Body Motion Options

For a 6-DOF body, Simcenter STAR-CCM+ provides several body motion options by which you can specify an efficient motion description for a certain application.

The available body motion options are:

Free Motion

A free motion describes a rigid movement with up to six degrees of freedom for 3D bodies. For 2D bodies, the degrees of freedom are reduced to three. You control which degrees of freedom are permitted, and which ones are frozen.

The free motion option provides a computationally efficient method to freeze inactive degrees of freedom. This method works well for most applications. In the following cases, however, the Multi-Body Motion option should be used instead of free motion:

  • cases involving one rotational degree of freedom which is not aligned to one of the principal axes of inertia
  • cases involving two rotational degrees of freedom

For cases involving two rotational degrees of freedom, you could consider the Four-DOF Maneuvering Motion option, if applicable.

All active degrees of freedom are specified with respect to the laboratory coordinate system. Rotations are defined as positive according to the right-hand convention around each coordinate axis. For the purposes of rotation, the laboratory coordinate system can be considered as though its origin were moved to the center of mass of the 6-DOF body.

For more details of the free motion formulation, refer to the Theory Guide section Equations of Free Motion.

One-DOF Rotating Motion

A one-DOF rotating motion object specifies a body rotation around a fixed axis. This motion is available for both 2D and 3D bodies.

Each one-DOF rotating body can have its own rotation axis. Therefore it is possible to simulate several bodies having different rotation axes (for example, wind turbines in a wind farm).

Optionally, you can use a motion limiter to constrain the angular motion of the body between a minimum and a maximum angle, for example, for valve applications. The body is stopped when a motion limit is reached and the kinetic energy of the body is then destroyed. However, this physical situation is extreme and can have a significant influence on flow field residuals and convergence. Therefore, if a motion limiter is used, preferably apply the default option—a damped motion limiter with which the body stops more smoothly. For more information, refer to the Theory Guide section One-DOF Rotating Motion.

You can also specify the angular damping length over which the body is stopped. The angular damping length is effective if the time step is small enough for the body to make several steps over the damping length.

The axis of rotation must be constant in time. Only use steady coordinate systems in the Coordinate System property; unsteady coordinate systems lead to an unsteady axis of rotation. Body coordinate systems cannot be used, since these coordinate systems also change with time.

To report the rotation angle of the body, use the Rigid Body 6-DOF Body Rotation Angle report. Use the rotation angle report only with bodies undergoing One-DOF Rotating Motion since it reports zero for all other motions. For these other cases, use the 6-DOF Body Orientation report.

One-DOF Translating Motion

A One-DOF Translating Motion object specifies a body translation along a certain direction. This motion is available for both 2D and 3D bodies.

Each one-DOF translating body can have its own direction of motion; it is therefore possible to simulate several bodies having different translation directions.

Optionally, you can use a motion limiter to constrain the translating motion of the body between a minimum and a maximum displacement (for example, for valve applications). The body is stopped when a motion limit is reached and the kinetic energy of the body is then destroyed. However, this physical situation is extreme and can have a significant influence on flow field residuals and convergence. Therefore, if a motion limiter is used, preferably use the default option—a damped motion limiter with which the body stops more smoothly. For more information, refer to the Theory Guide section One-DOF Translating Motion.

You can also specify the damping length over which the body is stopped. The damping length is effective if the time step is small enough for the body to make several steps over the damping length.

The direction of motion must be constant in time. Only use steady coordinate systems in the Coordinate System property; unsteady coordinate systems lead to unsteady directions of motion. Body coordinate systems cannot be used, since these coordinate systems also change with time.

Axisymmetric Translating Motion

Axisymmetric translating motion is a one-DOF translating motion along the axis of symmetry, which is the x-axis. This motion is only available for 2D bodies.

To set up axisymmetric translating motion for 2D bodies, refer to Axisymmetric Translating Motion.

Four-DOF Maneuvering Motion

A Four-DOF Maneuvering Motion specifies the speed and direction of a body in the X-Y plane, while leaving the body free to maneuver in the other four degrees of freedom. This motion is only available for 3D bodies.

Each Four-DOF maneuvering body can have its own speed and direction of motion; it is therefore possible to simulate several bodies having different velocities. For each body, the speed and direction in the X-Y plane is constant in time. The option uses the laboratory coordinate system throughout.

When you run an Orientation report for Four-DOF Maneuvering Motion, use the Rotation X-Y-Z option for the Euler Angle Convention property. See also: 6-DOF Body Orientation.

To set up four-DOF maneuvering motion for 3D bodies, refer to Four-DOF Maneuvering Motion.

Planar Motion Carriage

The Planar Motion Carriage simulates a captive motion in the X-Y plane of the laboratory coordinate system. This mechanism drives the body along a prescribed trajectory in the X-Y plane, while the body is optionally allowed to move freely in the directions of heave, pitch, and roll. This motion is only available for 3D bodies.

There are three Planar Motion methods available:

  • Planar Motion Mechanism— Simulates the captive motion of a rigid body in the X-Y plane of the laboratory coordinate system along a sinusoidal path. The body moves with a constant velocity in the X direction and performs an oscillatory motion in the Y direction [948].
  • Rotating Arm Motion—Simulates the captive motion of a rigid body in the X-Y plane of the laboratory coordinate system in a rotating action. The result is a circular path of the body in the X-Y plane.
  • General Planar Motion—Simulates the captive motion of a rigid body in the X-Y plane of the laboratory coordinate system in a user-defined action. You prescribe a trajectory for the motion in the X-Y plane and a yaw angle. The trajectory and the yaw angle are functions of time. The corresponding heave, pitch and roll motions are computed by Simcenter STAR-CCM+.

The option uses the laboratory coordinate system throughout. The Initial Orientation Coordinate System of the body must be aligned with the laboratory system.

In Planar Motion Carriage, the 6-DOF body property Release Time only applies to the unprescribed degrees of freedom. The prescribed trajectory is imposed from the very beginning of the simulation, regardless of the release time.

The 6-DOF solver moves the body in the prescribed directions from the 1st time step, so that the fluid field can build up during release time including the contribution of the prescribed motion components. At release time, the unprescribed degrees of freedom such as heave, pitch and roll are also enabled.

When you run an Orientation report for the Planar Motion Carriage, use the Rotation X-Y-Z option for the Euler Angle Convention property. See also: 6-DOF Body Orientation.

To set up planar motion carriage for 3D bodies, refer to Planar Motion Carriage.

Equilibrium

The equilibrium body motion helps you find the quasi steady-state equilibrium position of a rigid body that is subject to fluid forces or other external forces. A marine "sink and trim" simulation is an example of such a case. This motion is only available for 3D bodies.

Use this option to position and orient the rigid body in the flow so that the forces and moments acting on it are balanced in the chosen directions. The equilibrium body motion option has up to 6 degrees of freedom. The body moves within the specified directions according to a numerical iterative procedure until the forces and moments in these directions are balanced.

The numerical procedure to move the rigid body is purely mathematical, that is, there is no true physical motion of the rigid body. During the procedure, Simcenter STAR-CCM+ moves the body in a stepwise manner, but does not solve an equation of motion. The body mass only plays a role for the gravity force which might need to be balanced to zero in the equilibrium state.

The numerical procedure for finding the equilibrium position operates iteratively:
  1. At a time-step, the numerical procedure calculates a translation and/or rotation step size.
  2. The rigid body moves (translates and/or rotates) by the calculated translation and/or rotation step size as determined by the numerical procedure. The flow must then adapt to this change in position.
  3. In this new position, inner time-steps are performed that calculate the forces and moments until Simcenter STAR-CCM+ reaches either the Force and Moment Tolerance or the Maximum Inner Time-Steps. (Inner time-steps are regular time-steps. They are called inner time-steps because they apply between incremental movements of the body).

    At this point, Simcenter STAR-CCM+ triggers any DFBI Equilibrium Forces Converged events that are associated with the body. These events in turn cause an update to any plots that depend on them.

  4. The 6-DOF body moves again by a certain distance as determined by the numerical procedure.

This process is repeated until the total sum of forces and the total sum of moments in the specified directions become zero. To reach good convergence of the procedure, forces and moments must converge well during the inner time-steps. You can influence the convergence behavior by changing the control parameters of the equilibrium body motion option. See also: Equilibrium Properties.

In order for you to plot results such as forces and moments over time, Simcenter STAR-CCM+ provides the DFBI Equilibrium Forces Converged event. This update event only triggers when forces and moments are converged within a motion step. Plots that update in response to this event should show a smooth profile throughout the solution time history. For more information, see DFBI Equilibrium Forces Converged Event.

You are advised to use the equilibrium body motion option only if the following requirements are fulfilled:
  • For your DFBI simulation, you aim to reach an equilibrium position of the rigid body. The simulation itself can be dynamic and time-dependent, but your objective is to obtain a quasi steady-state solution.
  • The forces and moments that act on the body depend on spatial position. This dependency is essential for the numerical procedure to work properly. A marine "sink and trim" simulation is an example of such a case.
  • User-defined forces and moments acting on the body do not explicitly depend on time. For example, when modeling springs, forces can implicitly depend on time through position, but not explicitly by defining a direct time-dependency.

To set up equilibrium motion for 3D bodies, refer to Equilibrium.

Multi-Body Motion

Multi-body motion allows you to simulate one or more 6-DOF bodies connected through mechanical joints or with their motions limited by mechanical constraints. It can also be used to compute single body motion, if the motion can not be simulated by free motion. This motion is only available for 3D bodies.

When using multi-body motion, you can limit motion in one or more directions of a specific coordinate system and connect bodies through mechanical joints.

Mechanical joints impose kinematic constraints on the motions of the bodies, which limit the degrees of freedom of the multi-body system. Additionally, you can constrain the multi-body system, for example, by preventing rotation and translation in specific directions.

Consider the following:
  • You can create mechanical joints between rigid bodies for which multi-body motion is activated.
  • Coupling of a rigid body to the environment using mechanical joints is also possible.
  • You can connect bodies with free motion and multi-body motion using couplings such as springs, catenaries, and contacts.
  • You can connect bodies that are coupled using mechanical joints with other couplings such as springs, catenaries, and contacts.
  • When coupling several rigid bodies using joints, the creation of closed loops is supported. For example, you can couple Body 1 to Body 2, then Body 2 to Body 3, and then couple Body 3 back to Body 1 as a closed loop.
  • You can create tree-like structures of bodies that are connected through mechanical joints.

To set up multi-body motion for 3D bodies, refer to Multi-Body Motion.