a consistent cell spacing. These modifications can be made using the GUI tools and red dots that aid adjustments.

For Section 2, a “Left triangle” will be needed and the left boundary of the section can be moved to match the geometric feature between the flat and angled portions on the exhaust side of the cylinder dome. The attachment points of the triangular regions can be moved similarly to those of Section 1. Then the cell density of the triangular regions and “Left triangle” can also be modified.

To minimize the amount of plastering during piston modelling in this chapter, we need to align the mesh line to match the bowl feature more closely:

•Go to the Geometry window

•Isolate the piston shells in the currently-active cell set

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•	Turn Off the Mesh plotting and turn On the Fill options
•	Go to the General Workspace window
•	Turn On the Mesh plotting and turn Off the Fill options
•	Select Dplot

To rotate Valve 1, you need to open the Section 1 Tool panel by selecting Section 1 from the Sections pull-down menu in the Create Template panel, as shown in Figure 20-19.

•Select Edit section parameters

•Select Show all section parameters

•Change the Valve rotation (section) parameter to 30 (see Valve Rotation, page 4-37 of the User Guide, on how to rotate the valve using a control point)

•Click Create to update the Valve 1 section

•Using similar steps, enter a value of 45 for the Valve rotation (section) parameter of Valve 2.

The resulting views before and after the valve rotation are shown in Figure 20-20.

Figure 20-19 Modified 2D parameters for Valves 1 and 2

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Before Rotation

After Rotation

Figure 20-20 2D Template before and after the valve rotation

With every modification made, the es-ice window will update the number of cells on each side of the shared interface. Only when they are equal is the user in a position to continue.

•Click Store in the Create Template panel to connect and smooth the mesh of the two sections together, as shown in Figure 20-21.

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