### 11.3.1. Modeling a Rigid Body as a Collection of 2D Regions

To set up a rigid body as a collection of 2D regions:

1. Insert and define a rigid body object.

   The rigid body object holds settings that define: the rigid body characteristics, information about external forces and torques, which degrees of freedom are to be allowed, and initialization settings for the rigid body. Many of the settings are with respect to the rigid body coordinate frame, described in Coordinate Frame in the CFX-Pre User's Guide. For details on the parts of the user interface relevant to this step, see Rigid Body User Interface in the CFX-Pre User's Guide. The rigid body object, once created, is listed in the Outline tree view.

2. Ensure that the mesh motion in the related domain(s) reflects the motion of the rigid body by setting the Mesh Motion option for the boundaries that contain the rigid body faces to `Rigid Body Solution`. (Mesh motion requires, as a prerequisite, the Mesh Deformation setting of `Regions of Motion Specified` for the domain. For details, see Mesh Deformation in the CFX-Pre User's Guide.)

3. As required, apply the Mesh Motion option `Rigid Body Solution` to other boundaries (Boundary Details tab) and subdomains (Mesh Motion tab) in order to move (consistently with the rigid body) parts of the mesh not on the rigid body faces. Properly done, this can help to maintain the mesh quality as the rigid body moves.

4. As required, apply motion constraints to any of the boundaries/subdomains that are governed by the rigid body solution.

   The Motion Constraints setting is found in the Mesh Motion group of settings, which is found in the Boundary Details tab for boundaries and the Mesh Motion tab for subdomains.

   As an example of applying a motion constraint, you might have a circular mesh interface surrounding the rigid body, and you might want the mesh on the outside of that interface to not rotate but still translate to follow the rigid body. In this case, you would apply the `Ignore Rotations` constraint on the outer boundary of the interface.

5. As required, adjust the settings that govern the rigid body solver. For a description of these settings, see Rigid Body Control Tab in the CFX-Pre User's Guide.

Release 18.2 - © ANSYS, Inc. All rights reserved.