## 11.2. Rigid Body Motion

ANSYS CFX computes the position and orientation of a rigid body using equations of motion. These equations can provide up to six degrees of freedom (“6 DOF”): up to three translational and up to three rotational degrees of freedom. You have full control over which degrees of freedom are allowed in a given simulation.

In the context of rigid bodies in ANSYS CFX, an orientation is represented by a collection of three Euler angles that follow the ZYX convention. Under this convention, a reference coordinate frame is reoriented so that it acquires the orientation of interest as follows:

- Euler Angle Z modifies the initial orientation by a rotation about the Z axis (using the right-hand rule to determine the direction).

- Euler Angle Y then further modifies the orientation by a rotation about the (modified) Y axis (using the right-hand rule to determine the direction).

- Euler Angle X then further modifies the orientation by a rotation about the (twice modified) X axis (using the right-hand rule to determine the direction).

Figure 11.1: Rigid Body with Initial Orientation shows a rigid body in an unmodified position. Figure 11.2: Rigid Body Reoriented with Euler Angles Shown shows the same rigid body after being reoriented. The mounting system has pairs of pins at each pivot location. The angle that develops between a pair of pins at a given pivot is the Euler angle associated with the axis on which the pivot was initially located.

Figure 11.1: Rigid Body with Initial Orientation

Figure 11.2: Rigid Body Reoriented with Euler Angles Shown

For theoretical details about the rigid body implementations in ANSYS CFX, see Rigid Body Theory in the CFX-Solver Theory Guide.

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