Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.
How are global evaluations of the displacement field done in a solid mechanics simulation?
Posted 1 avr. 2026, 03:57 UTC−4 Structural & Acoustics, Results & Visualization Version 6.4 1 Reply
Please login with a confirmed email address before reporting spam
I am doing an eigenfrequency study of a solid mechanics system. I have defined some variables, let's call them A and B, which depend on the "solid.disp" variable from the "Solid Mechanics>Linear Elastic Material" node (that is to say, the mechanical displacement magnitude) and the "displacement field", the dependent variable in the "solid mechanics" node. I then evaluate A and B under a "results>derived values>global evaluations" node. Now my question is, how will A and B be evaluated? The mechanical displacement should change as the mechanical system resonates, right? So is the solid.disp used in the global evaluation the maximum of the displacement magnitude or is it something else?
I am asking because I expect A > B and instead get A < B so maybe I am missunderstanding something about how the variables are evaluated. For both variables, A and B, involves an integral over the same spatial domain and B involves a derivative of the "displacement field".
Please login with a confirmed email address before reporting spam
A first question is whether there is any damping in your model or not. If there is, the situation is more complicated, since the eigemodes are then complex valued.
solid.disp is the norm of the displacement of the eigenmode (at the chosen phase, which by default is zero). As it is a norm, it is always positive.
Usually, this means that it represents the peak displacement (= amplitude) in each point. But for a highly damped problem, the peak displacement can be reached at different phase angles at different locations. In such a situation, the variable solid.disp_rms (RMS of displacement) can be useful since it is phase independent.
The components of the field itself (u, v, w) will in general have different signs at different locations in the structure. It is u, v, and w that may be complex valued.
solid.disp is defined as sqrt(real(u)^2+real(v)^2+real(w)^2)
If you integrate solid.disp, you are integrating a quantity that is always positive. Integrals of (u, v, w) and their derivatives may have arbitrary signs (and may even be complex valued).
A final note: The eigenmode displacement does not have a physical meaning (it is arbitrarily scaled). This is sometimes overlooked when interpreting operations on it. You can choose the scaling in the Output section in the settings for the Eigenvalue Solver node.
-------------------Henrik Sönnerlind
COMSOL
