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January 16, 2019, 09:42 |
Plotting Q-criterion
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#1 |
Senior Member
Nejc
Join Date: Feb 2017
Location: Slovenia
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Hi.
I'd like to visualize spots with dissipation losses in my domain (a transient k-omega SST pimpleFoam case with dynamic mesh). I have the following options:
I have found many examples of Q-criterion but my problem is that the values range from say -1e+17 to +1e+17. How do I choose values for isosurface plots so that they give me the most information? Thanks! |
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January 16, 2019, 09:51 |
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#2 |
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Anton Kidess
Join Date: May 2009
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AFAIK there is no real way to chose the value for the isosurface - you choose what looks good (see [1]). However, your value range seems unbounded, which makes me believe you may have a small error in your computation.
[1] Dubief, Y., Delcayre, F., 2000. On coherent-vortex identification in turbulence. Journal of Turbulence 1. URL http://dx.doi.org/10.1088/1468-5248/1/1/011
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January 16, 2019, 15:58 |
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#3 |
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Nejc
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Thanks for the reply.
Actually the values are more like -1e+7 to 1e+7; does that make any difference? What range should Q-criterion usually be in? |
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February 18, 2019, 11:07 |
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#4 |
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Lucky
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The range of Q-criterion is normally some finite negative number to positive infinity. That you have a nice symmetric negative 1e7 and positive 1e7 suggests you might have some bug. But it might still be correct. Anyway, just check.
The isosurface of Q-criterion you should plot is "some positive number that you like" Q-criterion is also dimensional in its pure definition, so one type of flow you take Q=1 and another type of flow you might take Q = 1000. You could normalize the Q-criterion to help with this. But because this is so hard to decide (it's quite arbitary what this threshold is), the closely related lambda 2 criterion is often preferred over Q-criterion (where the threshold is 0). |
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February 18, 2019, 16:14 |
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#5 |
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Nejc
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Lambda2, you say.
Never heard of that, will try out, thanks a lot! |
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February 19, 2019, 04:20 |
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#6 |
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Anton Kidess
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The paper I linked in my post a month ago also explains the lambda2 criterion.
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February 19, 2019, 07:54 |
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#7 |
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Nejc
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...but unfortunately isn't available without paid access.
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September 11, 2020, 11:01 |
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#8 | |
Senior Member
MA
Join Date: Mar 2020
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Quote:
Hi LuckyTran, I am struggling with the same. Although I have created Q-criterion but half of the blade is taken away (maybe due to the flow separation) however I have to compare my RANSE Q-Criterion (STARCCM) with reference Q-Criterion (LES)as you can see in the attached images. Why the half of blade is taken away, is there anyway to fix this issue and represent the whole Q-criterion on the blade surface? |
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