Group Elements by Boundary

You can use the Group Elements by Boundary command to create groups that contain:

• Shell elements whose faces define the boundaries of a volume

• 1D elements like beams and bars that reinforce shell elements whose faces define the boundaries of a volume

You can then apply boundary conditions, such as pressure, to the elements in the group.

For example, you can use this command to group all of the shell elements whose faces form a compartment within the hull of a ship or the fuselage of an aircraft.

Because only visible elements can be grouped, before using the Group Elements by Boundary command, use the Show, Hide, and other visibility commands to limit your display.

Selecting elements to place in groups

In the Group Elements by Boundary dialog box, you first select a seed element, which is a shell element on the boundary of the desired volume. If the normal of the seed element points outside the volume of interest, reverse the direction.

When you click Find Elements, the software selects the following in this order:

1. The seed element

2. The shell elements that share an edge with the seed element and bound the volume of interest

3. The shell elements that share an edge with the shell elements in the previous step and bound the volume of interest, and so on

The software continues to select shell elements until the volume closes, free element edges are detected, or all the shell elements in the model are selected. Free element edges exist at interior features like bulkheads and gussets.

When the software finds free element edges, it evaluates the mesh to determine whether the free element edges form a closed loop, as for the mesh of a bulkhead.

If the free element edges form a closed loop, the behavior of the software depends on the Find Closed Loop check box setting.

• Find Closed Volume prompts the software to select the shell elements up to and including those in the mesh of the closed-loop feature and stops.

• Find Closed Volume prompts the software to continue selecting shell elements past closed-loop features and other interior features that it detects. The selection process stops after the software determines that the volume is closed or all the shell elements in the model are selected.Note: When the Find Closed Volume check box is selected, the possibility of a runaway condition where the entire model is selected can occur. If a runaway condition does occur in a very large model, in the Work in Progress dialog box, click Stop to terminate the selection process.

If the free element edges do not form a closed loop, such as for the mesh of a gusset, the software continues to select shell elements past the interior feature regardless of the Find Closed Volume check box setting.

Grouping shell elements

After the selection process is complete and you click OK or Apply, the software places the selected shell elements in groups in the Simulation Navigator Groups folder.

The software considers any selected shell elements that mesh an interior feature and have both faces interior to the volume of interest as non-manifold. They are placed in the Non-Manifold Connected group. However, if the software determines that all the shell elements in the model are non-manifold, the software does not group any of the elements.

Tip:

To view the non-manifold elements in the graphics window, click Preview Removed Elements.

The software places all the other shell elements that it selects in either one or two groups.

• If you select the Create two groups based on normals check box, the software divides the shell elements into two groups. One group contains the elements whose normal points into the volume of interest. The other group contains the elements whose normal points outside the volume of interest.

• If you clear the Create two groups based on normals check box, the software places the shell elements in a single group, regardless of the direction of their normal.

The naming of these groups is controlled by the Grouping customer default settings.

Grouping 1D elements

During the selection process, the software selects 1D elements that satisfy both of the following criteria:

• The connectivity of the 1D element is defined between nodes that are included in the connectivity of a shell element, and the shell element is selected during the selection process.

• The centroid of the 1D element cross-section at both ends of the 1D element either coincides with the neutral plane of the shell element or is offset inside the volume of interest.

Because you cannot disable the 1D element selection capability, turn off the visibility of 1D elements that you do not want the software to select.

After the selection process is complete and you click OK or Apply, the software places the selected 1D elements in the Non-Manifold Connected group.

Modifying groups

Before you click OK or Apply to create the groups, you can add elements to a previewed group.

1. In the Group Elements by Boundary dialog box, click Preview Groups.

2. Click the appropriate Select Element button to select the elements to add.

Note:

After you create the groups, you can use the Edit Group command to add elements to the groups.

Where do I find it?

Group Elements by Boundary command

Application	Pre/Post
Prerequisite	A FEM or Simulation file as the work part and the displayed part
Command Finder	Group Elements by Boundary

Group Elements by Boundary defaults

Menu	File→Utilities→Customer Defaults
Location in dialog box	Simulation→Pre/Post→Grouping

How do I

Create a group by manually selecting the contents

Create a group automatically from an element quality check

Create groups with the Automatic Group command

Copy a group

Edit a group

Apply a force to the entities in an existing group

Use groups to control model visibility

Learn more

Groups

Automatic group command

Creating new groups from existing groups using Boolean operations

Group selection options

Smart selection methods with groups

Controlling the display of nodes in groups

Quick links

Command reference

Pre/Post video examples

Bulk Entry Descriptions

Simcenter 3D tutorials

Browse Simcenter 3D help by product area

Group Elements by Boundary, Simcenter 3D 2021.1 Series

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