In COMSOL 4.3b, the contact pair conditions were changed to improve manual tuning of the augmented Lagrangian contact formulation.
In the augmented Lagrangian method, the penalty factor controls how “hard” the interface surface is during the iterations, but it does not directly affect the converged results. You can consider the penalty factor as a spring giving a resisting force if the boundaries in the contact pair have an interference. A larger value gives faster, but less stable, convergence to the state where there is no interference. A too large value can create convergence difficulties, particularly if the interference is large.
Manual tuning of the contact pair condition
There are now three settings for the Penalty factor control: Preset, Manual tuning and User defined.
If Preset is selected there are two presets for the Penalty factor control in the Augmented Lagrangian formulation: Stability and Speed. If the contacting parts are in contact at the start of the simulation, Speed is the preferred setting.
By setting the Penalty factor control to Manual tuning you can manually fine-tune the parameters used in the Penalty factor.
- The Penalty factor multiplier, , is used to scale the penalty factor to an appropriate value. Increasing this value gives a larger penalty factor.
- Use relaxation has the settings Always, Never and Conditional. When using relaxation the penalty factor is decreased during the first iterations in each parameter step. In this way convergence problems caused by very high reaction forces due to large interferences can be avoided.
- The Initial relaxation factor, , is used to avoid large penalty factors. This factor multiplies the penalty factor in the first iteration.
- The Number of iterations with relaxation setting, , determines how many iterations it will take to reach the full contact factor. The full value of the penalty factor will be used in iteration .
- If Use relaxation is set to Conditional you will have to define a Suppression criterion. This is a Boolean expression which suppresses relaxation when fulfilled. You could, for instance, use an expression like
load_parameter > 0.5to only use relaxation for the values of the load parameter smaller than 0.5. This is useful if you know that there will be large interferences only during part of your simulation.
- The User defined option allows you to define your own expression for the Penalty factor. This option allows you to create any type of expression for the penalty factor.
- It is also possible to add a Cutback criterion. At times the solver will spend a lot of time iterating on an incorrect solution from which it is impossible get back to the true solution path. The cutback criterion lets you create an arbitrary Boolean expression which will force the solver to immediately stop iterating and instead resume the simulation with a shorter parameter step.
For general guidance on contact modeling, see Knowledgebase Article 1102
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