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Blurp2x1 April 25, 2013 04:09

CPU& ram config to achieve acceptable simulation time
Dear All,
thank you for reading this.

I am have an engineering background (themodynamic) and do quite some natural ventilation study's. Recently we get larger and larger demands in the usage of cfd-scale. Now we have master planning projects that require airflow simulations (isothermal) on small neighborhoods to city scale. The small neighborhoods require up to 10 to 20 Million cells. the city analysis go up to 40 to 50 million cells.

Due to these two work scopes new investments are needed in our company.
Basically we cant afford to wait 4 to 5 days till a simulation is finished. Is there a good relationship in terms of nmbr of meshing cells , cpu+ram and computation time ( we use k-epsilon , Reynolds and Navier stokes equations mostly)

Advice on this field would be real valuable and insightful. If more info is needed for a good advice i will give so. Thank you for thinking with me on this.

evcelica April 25, 2013 09:02

The software you plan to use, and your licencing for how many cores you will be able to run will help us give you recommendations.

Blurp2x1 April 25, 2013 09:18

The software will be star ccm or fluent from ansys
We will license for a powerlicense probably which gives no restrictions on the amount of cores ( in the case of star ccm.) but only if we need it.

The goal is to get the simulation time done within 2 days or three days. Today's discussion we made clear we will focus on the 10 to 20 M cells max.

Again, some guidance on the specs needed to establish this would be a real help.


Blurp2x1 April 26, 2013 02:50

Would [ Nmbr of cells / cores] *42= hrs of simulation time be a good one?

assuming the we dont have a difficult solver and enough Ram on board?

example : (15 million cells / 16 cores) *42 = 39 hrs

CapSizer April 27, 2013 12:38


Originally Posted by Blurp2x1 (Post 423252)
Would [ Nmbr of cells / cores] *42= hrs of simulation time be a good one?
assuming the we dont have a difficult solver and enough Ram on board?
example : (15 million cells / 16 cores) *42 = 39 hrs

Well, it doesn't sound unreasonable, but we are talking very approximate numbers here. What you haven't stated is if you are using a transient solver with heat transfer switched on or not. A simple, steady state, no energy equation solution would be much faster than your number above. You've mentioned isothermal? But transient or steady makes more than an order of magnitude difference, and if you are doing transient, your total time that you are simulating is the determining parameter.

16 cores is not a very good number, from a software license point of view. It is too small to make good use of a power session license, and Ansys HPC goes (I think) 8-32-128-512. From that point of view, you might as well have 32 cores, because software costs totally swamps the hardware expenses.

Blurp2x1 April 28, 2013 21:54

Thank you for the clarity.

Yes our work will be 90% steady state + the calcs will be wind driven without any heat solving in the RANS calcs. The goal is to get an understanding which type of pc is needed for our work. We want to invest in computing hardware that will do the projects that has 20M cells within 2 or three days.

Budget wise we can invest for computer + licensing till +/- 60,000 US Dollar.
But if really needed it is stretchable.

The next step will be (this week) to get familiar if we need to get a cluster (wich sounds a bit alien to me). It would be real helpful if people can pinpoint if there are plug/n/play pc's on the market, or if it's a real process to aquire such.
We have a contract with Dell, but if they don't have pc's in this range other producers have to be approached.

It would be great if a few words could be shared with me. This guides me in the correct direction. But allready thanks for the advice that is given.

warmest regards ,


CapSizer April 29, 2013 01:47

In your position I would look at a Dell workstation with dual Xeon E5-2650 CPU's and 16 X 4 GB 1600MHz memory modules. That will give you 16 cores, and will easily meet your time targets. If you buy Ansys HPC up to 32 cores, it will mean that you cannot fully utilise the license, but at least you can fully utilise the hardware. If you require more performance, there is the option of adding another similar workstation and networking them together. You could consider hooking them up point to point with Infiniband network cards, which will save the considerable expense of an Infiniband switch. Get a network professional to sort that connection out for you.

Blurp2x1 April 30, 2013 03:32

Thank you for the insightful comments and help you shared with me.

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