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Fluid Structure Interaction, Solid/Fluid Impacts, Compressibility 1

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benzflieger

benzflieger

Mechanical
Jun 8, 2009
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I have a 7ft diameter x 15ft deep cylindrical basin of water that has the side and bottom formed from 1/2" welded steel plate. This open topped basin is buried in the ground with the open end "flush" with the ground.

I have been tasked with doing an analysis to determine what the effects on the basin wall material would be if a 2500lb plate with a diameter of 1.5~2.0 ft is dropped onto the free surface of the water. The drop height is 30ft, so I know that the velocity of the plate just before impact is v_impact = sqrt(2gh) = 44mph.

I know that this impact is going to impart a massive pressure shock wave into the water and that the effects of this wave will result in stresses in my basin casing.

My question is this...
Does anyone out there have experience in this type of analysis? I am looking at Algor Pro Sim & NEi Femap+LS-Dyna as possible FEA packages. Without getting into fisticuffs in the responses over who has the nicest GUI, or best support or, best pre-post processors, etc. can anyone offer any practical advice on how start approaching this problem?

Specific Questions I have are:
1. Should I be considering fluid compressibility. (my hunch is YES! since this is necessary to consider a wave propogating thru fluid. My understanding is that this would rule out Algor Pro Sim because it will only consider incompressible fluid)
2. Any thoughts on how this problem should be broken up into sections? i.e. time period just before impact, impact, time following impact, etc.
3. Can anyone point me in the direction of helpful literature, papers, other resources that would give insight into how to approach this problem?
4. Without getting into pro-con discussions of specific FEA packages, can anyone offer advice on FEA strategies.
5. What are some ways that I can simplify this problem to make it more straight forward? What are some key assumptions that can improve my chances of finding solution in a timely mannner?

Thanks,
 
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I would look into bottle drop test simulations. Many of the software webpages will have a similar example. I would add Abaqus to the list. If you want compressibility than I think you need FSI so Comsol may be a good option. In investigating see if the solver can handle axisymmetric which will greatly reduce your solve time. I hope this helps.

Rob Stupplebeen
 
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Agree on the recommendation to look at Comsol, they have some pretty neat combined-physics solver packages. Not sure I'd agree about a "massive" pressure shock wave, but it's not my problem to solve, either. Good luck!
 
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The articles here might be useful, but require payment:
Of note, related to the comment about wind resistance, is the notion of trapped air under the plate modifying the impact behavior.

Of course, your plate is rather small, so that might not be terribly relevant.

The air resistance is relative small, at about 67 N, compared with the weight at 11121 N

A free article on the web you can download:


TTFN

FAQ731-376
 
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If I understand the problem correctly you have a 2ft diameter plate being droped into a 7ft cylindrical tank. I think you would need a combination of FSI and VOF. The VOF analysis is a CFD type of analysis that simulates the flow and splashing that occurs between a liquid and a gas (i.e. air). Additionaly, you can model the moving plate as a moving solid in the CFD simulation and model the impact with the water. The Fluent tutorials and CFX tutorials have sample exercises of both moving solids in fluids and VOF simulations.

I wouldn't include compressibility for the same reasons Greg mentioned. Plus, if I am seeing it correctly it might be considered more conservative to treat the liquid as incompressible.

However, taking a step back for a second...if the tank is completely buried in the ground such that only the open portion is flush and the dropped plate is smaller than the opening of the tank ... how significant would the stresses be on the sides and bottom of the tank, given that the tank is roughly constrained radially by the soil and (from what I can tell) most of the kinetic energy of the plate would be converted to kinetic energy of the splashing water. I may be all wet on this (I know...bad joke), but it might be something to consider.

Either way...nice project and I hope you don't need results by the end of the week!

Good luck,

Steve

 
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Thanks for the thoughts so far guys. We have decided to go with MSC MD Nastran. The package has Arbitrary Lagrangian Eulerian (ALE) analysis capability and so should be able to handle the fluid structure interaction. It should also handle the splash effects pretty accurately as well.

By the way, I got an update today that the plate being dropped I am dealing with is no longer 2ft diameter & 3000lbs. but 2.5ft diameter & 10,000lbs. :) I am expecting a big splash.

Anyone who could point me in the direction of literature giving insights on what the effects of "surface impact generated stress waves on submerged structures" is would be greatly appreciated.

Also, does anyone out there have any experience with using scale models mock-ups to develop some empirical data to correlate with FEA calculated results? Because this is a somewhat "unique" FEA study, without a whole lot of analogous DOCUMENTED work with which to compare my calculated results, I am considering scale model testing. The problem though is time and of course money. We are already spending a lot on the FEA code.

Thanks.
 
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Treat the water as incompressible. 44mph is way below the speed of sound in water.

Consider that in the first moment after impact the surrounding water will be rising at 3.9mph. What may happen next is the disk slides sideways (objects in motion tend to follow that path of least resistance) so unless your plat is guided I would worry more about how fast it is going when it hits the sides of the tank.

And just after it has punched a hole in the water that hole will collapse (cavity collapse) which is likely to be a more severe shock than the impact of the plate.

If the plate is actually heading down in its original attitude the cavity collapse might actually accelerate it again before it hits bottom.

Just my 2cents.

TOP
CSWP, BSSE

"Node news is good news."
 
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