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One study with an Eigenfrequency study step for computing the eigenfrequencies and corresponding eigenmodes.
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One study with a Frequency Domain study step. This study can mainly be considered as a placeholder which is a mandatory input to the model reduction.
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One study with a Model Reduction study step, in which the ROM is created. It references the two previous studies. In the common case that you have already computed the eigenmodes, you can change the setting of the Training study in this node to point to the old eigenfrequency study, and then delete the newly generated one.
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A Global Reduced Model Inputs node in which you define the control parameters for the ROM. All loads that represents random excitations must have a value multiplied by one of such parameters.
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A Frequency Domain, Modal Reduced-Order Model node. This is a placeholder for the ROM to be created.
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A Random Vibration node. Here, you prescribe the PSD functions and the cross-correlation functions, if needed. They can be function of frequency only (the internal variable freq); it is not possible to prescribe a coordinate dependency.
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1
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Add a Random Vibration (PSD) study.
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2
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In the Global Reduced Model Inputs node, add all required control parameters. The values that you assign to the parameters are not important. Control parameters are used in the same manner as ordinary parameters in expressions for the loads in the physics interface. You need as many control parameters as you have different PSD functions.
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4
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If you want any scalar outputs, define them as Variables under Definitions in the component. You can use, for example, probes or functions like at3() to create scalar outputs. It is also possible to compute scalar results using, for example, Point Evaluation during result presentation. Such evaluations will however require a larger computational effort than scalar outputs that are part of the ROM definition.
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In case you want to reuse a previously computed eigenfrequency study, go to the Model Reduction study step. Change the settings for Training study in the Model Reduction Settings section to point to the correct eigenfrequency study.
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6
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Run the study containing the Model Reduction study step. It will automatically create a ROM that can be used for all further evaluations.
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7
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Define the PSD and cross-correlation functions required for representing the loading as functions under Global Definitions. These functions should depend on frequency only.
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It is common that an input PSD is provided in terms of straight lines in a log-log diagram of PSD value versus frequency, f. To mimic this behavior, you can use an interpolation function (say int1) where you enter pairs of log(f) and log(PSD) values. You then reference this function through an expression like exp(int1(log(freq))). This will provide a linear interpolation in a log-log space.
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10
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If the correlation type is Cross-correlated, enter also the off-diagonal part of the Correlation matrix.
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To make these settings available for result evaluation, run Update Solution for the model reduction study.
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