Select an option from the Expression evaluated for list:
Static solution,
Harmonic perturbation,
Total instantaneous solution,
Average for total solution,
RMS for total solution, or
Peak value for total solution. Each option is described below.
If Harmonic perturbation is selected, the
Compute differential check box is also available. If the check box is not selected, the expression is evaluated by taking the values of any dependent variables from the harmonic perturbation part of the solution.
If the Compute differential check box is selected (the default), the differential of the expression with respect to the perturbation is computed and evaluated at the linearization point. This is achieved by wrapping the expression in the
lindev operator. For expressions that are linear in the solution, the two options are the same.
This is the same as evaluating for Total instantaneous solution and then averaging over all phases of the harmonic perturbation. This is achieved by wrapping the expression in the
lintotalavg operator.
This is the same as evaluating for Total instantaneous solution and then taking the RMS over all phases of the harmonic perturbation. This is achieved by wrapping the expression in the
lintotalrms operator.
This is the same as evaluating for Total instantaneous solution and then taking the maximum over all phases of the harmonic perturbation. This is achieved by wrapping the expression in the
lintotalpeak operator.
To illustrate the effect of the different Expression evaluated for settings, consider a prestressed solid mechanics model. The solution has been obtained by first computing a prestressed state, in a Stationary study step, and then adding a small harmonic perturbation on top of the stationary solution, using a Frequency Domain, Perturbation study step. The linearization point solution,
u0, is therefore the stationary displacements from the prestress step, and the harmonic perturbation part of the solution,
up, is the small complex-valued harmonic displacements induced by the harmonic perturbation step. The instantaneous displacements are therefore
A common measure of the equivalent stress level in a solid is the von Mises stress, σe. This is a nonlinear function of the strain and therefore of the displacements. Assuming that the harmonic perturbation is small, the von Mises stress can be linearized around the linearization point
Evaluating using the harmonic perturbation solution with the Compute differential option selected returns the complex-valued amplitude of the linearized change in von Mises stress,
σep, due to the harmonic perturbation. For a perturbation that is small compared to the linearization point, the von Mises stress varies harmonically
Evaluating using the harmonic perturbation solution with the Compute differential option cleared returns the von Mises stress expression evaluated by substituting the complex perturbation solution,
up. This number,
σe(up), is essentially a nonsensical value because it involves products of complex-valued amplitudes.