Using Weak Constraints to Evaluate Reaction Forces

In the section, Select from the list in order to compute accurate distributed reactions. Extra variables that correspond to the reaction traction distribution are automatically added to the solution components.

With weak constraints activated, COMSOL Multiphysics adds the reaction forces to the solution components. The variables are denoted X_lm, where X is the name of the constrained degree of freedom (as, for example, u_lm and v_lm). The extension lm stands for Lagrange multipliers. It is only possible to evaluate reaction forces on constrained boundaries in the directions of the constraints.


	To compute the total reaction force on a boundary, integrate one of the variables X_lm using Volume Integration, Surface Integration, or Line Integration under Derived Values or under an Evaluation Group node. If the constraint is defined in a local coordinate system, the degrees of freedom for the weak constraint variables are defined along the directions of that system.

Since the reaction force variables are added to the solution components, the number of DOFs for the model increases slightly, depending on the mesh size for the boundaries in question. Boundaries that are adjacent to each other must have the same constraint settings. The reason for this is that adjacent boundaries share a common node.

Using weak constraints affects the structure of the equation system to be solved, and is not suitable for all types of equation solvers.

In the COMSOL Multiphysics Reference Manual:

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Weak Constraints

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Constraint Settings

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Derived Values, Evaluation Groups, and Tables

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Symmetric and Nonsymmetric Constraints