Using a Solution From Previous Study Steps

It is sometimes useful to run one simulation for one aspect of a model and then use the output of the first simulation as input into a second simulation, covering a different aspect. You can perform such a sequential study using multiple study steps in a single study or using multiple studies in the same model:

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Use two or more study steps in a sequence in the same study if you, for example, want to use a stationary or eigenvalue solution as the initial value for a time-dependent solution. You then add an Eigenvalue or Stationary study step node followed by a Time Dependent study step node. By default, the COMSOL software determines the values of the dependent variables in the fields for the physics heuristically depending on the model, so normally you do not have to take specific action. To specify the initial values of the dependent variables that you solve for, select the Initial values of variables solved for check box in the Values of Dependent Variables section. Then use the Method list to specify how to compute the initial values. Choose Solution and then select the solution to the stationary or eigenvalue problem solved using the previous study step. Doing so can be useful to specify a specific eigensolution to use as the initial value, for example.

Another case is when you want to solve for two different physics interfaces, one at the time, but use the solution from the first study as input data for the physics interface in the second study. You then select the physics interface to solve for and the physics interface to disable in the solver under Physics and Variables Selection in the Settings windows for each study step. Typically you solve for one physics interface (or set of physics) in the first study step and for the other physics interface in the other study step (you can also use two separate studies). To specify the values of dependent variables that you do not solve for, select the Values of variables not solved for check box in the Values of Dependent Variables section. Then use the Method list to specify how to compute the values of variables not solved for. Choose Solution and then select the study step and solution to use.

When a model modification is required in between the simulations, you can use two study steps where you use the setting available when you select the Modify physics tree and variables for study step check box to control what physics interface the study step solves (see Selecting Variables and Physics Nodes to Include); for example, if you want to change a boundary condition or expressions for the initial value. You then create two such nodes as separate analysis cases and use one in the first study step and the other in the second study step.

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An example when it is more practical to work with separate studies is when a study is inherently a multistep study, such as buckling in structural mechanics or modal analysis. Extending such studies with extra study steps normally becomes unnecessarily complex. A third example is when doing optimization, where the problem without optimization often needs to be solved and tested separately from a study with optimization enabled.

See Values of Dependent Variables above for details about the settings for controlling the values of the dependent variables that you solve for or exclude from the solvers. Also see Physics and Variables Selection below for information about controlling which physics interface to solve for.