COMSOL 6.3 - Adaptive Mesh Refinement (Time-Dependent Adaptation)

The node (

) is a solver attribute that handles adaptive mesh refinement together with a Time-Dependent Solver or References for the Solution Operation Nodes and Solvers. The adaptive mesh refinement creates multiple meshes for segments of a time-dependent simulation. Also see The Adaptive Solver Algorithms and Error Estimation.

The software performs adaptive mesh refinement in one geometry only. Use the list to specify that geometry. If you have specified the geometry in the time-dependent study step, which appears in the list above, this list is not available. Choose from the list to enable the list.

The following properties appear under :

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The can be controlled manually or automatically. Select (default) or . If is , the algorithm strives to assume the given value for the fraction of maximum refinement (default value: 0.2) by controlling the size of the time interval. If this setting is defined by the study step, the list is not available. A value of zero means no refinement of the base mesh and a value of one means refinement everywhere using the maximum element refinements. The shortening and lengthening of the interval is determined by the interval reduction and growth factors described below.

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The value in the (only available when is ) field (default value: 2.0) determines how the solver increases the time interval length. A value of 2.0 makes the interval length twice as large as the previous interval length when increased.

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By default, the solver determines the (unit: s) automatically (only available when is ) using an interval length that gives a total of 10 intervals. The length of the time interval is the simulation time before a refinement of the mesh takes place. Select the checkbox to specify a user-defined time interval length in the field (default value: 0.1 s).

	If the number of intervals (that is, the simulation time divided by the interval length) exceeds 100, the interval length specified here may not be respected. You can enforce such a large number of intervals by prescribing a minimal interval length in the Minimal interval length field (see below).

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By default, the solver determines the (unit: s) automatically (only available when is ). The length of the initial time interval is the simulation time before the first refinement of the mesh takes place. Select the checkbox to specify a user-defined time interval length in the field (default value: 0.1 s).

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By default, the solver also determines the (unit: s) automatically. The minimal length of the time interval is the shortest possible simulation time without performing a mesh refinement. Click to select the checkbox to specify a user-defined minimal interval length in the field (default value: 0.01 s). The interval length is reduced according to the if the solver fails to converge with the current interval length, in which case the minimal interval length setting can be useful.

The following properties are available under :

Use the list to control how to adaptively refine mesh elements. Select one of these methods:

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, to make the solver refine only the longest edge of an element by recursively bisecting the longest edge of edge elements that need refinement. This method is less suitable for models with nonsimplex elements. This is the default method.

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, to use the current mesh as a starting point and modify it by refinements, coarsening, topology modification, and point smoothing. Use the checkbox (selected by default) to control if mesh coarsening is used. If the mesh contains anisotropic elements (for example, a boundary layer mesh), it is best to disable mesh coarsening to preserve the anisotropic structure. If mesh coarsening is allowed, enter a (default: 5) if needed.

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, to set up a size expression describing the error and rebuild the meshing sequence using the size expression as input. Note that structured meshes, such as mapped and swept meshes, in general are not appropriately refined. This method is not supported on imported meshes. The size of the refined mesh is the minimum of the size of the original mesh (previous refined mesh) and the size defined by the refinement. Specify the (a value of 3 by default) to scale the refined mesh size in the regions where refinement is not needed.

If the adaptation method has been specified in the study step, the list is not available.

For the and adaptive mesh refinement methods, you can specify the maximum number of refinements of the mesh elements (default: 2) in the field. The field is only available if the list (see below) is set to . If the is set to , you can also specify a value between 0 and 1 in the field (default value: 0.2) for these adaptation methods. The value in the field acts as a reference value for a measure of the amount of refinement that the automatic time interval control tries to achieve. In general, a smaller value will lead to shorter adaptation time intervals. See The Adaptive Solver Algorithm — Stationary and Eigenvalue Adaptation for more details on the measure of the amount of refinement. Based on initial data, the value can be increased. If the value is increased this information will be printed in the log. Also, for these adaptation methods, select or clear the checkbox (the default is to not use this conversion).

Use the list to specify how the solver should select which elements to refine. Select:

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to minimize the L2 norm of the error by refining a fraction of the elements with the largest error in such a way that the total number of elements increases roughly by the factor specified in the accompanying field. The default value is 1.7, which means that the number of elements increases by roughly 70%.

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to refine elements whose local error indicator is larger than a given fraction of the largest local error indicator. Use the accompanying field to specify the fraction. The default value is 0.5, which means that the fraction contains the elements with more than 50% of the largest local error.

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to refine a given fraction of the elements. Use the accompanying field to specify the fraction. The default value is 0.5, which means that the solver refines about 50% of the elements with the largest local error indicator.

Select the checkbox if you want to save the solution for all new adaptively refined meshes. If cleared, the adaptive mesh refinement solver will only store solutions and meshes for adaptation time intervals that contain requested output times. This option is useful if you have many time adaptation intervals but are only interested in a few output times.

Use the list to control how the error estimate is computed.

For time-dependent adaptive mesh refinement you need to specify a user-defined error estimate. From list, choose , which is the default and the only option for time-dependent adaptation. You can add error expressions to the table below using the (

) button. The error expression can be any expression, including field variables and their derivatives, defined in the domain. This method will adapt the mesh where the error expression becomes large; the adaptation is typically not optimal. Select the checkbox for the error expressions that should be part of the error indicator. Use the (

), (

), and (

) buttons as needed to rearrange and remove error expressions.