Modal analysis with wetted elements

[enriquelvarez]

Hi everybody again,

I'm trying to understand the difference between two parametres related to calculating resonance frequencies in a modal analysis.

I'm doing a modal analysis of a wetted structure. The fluid is air. Three optional parametres can be chosen: "PARAM, VMOPT,1","PARAM, VMOPT,2" and default option is "PARAM, VMOPT,0"

What I got is: option 2 calculates dry frequencies and afer that, fluid mass is included in mass matrix and wet frequencies are calculated. On the other hand, option 1 includes fluid mass from the beggining and wet frequencies are calculated. Effective mass from both options are different.

In addition, using default option (option 0), I got the same results as option 1 concerning frequencies and effective mass.

I suppose that effective mass from options 0 and 1 includes fluid mass

So questions are:
1-What is the difference between "PARAM,VMOPT,1" and "PARAM,VMOPT,0"?
2-What is the effective mass results given by "PARAM,VMOPT,2"?

Thanks very much

Enrique

 

[40818]

If VMOPT=1, then the virtual mass is included in the amss matrix at the same time as all the 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
If VMOPT=2, the modes of the structure or component without the fluid are computed first ("dry" modes). the fluid effects are added in the modal basis during the residual flexibility computation to produce the "wet" modes for the component. Both eigenvalue tables are printed, allowing comparison of the dry and wet modes. The wet modes are used in modal dynamic analysis.
The cost savings results from the dense virtual matrix being kept out when computing dry modes in the physical basis. its presence can increase memory and computation times by an order of magnitude. The VM is added only in the smaller gerneralised basis used in the residual Flexibility Computations. The approximations introduced by this approach are geneally small due to the homogeneous nature of the fluid. This approach was provided in earlier Nastran versions with the vma.v* series of SSSALTERS. it is the preferred method when the number of wetted elements exceeds several hundred, for reasons of efficiency.

As regards the effective mass, then i would have thought the differance would be mainly due to the fluid mass being accounted

 

[enriquelvarez]

Hello again 40818,

Thank's very much. Anyway, is there any difference between VMOPT,1 and VMOPT,2? That's the only point it is not still clear to me.

Best Regards,

Enrique