## 20.5. Rigid Body Control Tab

The following options handle control of the rigid body solver.

The Update Frequency setting controls when the rigid body solver updates the position of the rigid body. The options are:

- `Every Iteration` (steady-state simulations only)

  This option causes the rigid body solver to update the position of the rigid body at the beginning of each iteration.

- `Every Time Step` (transient simulations only)

  This option causes the rigid body solver to update the position of the rigid body at the beginning of each time step.

  This option is recommended only for weakly-coupled systems (for example, where fluid flow changes within a single time step have little effect on the rigid body motion) or when the time step is very small.

- `Every Coefficient Loop` (transient simulations only)

  This option:

  - Causes the rigid body solver to update the position of the rigid body at the beginning of each coefficient loop within each time step.

  - Is recommended when there is tight coupling between the fluid flow and the motion of the rigid body.

  - Is more computationally expensive than the `Every Time Step` option

  - Can cause issues if the flow solver requires several coefficient loop iterations to stabilize the applied forces/torques between calls to the rigid body solver.

- `General Coupling Control`

  This option causes the rigid body solver to update the position of the rigid body during each "stagger" (or coupling) iteration.

  During every stagger iteration, the following sequence occurs:

  1. The rigid body solver reads the latest calculated forces and torques.

  2. The rigid body solver solves to determine the new rigid body state (position, orientation, velocity, and so on).

  3. The CFX flow solver reads the new rigid body state.

  4. The CFX flow solver solves all the field equations, including mesh motion where appropriate, by performing outer loop iterations (steady-state cases) or coefficient loop iterations (transient cases) until either the maximum number of iterations/coefficient loops is reached, or until all the field equations have converged. The mesh motion solution uses the latest rigid body state in order to calculate the mesh motion on all boundaries or subdomains set to use the rigid body solution. If the rigid body is an immersed solid, then the latest rigid body state is used to calculate the motion of the immersed solid domain.

  Stagger iterations are repeated until the maximum number of stagger iterations is reached or until the time step (transients runs) or simulation (steady-state runs) is deemed to have converged, when the following criteria are met:

  - The rigid body solver has converged (angular equations only),

  - The force, torque and mesh motion data transferred between the CFX flow solver and the rigid body solver satisfy the appropriate convergence criteria, AND

  - All the CFX field equations (mesh motion, mass and momentum, energy, and so on) have converged to the appropriate convergence criteria.

  When the time step (transient runs) is converged, then this guarantees an implicit solution of all solution fields for each time step, providing the most stability when there is a strong coupling between the fluid flow and the motion of the rigid body.

  The Internal Coupling Step Control settings set limits on the minimum and maximum number of stagger iterations.

When the `Every Iteration`, `Every Coefficient Loop`, or `General Coupling Control` option is selected, the Internal Coupling Data Transfer Control settings are available. These settings include:

- Mesh Motion Data Transfer Control

- Force Data Transfer Control

- Torque Data Transfer Control

For each of these Internal Coupling Data Transfer Control settings, you may:

- Optionally specify an Under Relaxation Factor for aiding convergence. The default value is 0.75 for all three transferred quantities. If a case has difficulty converging due to overshoots in the force, torque and/or mesh motion quantities passed between the two solvers, then the under-relaxation factor for one or more quantities can be reduced to damp down the overshoots. Values that are too small will tend to increase the number of iterations needed for convergence. On the other hand, for a case where the force, torque and mesh motion are steady and do not change much from time step to time step, it may be possible to raise the under-relaxation factors to 1 to speed up convergence.

  Note: The rigid body solver does not apply under-relaxation to Mesh Motion Data Transfer Control in the following situations:

  - For all time steps when Update Frequency is set to `Every Time Step`

  - For the initial coefficient loop when Update Frequency is set to `Every Coefficient Loop`

  - For the initial stagger iteration when Update Frequency is set to `General Coupling Control`

  Note: The rigid body solver also does not apply under-relaxation to Force Data Transfer Control and Torque Data Transfer Control, in the following situation:

  - For all time steps when Update Frequency is set to `Every Time Step`

- Optionally specify a Convergence Target.

  The convergence target is the target value of the convergence measures described below.

  - For Mesh Motion Data Transfer Control, the convergence measure is the maximum of:

    - The distance from the previous to the current center of mass, divided by the greater of the previous and current distances from the center of mass to the rigid body coordinate frame origin. Here, “previous” refers to the previous stagger iteration, coefficient loop, and so on — whenever the rigid body solver was last called.

    - , where , n refers to the current iteration (stagger iteration, coefficient loop, and so on — whenever the rigid body solver was last called), refers to the previous iteration, and is the orientation quaternion, with q0 being the scalar component and q1 , q2 , q3 being the vector components. Here, all quantities are with respect to the rigid body coordinate frame.

  - For Force Data Transfer Control, the convergence measure is:

    The magnitude of the change (from the previous value to the current value) in net force divided by the greater of the previous magnitude of the net force and the current magnitude of the net force, where “previous” refers to the previous stagger iteration, coefficient loop, and so on — whenever the rigid body solver was last called.

  - For Torque Data Transfer Control, the convergence measure is:

    The magnitude of the change (from the previous value to the current value) in net torque divided by the greater of the previous magnitude of the net torque and the current magnitude of the net torque, where “previous” refers to the previous stagger iteration, coefficient loop, and so on — whenever the rigid body solver was last called.

  The convergence measures are plotted on the Rigid Body Convergence tab in CFX-Solver Manager when running a simulation that involves the rigid body solver. For details on the plots available for such runs, see Monitor Plots related to Rigid Bodies in the CFX-Solver Modeling Guide.

The Angular Momentum Equation Control settings are:

- Integration Method

  An iterative process is used to calculate the solution to the angular momentum equation.

  Choose one of the following options to control the integration method for this process:

  - `First Order Backward Euler`

    This option provides first-order accuracy, and is vulnerable to gimbal lock problems. For details, see First Order Backward Euler in the CFX-Solver Theory Guide.

  - `Simo Wong`

    This option provides second-order accuracy. For details, see Simo Wong Algorithm in the CFX-Solver Theory Guide.

    The Iteration Convergence Criterion > Iteration Convergence setting controls the degree to which an orientation must remain the same between successive iterations in order for the solution to be considered converged. The value is a normalized quantity; the default should be sufficient for most purposes.

    The Maximum Number of Iterations > Max. Iterations setting (which controls the maximum number of iterations used by the Simo Wong integration method) can be increased if you see any warnings in the solver output that indicate a lack of convergence for the integration method.

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