COMSOL 6.2 - Time Dependent

) study and study step are used when field variables change over time.

For example, in electromagnetics, it is used to compute transient electromagnetic fields, including electromagnetic wave propagation in the time domain. In heat transfer, it is used to compute temperature changes over time. In solid mechanics, it is used to compute the time-varying deformation and motion of solids subject to transient loads. In acoustics, it is used to compute the time-varying propagation of pressure waves. In fluid flow, it is used to compute unsteady flow and pressure fields. In chemical species transport, it is used to compute chemical composition over time. In chemical reactions, it is used to compute the reaction kinetics and the chemical composition of a reacting system.

Selecting a Time Dependent study adds a study step node and sets up a solver with a Time-Dependent Solver. Use this study for a time-dependent or transient simulation using a Time-Dependent Solver for computing the solution over time. Also see The Relationship Between Study Steps and Solver Configurations. You do not need to consider the settings in the Time-Dependent Solver unless the simulation does not behave as expected. For example, the simulation behaves in an unexpected way when changing the end time, in case you may need to adjust the default settings for the time stepping and the tolerances. You may also need to output more time steps and use a strict time stepping and tolerance to fully resolve fast dynamics and to avoid aliasing effects.


	For time-dependent simulations, check that the initial conditions and the boundary conditions match at the start time. If they do not match, the model setup is likely unphysical, and the time-dependent solver can take very small time steps trying to reconcile inconsistent initial values.


	The Physics and Variables Selection, Values of Dependent Variables, and Mesh Selection sections, and the Include geometric nonlinearity check box, Auxiliary sweep settings, and Adaptive mesh refinement settings in the Study Extensions section are described in Common Study Step Settings. There is also detailed information in the Physics and Variables Selection and Values of Dependent Variables sections.

Study Settings

Select a . This option only appears if there are active least-squares objective functions defined in the model. The default is , which means that the time list defined in the field is used. The other possibility is , which means that the time list defined by least-squares objectives is used. If you use the latter possibility, you specify the instead (the default is 0).

Select a from the list (default: s) to use a time unit that is convenient for the time span of the simulation. Then specify the time interval for the output from the simulation in the field using the selected time unit. You can type a monotonically increasing list of individual values, for example, 0 1 2 5 10 20; use the range operator, for example, range(0,0.1,1.5), which (using seconds as the time unit) gives time steps from 0 to 1.5 s with a step size of 0.1 s; or use any combinations of such input.

When plotting the results from a time-dependent simulation, you can choose to plot the solution at any of the times specified in the field. You can also plot an interpolated solution at any intermediate time. The interpolation used between times is a cubic Hermite spline; that is, the interpolation uses both the solution values and their time derivatives at two points: the closest output times before and after the time for which the interpolated solution is computed.

From the list, choose (the default) to use the tolerance suggested by the physics. Choose to override the suggested relative tolerance with a value that you enter in the field. The tolerance settings control the internal time steps taken by the solver, so selecting large time steps for the output times does not affect the accuracy in the time stepping.

Results While Solving

See Results While Solving in the Common Study Step Settings section.

Adaptation

Select from the list to activate time-dependent mesh adaptation. Then choose the geometry in which you want to use adaptive mesh refinement from the list. If you want to use adaptive mesh refinement in only part of that geometry, select the geometric entities for the adaptation in the Geometric Entity Selection for Adaptation that becomes available below this section.

Under , choose (the default) or from the list.

Under , choose a method from the list:

•

(the default)

•

Depending on the choices for these settings, see Adaptive Mesh Refinement (Time-Dependent Adaptation) for additional settings for adaptive mesh refinement in the subnode under the node in the generated solver configuration.

Geometric Entity Selection for Adaptation

From the list, choose the geometric entity on which you want to do adaptive mesh refinement: (the default), , , or (3D only). For example, selecting can be useful if the model includes a physics interface defined on boundaries (surfaces) and you want to base the adaptation on that physics interface. For all levels except , select the geometric entities to include using the list and selection tools below.

Study Extensions

This section contains some optional extensions of the study. The options are mutually exclusive and only one of the check boxes can be selected. See Auxiliary Sweep for its settings. Auxiliary sweeps are not available when the in the section is set to .

Automatic Remeshing

Select the check box if you want the solver to remesh automatically when the quality of the mesh becomes poor in a Time Dependent study. Select the geometry to use for the automatic remeshing from the list. With automatic remeshing active, the solver adds an subnode under the node. In that subnode, you specify the mesh quality expression that determines when to remesh.