## 11.7. Limitations to using Rigid Bodies

The following is a list of limitations that apply when using rigid bodies:

- When modeling an immersed solid governed by the rigid body solver, you cannot set initial conditions (as you can for the rigid body object).

- Rigid bodies cannot be modeled in an ANSYS Multi-field (MFX) simulation or a System Coupling simulation.

- There is no collision modeling to detect when a rigid body contacts a solid wall or another rigid body.

- Rigid bodies cannot be modeled in a rotating domain.

- The built-in spring models are limited. However, you may be able to use CEL expressions to apply forces and torques that simulate a general spring. When working with CEL, you can use the `rbstate` function to get information about the rigid body position and orientation. For details, see rbstate in the CFX Reference Guide.

- Only simple motion constraints are possible. For example, there is no straightforward way to constrain the motion of a rigid body so that it rotates about a point other than its own center of mass.

- The rigid body coordinate frame definition should not be changed when a case is restarted.

- Although the rigid body orientation is internally tracked with quaternions, the RBSTATE callbacks report the orientation in terms of Euler angles with respect to the global coordinate frame. Euler angles suffer from the well known limitation of “gimbal lock” and can therefore be misleading. This does not indicate a problem with the rigid body orientation.

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