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*DYNAFREQ
Data format:
A single line with5 parameters, as follows:
OMEG_INI OMEG_END D_OMEG CONV DAMP
Status:
Optional.
Purpose:
To perform a frequency-domain dynamic analysis based on the modes of structure.
Restriction:
If a frequency analysis is requested, a modal one must be previously defined (see *MODAL).
Frequency analysis on random waves is not yet implemented.
Details :
| Parameter | Description |
|---|---|
| OMEG_INI | Initial angular frequency (rad/s) |
| OMEG_ENDI | Final angular frequency (rad/s) |
| D_OMEG | Angular frequency step (rad/s) |
| CONV | Convergence criterion (Default = 1E-06) for the iterative procedure dealing with the nonlinear damping of the Morison drag forces. |
| DAMP | Constant damping rate (the value given in *MAT is not taken into account for the frequency analysis) |
Note
- At a given frequency, the complex solution of the equations of motion is projected on an appropriately selected subspace of eigenmodes :
\(X(\omega) = X_{stat} + x(\omega) = X_{stat} + \left\{\sum_{imp}\overline{a}_{imp} \overline{x}_{imp} + \sum_{\lambda} \overline{a}_{\lambda} X_{\lambda} \right\}e^{j\omega t}\)
where ximp are the prescribed motions and xl the eigenvectors.
In order to define the chosen subspace of eigenmodes, a modal analysis is then required (see *MODAL). The choice of the mode must be coherent versus DYNAFREQ values After projection, The goal is to calculate the complex coefficients al through an iterative procedure. The procedure is stopped when :
\(\Vert \overline{a}^k_{\lambda} - \overline{a}^{k - 1}_{\lambda} \Vert \leq SEUIL \Vert a^k_{\lambda}\)
Examples : *DYNAFREQ 0.1 1.5 0.1 1.e-6 0. (between 0.1 and 1.5 rad/sec by step of 0.1 rad/sec)
