Warning: New Constraint Force Nodes Detected

Problem Description

I get this error message: Warning: New constraint force nodes detected. What does it mean?

Solution

As is indicated in the message, this is a warning message and not an error message, meaning that your model may be just fine. The message "New constraint force nodes detected: these are not stored" means that the solver detects the appearance of a new constraint force during the solution of a parametric or time-dependent problem, or a nonlinear study. This is nothing to worry about unless you plan to use reaction forces for visualization or postprocessing.

As an example, consider the situation where the following nonlinearity is added to a constraint in a time dependent diffusion problem

c = c_0 + (t>t_0)*T

Here, the concentration starts to depend on the temperature (which is a dependent variable) at t = t_0.

You can get rid of the warning message as follows:

1. Try to identify which constraint you think could be causing the new nonlinearity (for example a Concentration feature if you are working with mass transport), and click on this constraint feature in the Model Builder.

2. At the top of the model builder, click the Show button and make sure that Advanced Physics Interface Options is selected.

3. This will allow you to see the Constraint Settings section in the constraint feature discussed here. Expand this section and make sure that the Use Weak Constraints option is selected.

Please note that you sometimes can run into problems when switching to Weak Constraints. As an example, Iterative solvers may run into problems.

It is also not possible to have a boundary using Weak Constraints adjacent to one that does not have the checkbox selected. This may force you to enable Weak Constraints on more boundaries than you initially intended.

Some physics interfaces contain built-in nonlinear constraints. It is for example common to get this warning in a geometrically nonlinear analysis with the Shell interface.

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