The Percent memory allocation on the Analysis Parameters dialog box controls how much of the available RAM is used to read the element data and to assemble the matrices. (When the value is less than or equal to 100%, the available physical memory is used. When the value of this input is greater than 100%, the memory allocation uses available physical and virtual memory.)
Define Power Spectrum Densities
The power spectrum density is defined in the Random Vibration Analysis Input dialog box which is accessed by pressing the Analysis Data button on the Analysis Parameters dialog box.
Since the Random Vibration analysis uses the results from a modal analysis, specify which design scenario in the current model has the modal results with the Use modal results from Design Scenario field.
Note: The Model Units of the random vibration analysis must be identical to the Model Units of the modal results.
Then you must specify the type of PSD that is defined by selecting the appropriate radio button in the Input Power Spectrum Density (PSD) Type section. Next you must define a damping ratio in the Damping Ratio field. Next you must define the global direction of the PSD by selecting the appropriate radio button in the Excitation Type section. Finally you can specify the coordinates that define the PSD using the Frequency (Hz) and Amplitude columns. You can also import a PSD from a csv file by pressing the Import button. At least four points are necessary to bound the PSD.
Tip: The values in the comma-separated file need to match the PSD type and the active Display Units. Change the Display Units if necessary before importing the data. For example, a value of 314 is imported as 314 in
2
/sec per Hz if the Display Units are in English (in) when the PSD type is Accel Squared/Hz vs. Freq (Hz), and 314 m
2
/sec per Hz if the Display Units are in metric SI.
Control How Results Calculate
There are two types of calculations that can be performed during the random vibrations analysis. This is specified by selecting the appropriate radio button in the Modal Spectral Matrix Calculation section.
- The Approximation Method button neglects the cross-mode effect and uses a closed-form integration results with piece-wise constant PSD.
- The Std. Numerical Integration Method radio button includes the cross-mode effect if the None option is selected in the adjacent drop-down menu. If you do not want the cross-mode effect included, select the Neglect Cross-Mode Effect option. If you want the power spectrum density to be output as a result in the Results environment, select the Power Spectrum Density option. (To view the result, see the paragraph Power Spectrum Density on the page Results: Results Environment: Results Menu: Linear Results Menu for details.)
Note:
- When cross-mode effects are included, the sign of the participation factors (given in the modal analysis log file) and spectral matrix factors (given in the random vibration summary file) can have an effect on the results. Although the magnitudes of these factors are the same, combining terms of opposite signs (negative and positive) can change the results. Of course, the degree of the effect depends on the magnitudes of the factors.
- The signs of the factors are dependent on whether the mode shape is positive or negative, and this is an uncontrollable outcome of the solver used during the modal analysis.
If you activate the Cluster check box, the modal affects of similar frequencies is added directly. If the difference between consecutive frequencies relative to the first of the frequencies is less than the value specified in the Cluster field, the effects are combined.
Control Data in Output Files
Before the analysis is performed, you can indicate to output additional results. Use the Output Controls section of the Analysis Parameters dialog box to indicate what results to write out.
- Activate the Displacement data check box or the Stress data check box to get the relevant results in the summary file or stress text file, respectively. Since this output can be quite voluminous, and the results available in the Results environment are not affected by these settings, it may be better to use the Results environment if only selected results are needed.
- Activate the Stress/strain at midside nodes (binary) check box to get the stress and strain at midside nodes output to the binary result files. These results can then be displayed in the Results environment. (Midside nodes are an option that can be activated for certain element types using the Element Definition dialog box.)