### 18.1.2. Basic Settings Tab

The Basic Settings tab for the rigid body object has the following settings:

#### 18.1.2.1. Mass

Specify the mass of the rigid body. The mass is used in the calculation of translational acceleration due to applied forces. It is also used in combination with the specified gravity vector (the gravity vector specified with the rigid body, not with any buoyancy model) to calculate the force due to gravity. For details, see Gravity.

The force due to gravity acts through the specified center of mass. For details, see Center of Mass.

#### 18.1.2.2. Location

Specify all 2D regions (belonging to fluid domains) that physically contact the faces of the rigid body. The forces and torques exerted by the fluid on all of these faces will contribute to the motion of the rigid body. Note that if the rigid body is defined as an immersed solid, then the location is automatically just the location of the immersed solid, and no further location setting is required.

To have these faces move automatically in accordance with the rigid body solution, you must also specify that these faces have mesh motion provided by the rigid body solution. To do this, use the Boundary Details tab in the details view for the boundary containing the faces. Additional faces or mesh elements, which do not form part of the rigid body, can also have their mesh motion partially or wholly specified by the rigid body solution. To do this, use the Boundary Details tab for boundaries and the Mesh Motion tab for subdomains. Applying the rigid body solution to additional faces or mesh elements could be used, for example, to help control the way the mesh distorts as the rigid body moves.

#### 18.1.2.3. Coordinate Frame

You should specify a stationary coordinate frame that has its origin at the center of mass of the (physical) rigid body when the body is in its position at the start of the simulation. The coordinate frame you specify must have the same orientation as the axes used to define the mass moment of inertia of the body in its initial position (see Mass Moment of Inertia). In this documentation, this coordinate frame is referred to as the rigid body coordinate frame. The position and orientation of the rigid body are always calculated relative to this coordinate frame, and all other settings for the rigid body object (for example, gravity and degrees of freedom) are with respect to this coordinate frame, unless otherwise specified in this documentation.

If you choose a coordinate frame that does not have the position of the rigid body at the start of the simulation, then you must specify an initial position for the rigid body (on the Initial Conditions tab) that corresponds to the translation required to move the coordinate frame into the rigid body's initial position. See Initial Conditions Tab for more information. Note that initial conditions settings are not currently available for rigid bodies defined as immersed solids.

#### 18.1.2.4. Mass Moment of Inertia

Specify absolute values of the components of the mass moment of inertia for the rigid body with respect to 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.

The mass moment of inertia components are used to define the mass moment of inertia matrix described in Rotational Equations of Motion in the CFX-Solver Theory Guide.

For example, YY Component is defined as:

and XY Component is defined as:

where the center of mass is given by , and dm is a differential element of mass.

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