Fluid vessel modal analysis

[Adubeng]

Is anyone familiar with a nuts and bolts method for conducting FE modal analysis of a vessel or container filled with a fluid. I'm familiar with the fluid 80 element in classic ANSYS but I was hoping for more of a 'design level' approach for use in ANSYS WB. One method I've used in the past is modeling a solid fluid mass and attaching it to the bottom of the container to avoid adding stiffness to the structure. The problem I'm having is that the mode shapes are dominated by the fluid mass when the container is plastic rather than metal.

 

[GregLocock]

Is this completely filled or are slop modes of interest?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

I am reminded that the 15 dollar phrase is 'free surface' modes. Slop modes sounds a bit too earthy for a CAE report.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Adubeng]

It normally would not be completely filled, but then again 'free surface' modes are not of interest. Also, at the frequencies I'm expecting (150-300 Hz), I'm thinking that the slosh modes would not have much effect?

 

[GBor]

Some software packages will do Modal Analysis with Load Stiffening. You could apply a hydrostatic pressure to the structure and run this type of modal. Are you trying to take in to account the damping effect of the fluid?

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group

 

[Adubeng]

Right now I'm not interested in the damping effect of the fluid, only the added mass without adding stiffness to the structure.

 

[GBor]

What software are you using? Some allow a "non-structural mass" which could be added to the portion of the structure loaded by the fluid.

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group

 

[Adubeng]

I am using ANSYS and ANSYS WB. I am trying to develop a 'design level' tool so I would prefer to use WB. I know that WB has a point mass, but I do not understand how it is attached to the structure and how that will effect results.

 

[GBor]

Sorry, I missed the original statement about ANSYS.

I'm not that familiar with ANSYS, but another approach is to place an extra layer of plate elements on top of your structure. Set the modulus of elasticity to something very small (like 1) and manipulate the density to account for the weight of the fluid. This stops from stiffening the structure and is usually easy to generate by copying a surface on to itself.

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group