Meshing with the Trimming Method

Figure 19-1 Two-stroke engine geometry

Meshing with the Trimming Method

Generate a trimmed mesh for the two-stroke engine volume. The alternative to trimming is to use the mapping method, but given that mapping requires more computer time and user intervention, trimming is the recommended option.

The steps needed to generate a trimmed mesh are as follows:

1.Assign shells to geometry cell sets

2.Create a 2D template

3.Create a 3D template

4.Trim the 3D template to the engine geometry

5.Assemble the trimmed template

6.Run Star Setup

Assigning shells to geometry cell sets

The first stage of the trimming process is to assign surface shells to certain special geometry cell sets. These are then used to define the essential engine cylinder components. Shells are assigned to geometry cell sets using either via the training panel or via es-ice commands (if you are familiar with them).

•Select Panels > Directory from the menu bar of the Main es-ice window

•Enter the directory of the training user panel

•In the menu bar, select Panels > training to open the training panel shown in Figure 19-2

Figure 19-2 Training panel

First, define the G: cylinder shells 1 geometry cell set:

•Enter the following command to begin selecting the cylinder shells

CSet, Newset, Type, Cursor

•Pick the cylinder shells in the Geometry panel, as shown in Figure 19-3

Figure 19-3 Cylinder shell selection

•In the training panel, click Cylinder Shells to define all cells within the current cell set as the cylinder shells

•Enter the following command to re-display all shells:

CSet, All $CPlot

•Repeat the above process for the Piston Shells and Trimming Shells cell sets shown in Figure 19-4 and Figure 19-5, respectively. Note that the trimming shells include all the geometry surface shells, including the spark plug and the feature lines.

Figure 19-4 Piston shells

Figure 19-5 Trimming shells

Creating the 2D template

You can now begin creating the 2D template of the cylinder cross-section. First, set the General and Events parameters that define certain engine characteristics and operating conditions:

•In the Select panel, click Create Template

•In the Create Template panel, click General

•In the General parameters panel (see Figure 19-6), set the Base style to 0 or 1 Valve as this engine has no poppet valves

•Set the Cylinder radius to 50

•Click Ok to accept the settings and close the panel

Figure 19-6 Two-stroke General parameters

•In the Create Template panel, click Events

•In the Events parameters panel (see Figure 19-7), set the Crank angle start (deg) to 470 to begin the analysis at the instant before the exhaust ports open. This starting crank angle allows you to specify appropriate initial conditions in the cylinder by assuming the fluid composition has no residual exhaust gas.

•Set the Crank angle stop (deg) to 830 to cover a two-stroke engine cycle

•Set the Connecting rod length to 160

•Click Ok to accept the settings and close the panel

Figure 19-7 Two-stroke Events parameters

Note that in this case you can accept the default settings for Valve lift periodicity (deg) as the cylinder has no poppet valves. However, in other two-stroke engine cases, you may need to set this parameter to 360. Setting the valve lift periodicity to 360 also changes the periodicity of ignition in multiple-cycle cases to once every 360 degrees.

Next, create Section 1 of the 2D template. In this case, Section 1 defines the entire 2D template as the Base style was set to 0 or 1 Valve in the General parameters panel. Note that although the cylinder has no valves, you can use the valve settings to improve the cell distribution around the spark plug.

To create the 2D template:

•In the Create Template panel, select Section 1 from the Sections drop-down menu

•In the Section 1 Tool panel, select the Edit section parameters toggle button to access the valve-specific settings

•Set the Valve radius to 20 (see Table 4-10 in the User Guide for a definition of this and other Section parameters)

•Set the Valve inner radius to 18

•Set the Bottom ring radial cells to 5

•Set the Inner ring radial cells to 2

• Set the Outer ring radial cells to 24

•Click Create to create the 2D template, as shown in Figure 19-8

•Close the Section 1 Tool panel

•In the Create Template panel, click Store to store Section 1