### 18.1.3. Dynamics Tab

The Dynamics tab for the rigid body object contains settings for the following items that influence the motion of the rigid body:

- External forces

- External torques

- Degrees of freedom of movement

- Gravity vector.

With the exception of Rotational Degrees of Freedom all dynamics settings are in comparison to the rigid body coordinate frame.

The Dynamics tab has the following settings:

#### 18.1.3.1. External Force Definitions

External forces other than gravity are specified in the External Force Definitions frame. Use the Add new item and Remove selected item icons to add or remove external force definitions from the list.

Each external force definition has one of the following options:

- `None`

  This setting deactivates the external force.

- `Spring`

  These settings enable you to set up a spring force that is applied to the rigid body. Specify the neutral position with respect to the rigid body coordinate frame via the Linear Spring Origin settings. Whenever the center of mass of the rigid body is at the Linear Spring Origin, the applied spring force is zero. The spring force develops as the center of mass of the rigid body moves away from the neutral position, with a force-to-displacement ratio specified by the Linear Spring Constant. For details on how the center of mass is located, see Center of Mass.

- `Value`

  These settings enable you to specify an external force using Cartesian force components (along the principal axes of the rigid body coordinate frame).

The external forces are applied through the center of mass of the rigid body so that they cause translational accelerations only (and not any rotational accelerations).

#### 18.1.3.2. External Torque Definitions

External torques are specified in the External Torque Definitions frame (in the details view). Use the Add new item and Remove selected item icons to add or remove external torque definitions from the list.

Each external torque definition has one of the following options:

- `None`

  This setting deactivates the external torque.

- `Spring`

  These settings enable you to set up a spring torque that is applied to the rigid body. Specify the neutral orientation (with respect to the rigid body coordinate frame) via the Equilibrium Orientation settings. You define the orientation using three angular displacements (specifically, Euler angles), which are applied by the software using the ZYX convention:

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

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

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

  These Euler angles are described in more detail in Rigid Body Motion in the CFX-Solver Modeling Guide.

  Whenever the orientation of the rigid body matches that specified by the Equilibrium Orientation settings, the applied spring torque is zero. The spring torque develops as the rigid body rotates away from the neutral orientation, with a torque-to-angular-displacement ratio specified by the Torsional Spring Constant settings.

  The torque develops due to changes in Euler angles, but is applied on the corresponding axes of the rigid body coordinate frame. The torque spring implementation described here is recommended for use only when both the equilibrium orientation and the rigid body orientation can be described with small Euler angles (a few degrees).

- `Value`

  These settings enable you to specify an external torque using Cartesian torque components around the principal axes of the rigid body coordinate frame.

#### 18.1.3.3. Degrees of Freedom

##### 18.1.3.3.1. Translational Degrees of Freedom

This setting determines the combination of axes along which the rigid body may translate. In this context, the axes are those of the rigid body coordinate frame.

##### 18.1.3.3.2. Rotational Degrees of Freedom

This setting determines the axes about which the rigid body may rotate. In this context, the axes are of a coordinate frame that:

- Has the same initial orientation as the rigid body coordinate frame

- Has its origin at the rigid body center of mass

- Moves rigidly with the rigid body.

#### 18.1.3.4. Gravity

Specify a gravity vector in the rigid body coordinate frame that defines the downward direction and the magnitude of free-fall acceleration due to gravity.

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