[fbelga]

Hello,

I've only started working professionally with CFD (ANSYS Fluent) about a year ago in my previous job. Now I'm planning on investing in a home workstation, as I intend to develop some work on my own.

Throughout this year I basically worked with two CFD problems: 1) Studying the hydrodynamics of a wave energy converter and 2) Determining drag resistance of ships. In the first case we are talking of a time domain simulation, around 1.6e5 cells and a domain length of 2 kilometers. In the second case, it much varied depending on the situation. Both wer ran in this pc:

i7-8700k CPU @ 3.7 GHz (12 CPUs), ~3.7GHz
32 Gb RAM
Quadro P2000

I don't have any complains about it and I don't have any other pc to compare it to. I remember I left the simulations running by the end of the day. In the morning they were done. However I don't recall the exact running time.

Anyway, a few notes and ideas:

1) The majority of the work I'll be developing in CFD will be within the areas of ship flows, ship resistance, propulsion and seakeeping.

2) Not only I'm an inexperienced CFD user but also know little about hardware in general. I expect to get better at both.

3) I see this as an investment for my future professionally-wise. So, whatever workstation I acquire, I expect it to last a reasonable amount of time, even though I know new hardware (and better) comes out quite frequently. Basically I would prefer A) to get something that could be considered "overkill" for the type of work I'll be performing in the beginning, rather than B) buying something cheaper (probably more suited for my current skills) and updated it later on. But I could use some advice on this topic.

3) I've been digging into the refurbished market lately. I believe that up to 1400€ (my budget) I could get something really really good. Or at least that's my idea. Few workstations I've checked recently for that price:

Hp Z840
2x Xeon E5-2690 v3 (8 cores/ 16 threads)
2.60GHz (3.50GHz Turbo)
64GB (8x 8GB) RAM (512 GB Max)
Quadro K2200 4GB
HDD 1TB 3.5" SATA 7.2K

Hp Z840
2x Xeon E5-2667 v3 (16Cores/ 32 Threads)
3.20GHz (3.60GHz Turbo)
128GB (8x 16GB) RAM DDR4
Quadro K6000 12GB
512GB SSD

Hp Z840
2x Xeon E5-2637 v3 (4 cores/8 threads)
3.50GHz Turbo
128GB (8x 16GB) RAM DDR4
Quadro K4200
256GB SSD + 1,8 TB HDD

Any thoughts on these?

4) Finally, for the type of work I've described, considering my level of experience and considering that this is an investment for the future:

• Are there any particular specs for workstations you would suggest?
• What do you think of the alternatives I found and how do you think they perform against the i7 I showed you in the beginning?
• What are your thoughts on refurbished workstations?
• What are your thoughts on my budget? Is this suited? Is it overkill? Would you consider lowering this budget? I want to guarantee that I'm buying the best for that price and there are way many options...

Thanks so much for reading this and for the help if you can,

Best regards,
Filipe

[flotus1]

The most important question you need to answer is: are you limited by the number of parallel licenses you have for Ansys Fluent.

btw, Xeon E5-2690 v3 in your first example is a 12-core CPU.

[Simbelmynë]

To add to Alex' question:



Do you plan to submit cases continuously to your server or perhaps run several cases at the same time?



How long time do you spend on post-processing for each case?

[flotus1]

What is this question aimed at?
The fact that a large portion of the time spent in the whole workflow of many CFD simulations is user time? I.e. can not be sped up, no matter how fast the hardware?

[Simbelmynë]

Yes, it may be that way. This was directed at what the OP wrote. It just seems that the OP starts a simulation at the evening that is finished the next morning. So here the simulation pattern/scheduling is important. If the OP can make large batch jobs, running parametric sweeps then obviously any improvement in hardware will reduce total turn-around time. If, on the other hand, it is impossible to start a new simulation without looking at the results from a previous simulation, then the improvement needs to such that several simulations (+ post processing) can fit into one working day.


So if a simulation takes 8 instead of 10 hours, when you only run one simulation each night, then it may not matter much.



This is the reason of my question.

[fbelga]

Yes, you are right, that will be the case most of the time, i.e., running a simulation at the evening that is finished next morning. However, i won't necessarily need to post process the results before running a new simulation. As I told you my main focus will be on a quite typical problem: ship flows for resistance/drag prediction (usually steady state, no waves). I imagine my routines to set-up a case (build mesh, select solver, etc) and process the results will become quite standardized (and validated) after a while. Thus, user time might indeed reduce a lot. Running simultaneous simulations could be useful though, if possible. I never ran more than one at once while working in Fluent.

Was my answer helpful?

Thanks a lot

[fbelga]

Alex,

Could you explain what parallel licenses mean? Is it running more than one simulation at a time? If yes I am not sure... As this is intended to develop some work on my own, I'm still studying possibilities regarding software, aka licensing. Many colleagues of mine already turned to OpenFoam because of this and I might give it a try as well. Would the parallel license problem apply here as well, since OpenFoam is open source?

Thanks a lot for your reply and for the correction.

[flotus1]

Most of the commercial CFD codes limit the amount of threads they can run in parallel. E. g. with Ansys Fluent, a basic license allows you to run a single simulation with 8 threads max.
In that case, buying a machine with 24 cores would not make much sense.
Since you seem to be leaning towards open source and in-house codes, we don't have to worry about this.

The last workstation in your list does not have enough cores.
The first one has a decent amount of CPU performance, but the overall specs (less RAM, missing SSD, cheaper graphics card) are worse.
The second machine has the highest CPU clock speeds, and still enough cores. Which is what I would prefer for code development. Most codes don't scale too well in early stages of development, so having faster cores is preferable. The graphics card is probably overkill, but if the price is the same, who cares. An SSD on the other hand is a must-have for a workstation in 2019.

[fbelga]

Thanks a lot for your input Alex! Then, what would you consider enough cores and at the same time which CPU clock speeds you consider minimum? How do you balance both? More cores with lower speeds or lesser but faster cores? Take the second machine for reference: 2 processores, each with 8 cores, 3.20GHz/3.60GHz.

Thanks

[Simbelmynë]

There is no direct answer to core vs CPU frequency. Normally the thing that matters most is that you can fully utilize the memory bandwidth of the system.


If we limit ourselves to CPUs from the same family then


You need at least 2 cores per memory channel as a rule of thumb. However, some CPUs continue to give some minor improvements above this number so more cores might be better.



There are exceptions though, like the Xeon Bronze series which is way too low clocked, meaning it has too low computing power for the available bandwidth.



If parallel licenses cost a lot of money then it is usually better to go with fewer but more highly clocked cores (still trying to have 2 cores per memory channel).


2 CPUs effectively doubles the memory bandwidth and can in many cases almost double performance, compared to a similar single CPU system.



If you only have access to a serial license then you should just go with a state-of-the-art "gaming" computer that has a high turbo boost frequency.


The benchmark thread is probably the best place to find information about this.

[fbelga]

Thanks a lot for the feedback. I'll look into the benchmark thread! One last thing, one of the things I asked in my post was an opinion on one of those refurbished workstations for 1400€. Lets take Alex's choice for instance:

Hp Z840
2x Xeon E5-2667 v3
3.20GHz (3.60GHz Turbo)
128GB (8x 16GB) RAM DDR4
Quadro K6000 12GB
512GB SSD

Considering the price of 1400€ for a refurbished workstation, would you consider it a good deal? Or could I get better for the same amount? I'm not asking for a specific alternative, just an general opinion based on your experience.

Thanks!