FEA of fuel tank vibration with fuel in tank

[CaptainCrunch]

Hello,

I am thinking of ways to run a FEA vibration analysis of a fuel tank full of fuel. Actually I want ot examine the bracket which supports the tank and I think the fuel in the tank will make a significant differnence.

My problem is how to simulate the mass of the fuel in a tank, but have no modal participation of the actual fuel. In other words the mass and cg of the fuel is included but no modes or sloshing of the fuel occurs.

I first thought of actaully meshing the the fuel as a contniuum elemnents (fluid is a continua ...) but there are two problems: increased solve time in already big FEA model, and lots of low frequency modes due to the very low Youngs modulus I'd have to assign to water (to avoid adding stiffness to the tank).

I don't want to put in a liquid material, and I don't want to model any kind of sloshing.

My second thought, which I think is more promising is to alter the density of the fuel tank plastic, but keep the proper Youngs Modulus of the tank. The tank is roughly squarish so I think the cg of the tank / fuel would be close. This way would add the mass of the fuel but no extra stiffness. The modes of the tank may shift a bit but I can check that by running with and w/o fuel and see the difference.

Anybody ever deal with this problem? Ideas?

TIA,

George Vandyke

 

[GregLocock]

Add surface loading to each element proportional to its area. Crude but common. Should be OK for bracket resonances.

I think you are playing with fire if you are interested in flexural modes of the tank walls - fuel is incompressible so you'll get direct coupling of different walls, and some modes will be suppressed (those that result in net changes of volume) or more accurately pushed out of the frequency range of interest.




Cheers

Greg Locock

 

[CaptainCrunch]

Greg,

Thanks. I am also concerned with coupling opposite walls of a fuel tank with some type of spring element. I cross-posted this to the FEA group and some were recommending using spring elements from the walls to the fuel cg with a mass element at the cg. Then they reccomended tuning the springs so the fundamental freq was ~ 1 Hz. I think in the end I will try both methods: increasing the density of the tank with no elements representing fuel and then spring elems as stated above. Run the setup on a shaker table and see which one correlates to test.

George

 

[DerekBrighton]

can you just assume the fuel acts as a mass loading on the tank? Each element of the tank has a column of fuel normal to it to the tank center.