# Rough estimate for cost of DNS at Re=400000

**URL:** <https://pyfr.discourse.group/t/rough-estimate-for-cost-of-dns-at-re-400000/883>\
**Category:** Just Starting\
**Tags:** hpc\
**Created:** [21 July 2023 23:09 UTC](https://pyfr.discourse.group/t/rough-estimate-for-cost-of-dns-at-re-400000/883 "2023-07-21T23:09:52Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![steve](https://avatars.discourse-cdn.com/v4/letter/s/f19dbf/32.png) [@steve](https://pyfr.discourse.group/u/steve)\
**Post date:** [21 July 2023 23:09 UTC](https://pyfr.discourse.group/t/rough-estimate-for-cost-of-dns-at-re-400000/883/1 "2023-07-21T23:09:52Z")

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I am looking for an order of magnitude estimate for the cost or running a scale resolved DNS solution on an object with transition and significant separation at a Re of 400k. For units I am thinking perhaps A100 hours may be a good choice. For example, how many hours would it take to run a sphere at Re=400,000 on (32) A100’s? 1 hr? 10 hrs? 100 hrs? 1,000 hrs? 10,000 hrs? I would want enough time steps to get a reasonable estimate of the average drag coefficient.

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**Author:** ![fdw](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@fdw](https://pyfr.discourse.group/u/fdw)\
**Post date:** [22 July 2023 00:01 UTC](https://pyfr.discourse.group/t/rough-estimate-for-cost-of-dns-at-re-400000/883/2 "2023-07-22T00:01:12Z")

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Baseline resolution requirements can be had [here](http://aero-comlab.stanford.edu/Papers/akbarzadeh_scitech_2023.pdf) which concerns flow over a sphere. These simulations were mostly performed on 32 V100 GPUs (slower than an A100) and while the precise amount of time varied between set ups I recall them taking about a week on average.

Regards, Freddie.

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**Author:** ![steve](https://avatars.discourse-cdn.com/v4/letter/s/f19dbf/32.png) [@steve](https://pyfr.discourse.group/u/steve)\
**Post date:** [25 July 2023 16:22 UTC](https://pyfr.discourse.group/t/rough-estimate-for-cost-of-dns-at-re-400000/883/3 "2023-07-25T16:22:28Z")

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Freddie,  
This was very helpful. I have a lot of experience testing spheres with a very accurate system (not a wind tunnel). The subcritical CD in the paper is within 1% of what I quote based on experimental results.  
Thank you,  
Steve