An impressive CFD simulation on a Cray XE6 supercomputer

Discussion in 'Hydrodynamics and Aerodynamics' started by daiquiri, Nov 2, 2012.

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daiquiriEngineering and Design

Just saw this video of a CFD simulation of a flow around a transom-stern vessel: http://www.youtube.com/watch?NR=1&v=cenIBqda8zA&feature=endscreen

The qualitative result is really impressive, and the flow features appear to agree very well with those of a model tested in a towing tank (also shown in the video). Too bad they don't say anything about the numerical results, forces and moments. How do they compare to the towing-test data?

The simulation is done on a Cray XE6 supercomputer, here is the webpage of the beast: http://www.cray.com/Products/XE/CrayXE6System.aspx
I am reading some data there: 50 kW of power absorbed and 5000 cu.m/h of airflow for cooling, for every cabinet. It means that a beast made of 8 cabinets shown in the brochure requires 400 kW and 40.000 cu.m/h of airflow? Wow, those folks have to pay some nice electric bills at the end of the month. A 20 hrs computation should cost, how much, a \$1500-\$2000 just for the electric power consumed?

This video has left me pretty much stuck. Evidently we now have the computing power to perform qualitatively excellent full-NS calculations around vessels travelling at high displacement speeds. But what about the quantitative side of the results? Do we have now a reliable tool for prediction of flows around transom-sterns, and the associated forces and moments?

Anyone here who might have a first-hand insight into the state-of-the-art of this type of hydrodynamic research, and who could tell us something about the level of accuracy attained so far?

Cheers

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J FeenstraJunior Member

that electric bill really is significant, I always hear people say that CFD is cheaper and faster than model testing, but this really put's it in another perspective.

Just for prediciting the restistance I believe that CFD is rather accurate, I'm currious to know if people use CFD for prediciting seakeeping capabiltiy

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daiquiriEngineering and Design

Flow around a transom stern has always been a problem so far, that's why I was asking. Cheers

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Leo LazauskasSenior Member

Another very beautiful simulation!
It seems to be a recent refinement of one I pointed to several months ago...

I agree with you that it would be nice to see some predictions of forces and
moments to go with the nice pictures. My alter-ego "Ferocitus" commented

From my point of view, it is important to emphasise that it is asking a lot
from simple linear codes (e.g. Michlet, Hullspeed, Hydros etc) to even come
close to that level of detail for the near wave field. However, given the
uncertainties in measurement, those theories, combined with some empirical
equations, can do quite well in predicting forces and moments.

Furthermore, linear codes can do very well in predicting the wave field very
far from the ship, say > 5 shiplengths back, which the Cray simulation might
struggle with, unless it reverts to linear methods for that part of the field.

Note, too, how there is no splash at the bow. I appreciate that they are
concentrating on the stern flow, but splash and unsteadiness at the bow
would make for a very messy flow near the stern at high speed. They seem
to have side-stepped that complication. For good reason!

So, yes, it is "state-of-the-art", but IMO it's still a long way from modelling
"reality". I have read in some other forums that this group is also interested
in simulations for video games and movies. For those applications,
pathological accuracy and force calculations are irrelevant.

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I think Leo summed it up nicely.
Impressive..but jury is still out.

From Leo's thread on latest ITTC, here is their state of the art, as such on CFD:

Trouble is, it is no different from the fancy plots of FEA of composite materials in their through thickness. They look great, but it is impossible to verify. Since to have a strain gauge in the layers, is not exactly easy (the gauge being generally thicker than the cloth!!), and the mere presence of it alters the strain anyway.

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daiquiriEngineering and Design

AH, it would be interesting to compare the costs of the two approaches (towing tank vs CFD) too.
Let's put aside (just for a moment) the costs relative to the purchase of the equipment and the construction and maintenance of the test/computation facilities in two cases. How much, based on your experience, could a similar test program cost when performed in a towing tank?

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daiquiriEngineering and Design

Good observation, I take note of that. It looks as the hull in the simulation is a light displacement like a shallow sailboat hull, or a moderately high LDR hull, like the A-Yacht: http://www.youtube.com/watch?feature=player_detailpage&v=vbucJO3zDCE#t=7s so it makes no significant splash at the bow in the speed range considered. As you said, evidently they wanted to concentrate on transom-stern flow only.

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Leo LazauskasSenior Member

AdHoc might give you some idea of costs, but I think it depends on what you want.
For example, do you just want video of the flow filed, or do you want
laser-scans of the wave elevations on a fine grid so you have the elevations
in a digital format? Many tanks won't have that capability.

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Leo LazauskasSenior Member

And the transom stern is not all that deep. I suspect that a larger transom
might make for a less attractive video because there would be a lot more
wave-breaking and viscous debris flying around.
In any case, I'm glad I'm not paying the electricity bill!

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daiquiriEngineering and Design

A nice example of theory of relativity applied to fluid dynamics.

I get your point Leo. I am not trying to find out which method is ultimately better - I'm sufficiently smart to understand that there is no such thing as "the best". What I am trying to understand is - how reliable and accurate is the CFD nowadays, with present computational power, and where does it becomes competitive with (or can even substitute) classic towing tank tests.

Your reply has given a hint about one aspect of fluid dynamics research where CFD is indeed valuable - the visualization of flow details. The great videos posted by Mikko Brummer in the thread http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/ultimate-validation-44809.html are another fine example of this visual delivery capability of CFD. Very hard to achieve such level of visual information with empirical methods.

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Leo LazauskasSenior Member

I can only point you to the latest ITTC reports because that is by far the
best chance we have of getting some objectivity on the matter.

Those reports have some minor errors, but they really are a terrific summary
of current techniques and procedures. IMO they are the most significant documents
available on hydrodynamics and experimental methods.

I agree completely. It is just a case of choosing the right tool for the job.
Using Cray's at the preliminary design stage would be an expensive over-kill.

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Exactly.

We were forced by a client to use their preferred test tank. I wont mention their name.

This was almost 10years ago, they charged €50k for simple tank testing. I was in attendance, and during the higher speed runs, there was a huge bow wave and all hell broke out....they (test tank) went back started doing endless CFD simulates about changing the entrance angle and then the customer rep was up in arms..blah blah blah....and on it went..was a nightmare, politically.

Solution, simple, just alter the waterline, so she is "trimmed aft"...ie move the lcg/lcb aft....problem solved. Their CFD dint tell them that.....just plain old common sense

BTW..our preferred test tank is some 70% cheaper!!

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daiquiriEngineering and Design

You mean, around \$15k ? Does it include the costs relative to the creation of the model hulls?

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Leo LazauskasSenior Member

I can imagine that the experimental program that BMc and his colleagues ran
for their "Sea Train" would have cost much more than that.

Simple methods (e.g. Flotilla and other linear codes) can do fairly well in
predicting the resistance and wave patterns for such thin hulls and relatively
gentle air cushions, but I wouldn't trust computer simulations of the behaviour
of that long, articulated vessel in waves.

Sometimes you can save money by relying on CFD or other codes, sometimes
you have to resort to tank experiments. In both cases there are risks.

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daiquiriEngineering and Design

But you have already given some very good answers to my questions. What use can one make of them - it's up to reader's intelligence.

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