Linear in the COMSOL Multiphysics Programming Reference Manual
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Number of iterations. The default is 2.
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Select a Multigrid cycle: V-cycle (the default), W-cycle, or F-cycle. For Multigrid cycle, the settings are the same as for the geometric multigrid (GMG) and algebraic multigrid (AMG) solvers.
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Enter the Number of multigrid levels to generate (the default is 1 for Geometric multigrid and 5 for Algebraic multigrid).
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Lower element order first (any). The default. Generates first a multigrid level by lowering the order (by one) of any of the used shape functions. If there are no shape functions that can be lowered, the mesh is coarsened.
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Coarsen mesh and lower order. Combines lowering of the used shape function order and a coarsening of the mesh.
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Lower element order first (all). Generates a multigrid level by lowering the order (by one) of all the used shape functions. If this is not possible, the mesh is coarsened.
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Coarsen mesh. Does not change the order.
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Lower element order and refine (all). Generates a multigrid level by lowering the order (by one) of all the used shape functions. If this is not possible, the mesh is refined a number of times. The mesh solved for can, with this method, be a finer one than the one selected under the study node.
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Lower element order and refine (any). Generates a multigrid level by lowering the order (by one) of any of the used shape functions. If there are no shape functions that can be lowered, the mesh is refined. The mesh solved for can, with this method, be a finer one than the one selected under the Study node.
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Refine mesh. Does not change the order.
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Manual. Use this setting to select a multigrid level from the existing ones. You then specify the multigrid level to use in the Use multigrid level list. Use the Move Up (), Move Down (), Delete (), and Add () buttons to configure the list of multigrid levels. Use the Assemble on coarse level check box to assemble the discrete differential operators on the coarse multigrid level (selected by default).
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In the Use hierarchy in geometries list, select the geometries to apply the multigrid level to. Use the Move Up (), Move Down (), Delete (), and Add () buttons to configure the list of geometries.
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The Assemble on all levels check box is selected by default to assemble the discrete differential operators. Otherwise, these operators are formed using the restriction and prolongation operators. Click to clear the check box as needed.
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If None is selected, no coarse mesh is used in addition to the fine mesh. This can lead to severe reduction in convergence rate but saves memory.
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Enter a Maximum number of DOFs at coarsest level. The default is 5000. Coarse levels are added until the number of DOFs at the coarsest level is less than the max DOFs at coarsest level or until it has reached the number of multigrid levels.
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Enter a value or use the slider to set the Quality of multigrid hierarchy. Higher quality means faster convergence at the expense of a more time consuming setup phase. For instance, if the linear solver does not converge or if it uses too many iterations, try a higher value to increase the accuracy in each iteration, meaning fewer iterations. If the algebraic multigrid algorithm runs into memory problems, try a lower value to use less memory. The range goes from 1 to 10, where 10 gives the best quality. The default is 3.
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The Lower element order first (any) check box is selected by default. This setting provides the combination of GMG with lower order until order 1 is reached and then uses AMG to generate the coarser levels. The Assemble on the order-lowered levels check box, which is selected by default, then corresponds to the GMG option top assemble on all level. Using this setting is equivalent to using GMG with AMG as a coarse grid solver.
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Enter a Maximum number of DOFs at coarsest level. The default is 5000. Coarse levels are added until the number of DOFs at the coarsest level is less than the max DOFs at coarsest level or until it has reached the number of multigrid levels.
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The aggregation algorithm is based on a connection criterion, which you specify as a coefficient in the Strength of connections field. A node j is connected to another node i, if where ε is the strength of connection coefficient, and Aij is the submatrix of the stiffness matrix defined by the degrees of freedoms on node i and j, respectively. Loosely speaking, the strength of connection value determines how strongly the aggregation should follow the direction of anisotropy in the problem. The default value is 0.01.
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From the Null-space vectors list, choose Constant (the default) or Rigid body modes. For linear elasticity problems, always select Rigid body modes because it enhances the convergence properties significantly.
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Select the Construct prolongators componentwise check box for CFD applications and other not strongly coupled physics. It is selected by default in predefined solver suggestions for CFD. This setting is only available when the Null-space vectors settings is set to Constant.
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The Compact aggregation check box is selected by default to use an aggregation algorithm that forms, on average, smaller aggregates, which leads to a less rapid coarsening.
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By default, the Reuse prolongators check box is selected to avoid computing the prolongator matrix P when an old prolongator can be reused.
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Choose how to control the prolongator smoothing using the Smoothing list, which is active when the Prolongator smoothing check box (selected by default) is selected. The Auto option postpones the smoothing for sdim-1 levels, where sdim is the space dimension of the problem. If you choose Manual, enter the level to start smoothing at in the Start smoothing at multigrid level field.
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The final transfer operator, P, between the fine and coarse problems are smoothed by one application of Jacobi smoothing:
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By default, the Use filtering check box is selected. Filtering means that entries in the stiffness matrix have been dropped if they correspond to degrees of freedoms on a node that has no strong connections. Loosely speaking, filtering highlights anisotropy in the problem and results in a sparser coarse level problem.
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The Lower element order first (any) check box is selected by default. This setting provides the combination of GMG with lower order until order 1 is reached and then uses SAAMG to generate the coarser levels. The Assemble on the order-lowered levels check box, which is selected by default, then corresponds to the GMG option top assemble on all level. Using this setting is equivalent to using GMG with SAAMG as a coarse grid solver.
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The Physics from the list.
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Add a weak contribution method from the Add weak contribution list:
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