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

 

[gillesD]

Hey Captain,

I would vote for adding the fuel mass. That should work well for the 1st modes.

Gilles

 

[rasna]

I would model the fuel as a mass element with all inertia properties. I would then connect that mass element to the inner surface of the tank with rbe3 element (nastran) and then run a frequency response analysis in the frequency range of interest.

Rasna

 

[greyspace1]

George, agree with Rasna, usual thing is to put a node at the c/g of the fluid, then use an RBE3 (flexible) element to connect this to a reasonable spread of nodes over the tank. As a refinement add a duplicate node positioned at the fluid centroid, connect it via 2 horizontal springs and add the fluid lumped mass on the duplicate. Then you can get the fluid sloshing behaviour with only 1 additional node. Note do not connect vertically as the fluid is decoupled in this direction. The sloshing frequencies tend to be circa 1Hz if you want to tune the spring to give realistic results. I would not include inertia since the fluid decouples in these directions also. I think Blevins (Formulas for Natural Frequency and Mode Shape) has a sdof method to include fluid virtual mass.

The inclusion of an RBE3 element should not significantly affect run times.

Problem with enhancing the plastic skin density is that you may introduce fictitious modes of the tank.

 

[tld23]

Hi Captain,
I am not sure of the part and extent you want the fuel to take in the analysis. However a simple level you could represent the fuel load by a distributed pressure load that changes with fuel depth. This could represent the static load of the fuel. For bulk acceleration of the fuel an additional 'g' factor could be included in the calculation of the pressure.
If this is inadequate a number of codes have hydrostatic fluid elements that can represent fluids such as liquid fuel.


TERRY

 

[mloew]

CaptainCrunch,

Take a look at the following Technical Resource:

Physics first, using ANSYS to define the design: Real-life examples from the automotive supplier company Mannesmann VDO
Zlatko Penzar
Mannesmann VDO AG, Fuel Systems
Schwalbach, Germany

http://ansys.net/ansys/papers/end02111b_pdf.pdf

Specifically in Section 2:
Impact performance of closed tanks, filled with fluid
A simple procedure of calculating the crash or drop-test performance of the closed containers, partially filled with fluid, being applicable to automobile fuel tanks.


There may be some useful information there.

Best regards,

Matthew Ian Loew
"Luck is the residue of design."
Branch Rickey

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

 

[bkal]

ACI 350 code might give you some idea about modelling fluid in tanks subject to dynamic loading and how to calculate dynamic characteristics of impulsive and convective fluid components. You can also check papers by Housner G.W. Please note that above is mainly concerned with seismic events.