Applying VMOPT Method in Femap

[mashst24]

Hello,

I am running a normal modes analysis for a model that incorporates the MFLUID virtual fluid mass, and I would like to use the VMOPT method described in the NX Nastran Advanced Dynamic Analysis User's Guide to decrease memory usage and solution time. I know that I need to specify the parameter VMOPT=2 to separate the computation of the dry and wet modes, but I can't figure out how to do that in Femap. Is it something I specify in the master requests and conditions, and if so can somebody help me understand how to do so?

Thanks for your help!

 

[BlasMolero]

Hello!,
You need to play with PARAM,VMOPT in the NX NASTRAN BULK DATA:

For VMOPT = 0 or 2, mass is added in modal space to the reduced model. The global mass matrix is never augmented and this the grid point weight generator never "sees" the additional mass.

For VMOPT=1, mass is added to the global matrix. This makes the computation expensive for all but the smallest problems.

VMOPT	Default = 0
[b]• VMOPT=0[/b] (default) The modes of the structure only are first computed without virtual mass effects to obtain Ritz vectors to use in a generalized coordinate solution, 
similar in concept to the older Generalized Dynamic Reduction method. The modes are re-computed in the generalized basis after the VM effects are added. 
When an o-set exists (usually because of the presence of ASETi-type entries, or for super element reduction), the VM effects are added to the component modes (o-set level). 
When there is not an o-set the VM effects are added in the residual structure Phase II operations (a-set level), after the structure-mass-only eigensolution. 
The rigid body mass of the VM is output automatically, computed about the location of the grid point listed on PARAM, GRDPNT.

[b]• VMOPT=1[/b], The virtual fluid mass will be included in the mass matrix at the same time as all other mass elements. In other words, the component modes will reflect 
the virtual mass. By default, virtual mass is included after the component modes are computed. This is the most expensive option in terms of computer resource requirements 
and computation time. It is practical for only relatively small-size models, or when the VM is in one super element only, with most elements in the super element wetted.

[b]• VMOPT=2[/b], This is the more efficient method and is summarized in the following steps:
Step 1: Compute the normal modes of the structure without the fluid mass: the `dry’ modes. 
Step 2: Generate reduced virtual mass using the reduced, modal basis from the dry modes.
Step 3: Compute the normal modes of the reduced virtual mass problem from step two to produce the `wet’ modes.

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

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