Use a Model Reduction study node (
) to create a reduced-order model (ROM) based on a time-dependent or frequency-domain simulation (see
Reduced-Order Modeling). Reduced-order models are usually thought of as computationally inexpensive mathematical representations that provide the ability to run faster simulations using a small model that captures the dynamic behavior of the original model.
The creation of a reduced-order model can typically be divided into two steps: production of training data and model building. The resulting model can then be used for repeated simulations. When using the Modal method, the training data consists of solution vectors computed by some other study step or steps. If these study steps are located in a different source study, then the Model Reduction study step assumes that the source study has been computed and that updated solution vectors can be found at the position corresponding to the training study step in an enabled solver configuration. When the training study steps are part of the same study sequence, that is, precede, the Model Reduction step, the training data will be automatically recomputed each time the reduced-order model is rebuilt.
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To add a Model Reduction node, first select Reduced-Order Modeling in the Show More Options dialog box.
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There can only be one Model Reduction node in a study. When you copy a Model Reduction node, it is possible to paste it into another study without a Model Reduction node.
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Click the Compute button (
) (or press F8) at the top of the
Settings window to produce an instance of the reduced-order model under
Global Definitions>Reduced-Order Modeling.
The Settings window contains the following sections:
From the Method list, choose one of the following model-reduction methods:
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The Modal method (the default) supports inputs and outputs and makes it possible to export ROM matrices.
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The AWE (asymptotic waveform evaluation) method is an alternative reduced-order model that provides a fast frequency sweep (an advanced interpolation) and supports outputs only.
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From the Training study for eigenmodes list, choose an existing study for the eigenmode basis functions or choose
None.
From the Study step for eigenmodes list, choose
Automatic (the default) to use the last applicable solution in the solver sequence, or select any of the applicable study steps (such as
Eigenvalue).
From the Training study for constraint modes list, choose an existing study for the constraint basis functions or choose
None.
From the Study step for constraint modes list, choose
Automatic (the default) to use the last applicable solution in the solver sequence, or select any of the applicable study steps (such as
Stationary).
From the Unreduced model study list, choose a study that contains at least one study step of a type that can be reduced using the selected model-reduction method, or the parent study of the current Model Reduction node. When another study is selected, select an applicable study step from the
Defined by study step list.
From the Reduced-order model list, choose
New to create a new reduced-order model, or choose any existing and compatible reduced-order model (available under
Global Definitions>Reduced-Order Modeling).
If the model-reduction method is Modal, select the
Ensure reconstruction capability check box to enable reconstruction of the unreduced solution vector also when there are only linear outputs defined. For the
AWE method, reconstruction is always enabled. The reduced-order model can then also assign reconstructed values to some of the dependent variables not solved for. This is controlled by the table with
Reconstruction and
Reduced-order model columns in the
Physics and Variables Selection section in the destination study. There is a row for each physics interface that is not solved whose dependent variables can be reconstructed by at least one reduced-order model. The
Reconstruction column shows the physics interface name. The list in the
Reduced-order model column determines which reduced-order model (if any) should be asked to reconstruct the fields for this physics interface.
For the Modal method, select the
Store reduced matrices check box to store the reduced matrices from the model reduction in the solution data for exporting state-space matrices, for example.
The Use extra Compile Equations for Results check box is selected by default. This setting controls whether an extra, active
Compile Equations node will be added last in the default solver sequence. The extra
Compile Equations step is equivalent to an
Update Solution call for the unreduced model study step. Most importantly, it includes the newly created reduced-order model feature in the final solution output by the study so that the reduced-order model’s outputs can be evaluated directly using this solution. The extra
Compile Equation node is not necessary when the reduced-order model will only be called from some other study. In particular, when multiple reduced-order models are created in a sweep, it can be removed to save computation time.
For the AWE method, enter a value for the
Relative tolerance for adaptation (default: 0.01).
In this section, you define the model control inputs when you use the Modal method. The
Model Control Inputs table consists of three columns:
Reduced-model input,
Use, and
Training expression. The
Reduced-model input column shows all the variables defined in the
Global Reduced-Model Inputs node under
Global Definitions. When the variable is added to the
Global Reduced-Model Inputs it is automatically added to the
Model Control Inputs table. The
Use column controls which of the defined variables that should be used. In the
Training expression column, enter a training expression around which the model reduction will linearize the model. Note that the training expression value is not passed on to the training study steps: if their solution depends on reduced-order model input variables they have to be recomputed manually when changing the training value.
In the Outputs section, add outputs for the reduced-order model. You can add output variables by clicking the
Add Expression (
) and
Replace Expression (
) buttons to search through a list of predefined expressions. If you do not add a name in the
Variable column, the output is assigned a default variable name in the created reduced-order model.