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Estimate an averaging time from the characteristics of a problem? |
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March 1, 2020, 07:02 |
Estimate an averaging time from the characteristics of a problem?
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#1 |
New Member
Join Date: Jun 2016
Location: Birmingham, UK
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Hello, a generic question more about physics than CFD software.
I used to simulate an axial gas flow, with typically v = 3m/s and L = 1m. Averaging was done over 8 seconds of simulated time. So, it took 24 passages of a fluid particle through the axis to get a reliable average. Now I have a shear layer axis with, say, v = 10m/s, L = 20m. To travel that distance 20 times at that speed takes 40 seconds. So far I ran the case for 20 seconds max, which didn't give great results, so the idea above seems to make sense, but it could be completely coincidental. Any suggestions? |
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March 1, 2020, 14:21 |
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#2 | |
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
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Quote:
The convective time is one of the characteristic times you can have in a flow problem. You can think also to a diffusive time, an acoustic time... |
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March 2, 2020, 10:06 |
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#3 |
New Member
Join Date: Jun 2016
Location: Birmingham, UK
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Hi FMDenaro,
Thanks for your reply. By convective time I assume you mean the large eddy timescale, which is indeed what I base my estimation on. Basically I want to know if one can scale the time averaging duration of one problem to another similar problem but of a different size. In other words, "in problem A the averaging duration allows for 20 eddy passages through a probe. If this yields a good average velocity, can I adjust the averaging duration in problem B to 20 eddy passages and hope to get an equally good average?" |
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March 2, 2020, 10:21 |
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#4 | |
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,777
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Quote:
Of course assuming that the Mach number has no relevance in your problem. |
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