Body Motion Option Reference

Properties

X Motion, Y Motion, Z Motion

When On, translation motion is calculated along the axis with respect to the laboratory coordinate system. When Off, the degree of freedom in the selected direction is frozen.

X Rotation, Y Rotation, Z Rotation

When On, rotation around the chosen axis is calculated with respect to the laboratory coordinate system (as transposed to the center of mass of the 6-DOF body). When Off, the degree of freedom in the selected direction is frozen.

Properties

Rotation Axis Origin

Specifies the origin of the rotation axis.

Rotation Axis Direction

Specifies the direction of the rotation axis.

Coordinate System

Determines the coordinate system in which the origin and the direction of the rotation axis are specified.

Motion Limits

When activated, applies a damping moment that limits the rotation motion (see Eqn. (4899)).

Minimum Angle

Specifies the minimum angle ${\alpha }_{min}$ at which the body motion is stopped if the motion limiter is enabled. The angle is measured with respect to the initial orientation of the body. The initial angle is 0. See Eqn. (4899).

Maximum Angle

Specifies the maximum angle ${\alpha }_{max}$ at which the body motion is stopped if the motion limiter is enabled. The angle is measured with respect to the initial orientation of the body. The initial angle is 0. See Eqn. (4899).

Angular Damping Length

Specifies the angular length $\lambda$ (before the minimum and maximum angle) over which the stopping process takes place if the motion limiter is enabled. See 1-DOF Rotating Motion.

Properties

Translation Origin

Specifies the origin of the translational motion. This point is treated as the initial body position, which can be different from the initial center of mass.

Translation Direction

Specifies the direction of the translational motion.

Coordinate System

Determines the coordinate system in which the origin and the direction of the translational motion are specified.

Motion Limits

When activated, applies a damping force that limits the translation motion (see Eqn. (4897)).

Minimum Position

Specifies the minimum displacement $d_{\min }$ at which the body motion is stopped if the motion limiter is enabled. The displacement is measured with respect to the initial position of the body. The initial displacement is 0. See Eqn. (4897).

Maximum Position

Specifies the maximum displacement $d_{max}$ at which the body motion is stopped if the motion limiter is enabled. The displacement is measured with respect to the initial position of the body. The initial displacement is 0. See Eqn. (4897).

Damping Length

Specifies the length $\lambda$ (before the minimum and maximum position) over which the stopping process takes place if the motion limiter is enabled. See Eqn. (4897).

Properties

Course Velocity

Indicates the speed of the body in the X-Y plane. The default is zero.

Course Angle

Indicates the direction of motion of the body in the X-Y plane, given as the counter-clockwise angle from the X-axis. The default is zero.

Properties

Planar Motion Option

Specifies one of the following types of planar motion that you want your 6-DOF body to perform:

Planar Motion Mechanism Properties

Velocity X

Specifies the magnitude of the velocity along the X-axis with a constant value. The default is 1.0 m/s.

Amplitude Y

Specifies the maximum displacement of the body along the Y-axis. The default is zero. This corresponds to the sway amplitude $y_0$ in Eqn. (4925).

Oscillation Frequency

Specifies the number of cycles per second. The default is zero. This corresponds to the sway frequency $f$ in Eqn. (4925).

Additional Drift Angle

Specifies an additional drift angle of the body with a constant value. This value is added to the yaw angle.

Pure Yaw

When activated, the body rotates back and forth in the X-Y plane (yaw) so that the same face (the bow, for a boat) always leads along the sinusoidal path. The Additional Drift Angle is added if specified. The yaw angle is given by Eqn. (4926).

When deactivated, the body does not change orientation as it sways. This setting is the default.

Rotating Arm Properties

Center of Rotation

Specifies the coordinates of the point ${\mathbf{\text{r}}}_c$ around which the rigid body rotates, with respect to the specified coordinate system (see Eqn. (4928)).

Angular Velocity

Specifies the angular velocity $\omega$ of the rotating arm (see Eqn. (4928)).

Drift Angle

Specifies the drift angle of the body. The drift angle must be a constant value. The drift angle is added to the yaw angle, which is due to the rotating arm motion.

Coordinate System

Determines the coordinate system with respect to which the center of rotation is specified.

General Planar Motion Properties

Translation X

Specifies the translational motion of the 6-DOF body in the X direction with respect to the laboratory coordinate system. You enter the function as an Expression to specify the translation as a function of time.

Translation Y

Specifies the translational motion of the rigid body in the Y direction with respect to the laboratory coordinate system. You enter the function as an Expression to specify the translation as a function of time.

Rotation Z

Specifies the rotational motion of the rigid body around the Z axis with respect to the laboratory coordinate system. You enter the function as an Expression to specify the translation as a function of time.

User Defined Derivatives

To calculate the motion of a body defined by the General Planar Motion mechanism, the first and second order time derivatives of the prescribed trajectory are required. By default, STAR-CCM+ computes these derivatives numerically. If the accuracy of this numerical computation is considered insufficient, you can specify the first and second order derivatives explicitly.

Activated: Specifies that the first and second-order time derivatives of the user-defined trajectory are also user-defined. A new Planar Motion Derivatives sub-node is added to the simulation tree.

Deactivated: The first and second order time derivatives of the user-defined General Planar Motion trajectory are computed numerically.

Free Heave

When activated, this option enables the body to move along the Z-axis.

Free Pitch

When activated, this option enables the body to rotate around its own Y-axis.

Free Roll

When activated, this option enables the body to rotate around its own X-axis.

Properties

X Motion, Y Motion, Z Motion

For each chosen direction, the numerical procedure translates the body in the direction to achieve an equilibrium position. Directions are set with respect to the laboratory coordinate system.

X Rotation, Y Rotation, Z Rotation

For each chosen axis, the numerical procedure rotates the body around the axis to achieve an equilibrium position. Axis directions are set with respect to the laboratory coordinate system.

Control Parameters

Maximum Translation Step

Specifies the maximum translation step size during one time-step.

• If the algorithm calculates a translation step size that is smaller than the specified Maximum Translation Step, the calculated translation step size is applied to move the body.
• If the algorithm calculates a translation step size that is greater than the specified Maximum Translation Step, the calculated translation step size is split over a number of consecutive time-steps. In each of these time-steps, at most the Maximum Translation Step is applied. This is done until the entire predicted translation step size is reached.

Maximum Rotation Step

Specifies the maximum rotation step size during one time-step.

• If the algorithm calculates a rotation step size smaller than the specified Maximum Rotation Step, the calculated rotation step size is applied to rotate the body.
• If the algorithm calculates a rotation step size greater than the specified Maximum Rotation Step, the calculated rotation step size is split over a number of consecutive time-steps. In each of these time-steps, at most the Maximum Rotation Step is applied. This is done until the entire predicted rotation step size is reached.

URF Translation

Specifies an under-relaxation factor applied to the translational motion while computing equilibrium. This factor defines which portion of the internally calculated translation step size is actually applied to the body. Reduce this factor if you encounter convergence problems.

URF Rotation

Specifies an under-relaxation factor applied to the rotational motion while computing equilibrium. This factor defines which portion of the internally calculated rotation step size is actually applied to the body. Reduce this factor if you encounter convergence problems.

Maximum Translation Growth Rate

Specifies the maximum growth rate of the translation increment.

To increase stability of the simulation, the translation step size of the current motion step is limited by the translation step size of the previous motion step times the Maximum Translation Growth Rate. Therefore, the translation step size cannot increase arbitrarily from one motion step to the next, but its growth is limited by the maximum growth rate factor. Reduce this value if you encounter convergence problems.

Maximum Rotation Growth Rate

Specifies the maximum growth rate of the rotation increment.

To increase stability of the simulation, the rotation step size of the current motion step is limited by the rotation step size of the previous motion step times the Maximum Rotation Growth Rate. Therefore, the rotation step size cannot increase arbitrarily from one motion step to the next, but its growth is limited by the maximum growth rate factor. Reduce this value if you encounter convergence problems.

Force and Moment Tolerance

Specifies the convergence tolerance for the forces and moments. This value determines how much the forces and moments can vary before the next motion step is performed.

Maximum Inner Time-Steps

Specifies the maximum number of inner time-steps between two motion steps.

• If the Force and Moment Tolerance convergence tolerance is met during a time-step number that is lower than the Maximum Inner Time-Steps, the next motion step is performed.
• If the Force and Moment Tolerance convergence criterion is not met during a time-step number that is lower than the Maximum Inner Time-Steps, a motion step is performed after the Maximum Inner Time-Steps are reached.

When applying the body motion Equilibrium, a DFBI Equilibrium Forces Converged Event becomes available. You use this event to monitor the time when the equilibrium is achieved. For more details, refer to DFBI Equilibrium Forces Converged Event Reference.

Properties

X Motion, Y Motion, Z Motion

When On, translation motion is calculated along the axis with respect to the selected Coordinate System. When Off, the degree of freedom in the selected direction is frozen.

X Rotation, Y Rotation, Z Rotation

When On, rotation around the chosen axis is calculated with respect to the selected Coordinate System. When Off, the degree of freedom in the selected direction is frozen.

Coordinate System

Coordinate system in which X Motion, Y Motion, Z Motion and X Rotation, Y Rotation, Z Rotation are specified.

Any steady Cartesian coordinate system is supported, as well as the unsteady body coordinate systems of the Multi-Body Motion's body. Other unsteady coordinate systems, for example, managed coordinate systems, are presently not supported.

Properties

X Motion

Enables translation along the X-axis of the laboratory coordinate system.

Y Motion

Enables translation along the Y-axis of the laboratory coordinate system.

Z Rotation

Enables rotation about the Z-axis of the laboratory coordinate system, transposed to the center of mass of the 2D 6-DOF body.

Properties

Rotation Axis Origin

Specifies the origin of the rotation axis.

Coordinate System

Determines the coordinate system in which the origin and the direction of the 2D rotation axis are specified.

Properties

Translation Origin

Specifies the origin of the translational motion. This point is treated as the initial body position, which can be different from the initial center of mass.

Translation Direction

Specifies the direction of the translational motion.

Coordinate System

Determines the coordinate system in which the origin and the direction of the 2D translational motion are specified.

Properties

Translation Origin

Specifies the origin of the translational motion. This point is treated as the initial body position, which can be different from the initial center of mass.

Coordinate System

Specifies the coordinate system in which the origin is specified for the axisymmetric translational motion.