6-DOF Solver Reference

6-DOF Solver Properties

Verbosity

Provides additional output in the Output window. The options are:

• None: No additional output.
• Low: Provides output per iteration. Available only for multi-body motion.

  The output is the normalized drift from the constraint equations Eqn. (4934) for each joint that is defined as part of a multi-body simulation. For more information, see Constraint Stabilization.

  • Spherical Joint: Drift in position.
  • Revolute Joint: Drift in position and the angular drift.
• High: Provides the same output as the option Low, but per sub-step. Available only for multi-body motion. See also: Sub-Stepping Enabled.
• Diagnostics: Provides output of listed regions and boundaries associated with the 6-DOF body motion and move with it.

Maximum Number of Iterations

Specifies the maximum number of iterations within a time-step.

6- DOF Solver Expert Properties

Solver Frozen

When On, the solver does not update any quantity during an iteration. It is Off by default. This is a debugging option that can result in non-recoverable errors and wrong solutions due to missing storage. See Finite Volume Solvers Reference for details.

Sub-Stepping Enabled

Activates or deactivates the application of a smaller time-step for the solution of external forces on a 6-DOF body. This option can be activated to overcome convergence problems due to strong non-linear external forces such as catenary forces. This option must not be used generally, but only to enhance stability for these special cases. With sub-stepping activated, the fluid mesh and the fluid forces are updated in each inner iteration and not sub-step-wise.

Sub-stepping is presently only available for the Body Motion Option Free Motion and Multi-Body Motion (see 6-DOF Bodies Reference).

• Activated: Sub-stepping is activated. A Sub-Stepping sub-node is created where you specify the Number of Sub-Steps during a time-step. If you specify a large number of sub-steps, you are advised to increase the number of inner iterations to ensure that the sub-steps converge.

Multi-Body Constraint Stabilization

Activates the Baumgarte constraint stabilization. The constraint stabilization is required to stabilize the kinematic constraints that are imposed by a joint between two 6-DOF bodies.

By default this option is always activated. It is only applicable for multi-body motion simulations with multi-body joint couplings. For other simulation types, this option is ignored.

You specify the coefficients of the Baumgarte constraint stabilization in the Baumgarte Constraint Stabilization sub-node. For more details, refer to Baumgarte Constraint Stabilization Properties.

Legacy Force Update

Activates the update scheme that was used before Simcenter STAR-CCM+ 12.02. In this previous update scheme, forces, moments, couplings, and constraints were updated at the end of each inner iteration, but not additionally at the end of the time-step, except for the case that the number of inner iterations exceeded the maximum number of 6-DOF iterations.

When deactivated (default), forces, moments, couplings, and constraints are always updated at the end of the time-step, in addition to the updates during the inner iterations.

Baumgarte Constraint Stabilization Properties

Alpha, Beta

$\alpha$ and $\beta$ parameters of the Baumgarte constraint stabilization method in Eqn. (4943). The parameter $\alpha$ has a damping effect whereas the parameter $\beta$ corresponds to a stiffness. Typical values for both parameters range between 1–30. If you set the parameters to overly large values, the simulation can become unstable, especially if combined with a large time-step.

Setting $\alpha =\beta$ corresponds to critical damping which means that constraint deviations decay most rapidly. However, large values of $\alpha$ can lead to energy losses due to damping. Therefore, the default values use a small $\alpha$ in combination with a larger $\beta$ to stabilize the constraints effectively and to improve the energy conservation.

Multi-Body Constraint Solver Properties

The Multi-Body Constraint Solver node appears when at least one 6-DOF body is associated with multi-body motion. The expert properties that you adjust here are parameters of an SOR (Successive Over-Relaxation) Gauss-Seidel algorithm. The SOR Gauss-Seidel algorithm solves the constraint equations that are necessary for freezing motion components when setting Multi-Body Motion properties. The solver is not required for the numerical solution of the constraint equations for the mechanical joints, which have their own specialized solver.

Verbosity

Provides additional output in the Output window to monitor convergence of the multi-body constraint solver (SOR Gauss-Seidel algorithm).

Maximum Number of Iterations

For each inner iteration of the 6-DOF solver, specifies the maximum number of iterations allowed for the SOR Gauss-Seidel algorithm.

Residual Tolerance

The SOR Gauss-Seidel algorithm stops iterating when either the specified Residual Tolerance or the Maximum Number of Iterations are reached.

Successive Overrelaxation Factor

Factor to enhance convergence of the SOR Gauss-Seidel algorithm. Assign values between 1 and 2 (1.5 is the default). Reduce below 1 when you must stabilize the numerical solution.

Redundant Constraint Detection

Automatically identifies and removes redundant constraint equations of the multi-body system. Redundant equations occur when different joints and constraints contribute the same constraint equations. Redundant equations increase the complexity of the overall DFBI equation system and can lead to divergence of the constraint solver or intolerably large constraint deviation.

The possible options for detecting and removing redundant constraints are:

• Tight (default): Detects and removes only those constraints that are very close to redundancy. This option is sufficient for most of the cases.
• Relaxed: Detects and removes also constraints showing a strong tendency to approach redundancy. Use this option to overcome redundancies occurring intermittently during the course of the simulation.
• Loose: Detects and removes also constraints showing a weak tendency to approach redundancy. Use this option to overcome redundancies occurring intermittently during the course of the simulation.

A typical sign of an intermittent redundancy is a very large 6-DOF constraint deviation value $(\gg {10}^{-3})$ and the value increases rapidly. An intermittent redundancy may even disassemble the multi-body system.

In case of an intermittent redundancy, a warning message appears in the output window:

To resolve this problem, you set the option to Relaxed or Loose.