State Space
The State Space node () provides access to the raw data of a PDE in state-space form. To create state-space data, right-click the solver node and select Other>State Space. Specify the input and output and the state-space matrices and vectors that you want to access. Then save the model as a file for Java®. The saved Java file contains code for assembling the selected matrices and vectors that can be used to access the matrix rows, columns, values, and so on.
Input
In the Input parameters field, enter all parameters that affect the model as space- or comma-separated entries.
Output
The state-space node assembles matrices that describe a model as a dynamic system when Off is selected from the Static list:
If you select On from the Static list, a static linearized model of the system is described by
In the Output expressions field, enter all expressions that are to be evaluated as output from the model as space- or comma-separated entries. Select any of the MA, MB, D, and C check boxes, and if Static is set to Off, any of the MC, Null, ud, or x0 check boxes. Null is the PDE constraint null-space matrix, and ud is a particular solution fulfilling the constraints. x0 is the initial data. The solution vector U for the PDE problem can then be written
where u0 is the linearization point, which is determined by the current solution (that is, the solution computed by the previous feature in the sequence). The previous feature can, for example, be a solver or a Dependent Variable node. The Dependent Variable node gives control over which variables to solver for (compute the matrices for). The input linearization point is stored in the sequence after the state-space node is run.
For output expressions that are defined using time derivatives, the corresponding component in the output matrix C is always 0. In such cases, use the Modal solver instead.
Constants
In this section you can define constants that can be used as temporary constants in the solver. You can use the constants in the model or to define values for internal solver parameters. These constants overrule any previous definition (for example, from Global Definitions). The constant values are expressions and can, for example, include the range() operator, units, and global expressions. The constant name can be a new or an existing global parameter. The constant is temporary in the sense that it is only defined during the solver run. You cannot override parameters used in the following parts of the model:
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Mesh and geometry
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Selections
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Probes
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Functions
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Materials
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Pairs
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Solver sequences
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Parameter-dependent lists
Also, the Parametric and Time Dependent solvers overrule any definition of solver constants.
Constants settings for a solver node do not carry over to postprocessing.
Some examples of when it can be useful to define constants for a solver:
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When doing more advanced solver sequences, where constants need to be changed between calls (for example, in for-loops).
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When you do not want to change the global definition of a parameter or when you cannot or do not want to use a Parametric Solver feature.
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When you want to define auxiliary parameters that are part of the equations like CFLCMP or niterCMP and where the solver does not define these parameters.
Click the Add button () to add a constant and then define its name in the Constant name column and its value (a numerical value or parameter expression) in the Constant value column. By default, any defined parameters are first added as the constant names, but you can change the names to define other constants. Click Delete () to remove the selected constant from the list.
Log
This section, which is initially empty, contains a log from the run of the State Space node. This log is stored in the Model MPH-file. Select the Keep warnings in stored log to keep warning messages in this log so that the information in those warnings is also available when reopening the model.
StateSpace in the COMSOL Multiphysics Programming Reference Manual.