# Six Degree-of-Freedom Motion

In many applications, the influence of flow on position and orientation of a rigid body is of interest. This requires allowing complete freedom for the body to translate and rotate. The six degree-of-freedom motion (6DoF) on the SimScale platform provides such functionality, making a large variety of industrial and scientific simulation applications possible.

Solid body montion, including 6DoF motion, is currently only available in **transient incompressible single or multiphase flow simulations** only.

## Setup parameters

### Basic paramters

The following set of basic settings is required to determine the physical characteristics of the rigid body:

**Inner Morphing Distance** (\(m)\):

In many cases it is important that the quality of the mesh near the surface of the body be preserved, e.g. for turbulence models. Since the 6DoF solver deforms the mesh around the body, this distance can be used to specify the distance from the body below which no cell deformation is allowed.
**Outer Morphing Distance** (\(m)\):

Similarly, this distance specifies the extent of the mesh-morphing region around the body.
**Center of Mass** (\(m)\):

Specifies the coordinates of the center of mass of the object. Note that the coordinates need to be specified in the global coordinate system (i.e., that of the entire flow domain).
**Mass** (\(kg)\):

Specify the mass of the rigid body.
**Moment of Inertia** (\(kg*m^{2})\):

Specify the moment of inertia of the body in the *x*, *y* and *z* direction. (rotational equivalent of the mass). Take a look at this list for the moments of inertia of some simple shapes.
**Density**:

Provide the density of the **reference fluid**. Provide a value only for the incompressible case – for other cases, the value is picked automatically.
**Gravity** (\(ms^{-2})\):

Provide the (signed) value of gravity in the *x*, *y* and *z* direction.
**Initial Motion State**:

This setting allows you to specify the initial state of motion for the rigid body. The default state is *Stationary*, but one can also prescribe initial values for orientation, velocity, acceleration, angular momentum and torque. The orientation is specified in terms of a Rotation matrix.
**Acceleration Relaxation Coefficient** (-):

This factor relaxes the acceleration computed in the previous timestep. It can be used to stabilize transient simulations.
**Acceleration Damping Coefficient** (-):

This factor dampens the rigid-body’s acceleration in steady-state simulations.
**Report**:

Specify if the motion data should be reported in the solver log during the simulation.

### Motion constraints

It is possible to constrain the motion of the rigid body to certain translations and rotations. The following settings show how such constraints could be defined:

**Translation Constraint**:
*No Constraint*: The body is free to move in any direction.
*Point*: The body is fixed to a reference point.
*Line*: Motion of the body is restricted along the specified direction.
*Plane*: Motion of the body is restricted in a plane. The plane is specified by its normal vector.

**Rotation Constraint**:
*No Constraint*: The body is free to rotate.
*Fixed Orientation*: No rotation is allowed. The body translates in a fixed orientation.
*Axis*: The body is only allowed to rotate about a fixed axis.

### Motion restraints

Additionally, the motion of the rigid body could be restrained using physical components such as springs and dampers:

**Linear Spring**: Specify if there exists a linear spring restraint on the body’s motion.
*Fixed Anchor Point* (\(m)\): The point at which one end of the spring is fixed in space.
*Attachment Point to Body* (\(m)\): The point on the body at which the other end of the spring is attached.
*Stiffness Coefficient* (\(Nm^{-1})\): Stiffness coefficient of the spring.
*Damping Coefficient* (\(Nsm^{-1})\): Damping coefficient of the spring.
*Rest Length* (\(m)\): Rest length of the spring.

**Linear Damper**: Specify if a linear damping restraint needs to be applied on the body’s motion.
*Damping Coefficient* (\(Nsm^{-1})\): Specify the damping coefficient.

**Angular spring**: Specify if an angular spring restraint needs to be applied on the body’s motion.
**Linear Axial Angular Spring Restraint**: Specify an axial angular restraint on the body’s motion.
*Reference Orientation*: Orientation matrix for the rigid body. Equivalent to the rest length of a linear spring.
*Spring Axis*: Axis about which the angular spring works.
*Stiffness Coefficient* (\(Nm*rad^{-1})\): Stiffness coefficient of the spring.
*Damping Coefficient* (\(Nsm*rad^{-1})\): Damping coefficient of the spring.

**Linear Axial Angular Spring Restraint**: Specify an axial angular restraint on the body’s motion.
*Reference Orientation*: Orientation matrix for the rigid body.
*Stiffness Coefficient* (\(Nm*rad^{-1})\): Stiffness coefficient of the spring.
*Damping Coefficient* (\(Nsm*rad^{-1})\): Damping coefficient of the spring.

**Angular Damper**: Specify if an angular damping restraint needs to be applied on the body’s motion.
*Damping Coefficient* (\(Nsm^{-1})\): Specify the damping coefficient.

Last updated: August 4th, 2020

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