1 billions cells: useless?
 

hello, further to this:

http://www.cfdreview.com/articles/08.../1933248.shtml

one question is: is this really useful for design changes?

with 100 millions cells you resolve flow features < 1mm look at your ruler what is 1mm.

with 1 billion cells, could you be wasting 170 hours of computation for 1 run to resolve flow pattern of the size of a hairpin, whereas 17 runs with 10 times less cells at different speed or design you give you a better understanding of the thingy you are looking at.

especially for a boat which need to run under infinite conditions of wind/wave/sail choice.

Re: 1 billions cells: useless?
 

Turbulence structures can be much smaller than 1 mm and may be very important for very accurate predictions of drag. Without knowing the details, it is impossible to say if 1 billion cells was overkill or not.

Re: 1 billions cells: useless?
 

Would anyone bother to resolve the minute details of a wave breaking onto a shoreline, or would instead, it be simpler to concentrate on the global nature of the wave solution?

The concept of trying for fine & finer resolution, with billions of cells, to try & understand the turbulence phenomenon, is in my experience, impractical. Better to step a few paces back & observe the global nature of the flow solution.

If a momentum wave paradigm is used to understand flows, 2D simulations in the order 7.2Gb RAM, can be completed in around 2.5 days. No flow stabilisation techniques, or tricks, are used, nor are they necessary.

mw...

<www.adthermtech.com/wordpress3>

Re: 1 billions cells: useless?
 

""Our research indicates that this is among the world's first applied engineering simulations using a single mesh of more than 1 billion cells performed with commercial software,""

This statement is wrong. More than that i am not allowed to say.

Re: 1 billions cells: useless?
 

Would anyone bother to resolve the minute details of a wave breaking onto a shoreline, or would instead, it be simpler to concentrate on the global nature of the wave solution?

The concept of trying for fine & finer resolution, with billions of cells, to try & understand the turbulence phenomenon, is in my experience, impractical. Better to step a few paces back & observe the global nature of the flow solution.

If a momentum wave paradigm is used to understand flows, 2D simulations in the order 7.2Gb RAM, can be completed in around 2.5 days. No flow stabilisation techniques, or tricks, are used, nor are they necessary.

mw...

<www.adthermtech.com/wordpress3>

Re: 1 billions cells: useless?
 

i briefly looked at your work related to momentum waves. I find it interesting. But could not understand one thing (sorry if this is really silly question). Usually we solve poisson equation to get pressure. In the method you have mentioned do we solve poisson equation to get pressure.

Second , what about solution of air flow around bluff bodies with higher reynolds number. Is it applicable there too. I mean most of flow that i encounter are high reynolds number flows. This is why i am asking this.

Re: 1 billions cells: useless?
 

question wrote:

i briefly looked at your work related to momentum waves. I find it interesting. But could not understand one thing (sorry if this is really silly question). Usually we solve poisson equation to get pressure. In the method you have mentioned do we solve poisson equation to get pressure.

mw relies:

The penalty-based solver uses a weak variational formulation, which solves for u-velocity, v-velocity & pressure in one algorithm. Take a look at the FreeFem++ website, & pages 171 - 174 of their manual, for an explanation of the method.

question wrote:

Second , what about solution of air flow around bluff bodies with higher reynolds number. Is it applicable there too. I mean most of flow that i encounter are high reynolds number flows. This is why i am asking this.

mw replies:

My simulations are currently running around Re~1200 based on body characteristic dimension. This is currently at a velocity of around 120 m/s, for air - rather close to the incompressible limit for the incompressible NS equations.

I hope that helps.

mw...

<www.adthermtech.com/wordpress3>

Re: 1 billions cells: useless?
 

thank you for the reply.

I looked at the manual, and it seems they also solve a linear set of equations using conjugate gradient type of solver.

Which is what i want to avoid. (I know i can eschew this though).

Re: 1 billions cells: useless?
 

Question wrote:

I looked at the manual, and it seems they also solve a linear set of equations using conjugate gradient type of solver. Which is what i want to avoid. (I know i can eschew this though).

mw replies:

On page 227 of the manual, a number of different solvers are mentioned - CG is one of them. FreeFem++ also allows other sparse solvers to be used.

mw...

www.adthermtech.com/wordpress3

Re: 1 billions cells: useless?
 

Of course it's misleading/incorrect. That statement is nothing more than PR spin to make CFX sound like a good CFD solver. Funny how they don't show any figures or animations from their simulation.....

Re: 1 billions cells: useless?
 

Hi Sarah

How many prcoessors did you use to resolve the flow with one billion cells?

Re: 1 billions cells: useless?
 

you asking me.

I think i miswrote, i missed the commercial solver part in the statement. With commercial solvers like Fluent CFX etc yes it might be the first such run.

But if the talk is only about 1 billion cells, i have results of our simulation with me for about 1 year now. So 1 billion thing is already done.

I can not write more detail than this.

Re: 1 billions cells: useless?
 

Its not even the first...http://www.deskeng.com/articles/aaafsg.htm

Re: 1 billions cells: useless?
 

actually you are correct. And I was under the same impression this is why it was never a big thing.

I am currently working some code that shall handle large cases like 500 million or bigger, because we need efficient methods for our calculations.

I am think with current technology this large meshes is no biggy.

Re: 1 billions cells: useless?
 

So you work in motorsport or aerospace then I presume?

Re: 1 billions cells: useless?
 

You did not answer my question Sarah?

Re: 1 billions cells: useless?
 

An oddity found by a German research group (FeatFlow) is that with some of the legacy solvers (FVM - no names), the high mesh-density simulations often end up producing large solution errors - the smaller the mesh size, the larger the error.

So, even if a huge mesh exists, the solution mechanism can produce rubbish.

mw...

www.adthermtech.com/wordpress3

Re: 1 billions cells: useless?
 

That sounds like a solver which is not consistent (aka broken). I know nothing about the study, but a possible source of error is that the conditioning of the system decays as <code>1/h^2</code> for <code>h</code>-refinement and <code>1/p^4</code> for <code>p</code>-refinement. So increasing the resolution means that algebraic tolerances must be increased in order for computed results to show the improved truncation error.

As for <code>10^9</code>, implicit solvers for non-Newtonian flows with AMR and more than <code>10^10</code> elements is news. Explicit solvers have been doing this for years. The press release that started this thread lacks technical details and is clearly marketing fluff.

Re: 1 billions cells: useless?
 

But why would one billion cells produce rubbish results. If we use smagorinsky's model or no model at all with one billion cells, it should give us excellent results. How many processors we would require to run such a job though?

Re: 1 billions cells: useless?
 

You make a very significant assumption there - what kind of model is being used in their calculation.

Motorsport firms regularly use upwards of 500 million cells in their CFD of cars, but this is largely due to the complexity of the geometry involved, and I'd guess that much the same is true of the boat that was mentioned earlier. You can guarantee that the modelling used in these calculations will be RANS-based.

Using 1 billion cells on, say, a flow around an aerofoil with a RANS model should produce an extremely poor result - a significant proportion of the energy cascade will be resolved and the RANS won't be able to smear out the flow, leading to an oscillatory and essentially unconverged solution. Using LES or DNS on this case should of course produce good results, but the timescale involved to perform the simulation will be huge. You'd need upwards of 1024 processors to do the simulation as well.

Returning to the original point - is 1 billion cells useful in a design context - if the geometry needs that many cells then yes it is, but only for Reynolds-Averaged methods. Using a time-dependent method would probably result in a simulation time longer than the design cycle of the object of interest.

Re: 1 billions cells: useless?
 

This from CD-Adapco's most recent e-newsletter

"Find out how CD-adapco broke the Billion Cell Barrier over a year ago ..."

Words like "knee", "jerk" and "reaction" spring to mind.

Re: 1 billions cells: useless?
 

Yes, next thing you know adapco will be telling us that they invented polyhedral meshes first too...

JV

Re: 1 billions cells: useless?
 

The name 'legacy code' circa 2000 springs to mind :)

Re: 1 billions cells: useless?
 

sort of. We are supposed to calculate drag and lift. So mainly turbulence.

Re: 1 billions cells: useless?
 

Has it ever occured to you that may be my company does not want me to say anything on this regard.

Re: 1 billions cells: useless?
 

sarah-plain wrote:

Has it ever occured to you that may be my company does not want me to say anything on this regard.

For someone who shouldn't be talkative, you've certainly done your company proud. :)

Re: 1 billions cells: useless?
 

For someone who shouldn't be talkative, you've certainly done your company proud. :)

I have not written anything that is not in the public domain. Much much more than what i wrote is already published in papers. (further you do not know what my company is). So yaa if someone wish to guess, thats fine by me.

I just do not want to write it here.

Re: 1 billions cells: useless?
 

I just received a e-news letter from cd-adapco in which they mentioned that they broke 1 billion cell barrier year a go. So, Sarah, you have nothing to be proud of.

I totally agree with Andrew, it is useless to get mean results with one billion cells. I am sure RANS would have given you same results even with half of what you have used. Using RANS modelling with that many cells is pointless.

Re: 1 billions cells: useless?
 

underGroundMan,

See my Thu 20 post in this thread.

Don't you think it's quite a coincidence that CD decide to share this information now and then boast that it was over a year ago?

Re: 1 billions cells: useless?
 

Check the date and then stop making duck noises: http://www.deskeng.com/articles/aaafsg.htm

Re: 1 billions cells: useless?
 

<more duck noises>

So why is work that's that old in a "news" letter?

Re: 1 billions cells: useless?
 

I guess that the adapco marketing people wanted to show that ANSYS marketing people were telling fibs...

The words "don't" "know" "what" "you're" "talking" "about" come to mind.

Re: 1 billions cells: useless?
 

May be cd-adapco did not consider it a milestone at that time. But now when Sarah Palin's company started bragging about their work, cd-adapco reminded them how far behind they are!

Re: 1 billions cells: useless?
 

no the polyhedral meshing was copied from medical imaging softwares that existed from a very long time ago to discretize body part and to visualize them. The principle was is many open sources. It was just copied to CFD codes.

There is a also no point solving 1 billions if you cannot afford the licenses for the solver on all the processors.

Re: 1 billions cells: useless?
 

But now when Sarah Palin's company started bragging about their work, cd-adapco reminded them how far behind they are!

i think you need to learn to read.

1. How did you decide that our company bragging. 2. I certainly do not work with ANSYS, in fact i never thought it is big deal. 3. I am sure with todays resources it is not big deal.

Personally it would be big deal for me if i could get the same results in meshes less than 5 or 10 million cells. Speed and time is where my interest lies.

Re: 1 billions cells: useless?
 

While CD-adapco did not invent polyhedral meshes, it made them work for CFD which is not the same as copying. Building a mesh that a CFD solver can accept is a lot different than what is required for simple visualization. If it was simply copying, Ansys would also have a workable poly-mesh generator instead of just talking about having one.

In fact you can buy a license from CD-adapco for unlimited numbers of processors attached to a single analysis so if you have the hardware, you can afford the software.

Re: 1 billions cells: useless?
 

you need to be able to reference:

Area(i), Vertex(i), Volume(i), Connectivity(i) etc... and organize the arrays in the data structure to be able to find them in the solver...

It requires mainly cleanliness in the coding, but as you said nothing new from what was done in the medical field.

Re: 1 billions cells: useless?
 

Re: 1 billions cells: useless?
 

Sarah may be cd-adapco can help you in this regard. Considering they broke 1 billion cell barrier year ago, they are way ahead of you guys. I dont know why did you waste your resources on 1 billion cells when you were going to run RANS simulation? Even school kids can run such simulation.

Re: 1 billions cells: useless?
 

Sarah may be cd-adapco can help you in this regard. Considering they broke 1 billion cell barrier year ago, they are way ahead of you guys.

Are you three year old or what. Do you think there is some kind of competition going on to reach one billion cells.

English is not your first language, i have written multiple times that i thought it was not a big thing. So apparantly i thought many guys have already done it. (including F1 guys).

I dont know why did you waste your resources on 1 billion cells when you were going to run RANS simulation?

you know nothing about CFD.

Even school kids can run such simulation.

Yepp this statement very much indicates where you belong and what level you think at.