Random vibrational analysis (PSD) only supports Rayleigh damping.
The new Direct PSD (DPSD) supports element damping (such as local dampers) as well as frequency-dependent damping (for example in elastomer mounts). It is based on the direct usage of Frequency Response Functions (FRFs) calculated with either:
A mode-superposition harmonic analysis (HROPT, MSUP) using undamped eigensolvers or the QRDAMP eigensolver, or
A full harmonic analysis (HROPT,FULL).
When using mode-superposition FRFs, there is no need to expand the solution. This step is done during the DPSD procedure.
Using full harmonic FRFs is more computationally demanding but is required if frequency-dependent damping (TBDATA with TBFIELD,FREQ) is present.
The FRFs are extracted from files using a DPF Plugin.
Overview of DPSD Analysis
Input PSD
No damping is defined during the DPSD analysis as it is contained in the FRFs.
There are no specifics for the definition of the input PSD table (PSDFRQ, PSDVAL). Only one input PSD is supported.
The types of input PSD ( PSDUNIT) are:
Base excitation: DISP, ACEL, or ACCG when the preceding MSUP harmonic analysis has enforced motion defined (DVAL).
Nodal excitation: FORC, or PRES when the preceding MSUP or full harmonic analysis has force or pressure load.
Output Results
The Response PSD (RPSDs) are calculated as well as the 1-sigma for the nodal displacement solution. They are stored in two different results files.
If node scoping is used (OUTRES,NSOL with Cname component based on nodes), the solutions are output at the scoped nodes only.
The type of output solution is driven by the PSDRES command. It can be displacement (DISP), velocity (VELO), or acceleration (ACEL).
For base excitation, the absolute/relative result specification is inherited from the MSUP harmonic analysis. It is specified with KeyCal on the DVAL command.