Using Predefined Variables to Evaluate Reaction Forces
The results analysis capabilities include easy access to the reaction forces and moments. They are available as predefined variables. The reaction force variables are available only at the nodes, and not as a continuous field, so they are not suitable for graphic presentation.
To compute the sum of the reaction forces over a region, use Volume Integration, Surface Integration, or Line Integration under Results>Derived Values. The integration method discovers that the reaction forces are discrete values and applies a summation instead of an integration.
If you create an integration operator under Component>Definitions>Component Couplings>Integration to sum reaction forces, you must explicitly set Method to Summation over nodes.
Reaction forces are computed as the sum of the nodal values over the selected volume, face, or edge. Reaction moments are calculated as the sum of the moment from the reaction forces with respect to a reference point, and any explicit reaction moments (if there are rotational degrees of freedom).
Specify the default coordinates of the Reference Point for Moment Computation at the top level of the Settings window for the physics interface. After editing the reference point coordinates, you need to right-click the Study node and select Update Solution for the change to take effect on the reaction moment calculation. During postprocessing, you can modify the coordinates of the reference point in the Parameters section of a result feature..
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Reaction forces are not available for eigenfrequency analysis or when weak constraints are used.
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Reaction force variables are computed where there are constraints, that is Dirichlet conditions like Fixed Constraint, or Prescribed Displacement. Reaction force variables are found in the Reactions folder in the result menus, and have names like solid.RFx or solid.RMz for an x-directed force and a moment around the z-axis respectively.
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From the physical point of view, there can be other sources of reaction forces, such as the elastic and viscous forces from a Spring Foundation. Such forces are not included in the reaction force variables, but are placed in separate variables. You will find them in the Spring and damping forces folder in the result menus.
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Total reaction force variables are available. In these variables, reaction forces, spring forces, and damping forces have been integrated over the whole physics interface. These variables are found in the Reactions folder in the result menus, and have names like solid.RFtotalx or solid.RMtotalz for an x-directed force and a moment around the z-axis respectively. These variables should thus, for a stationary analysis, be equal to the total applied load.
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If reaction forces are summed independently for two adjacent boundaries, the total sum is not the same as if the reaction forces were summed for both boundaries in one operation. The values of the nodes at the common edge always contain contributions from the elements at both sides of the edge.
Derived Values, Evaluation Groups, and Tables in the COMSOL Multiphysics Reference Manual