What's the physical meaning of Q-Criterion?

Moreza7 · February 9, 2021, 13:12

Hello,

According to the definition, the Q-criterion defines vortices as areas where the vorticity magnitude is greater than the magnitude of the rate of strain.

Why does Q-Criterion show the vortices? Why should Q>0 show a vortex? From a mathematical point of view, it's just a number!

Also, suppose that we plot Q=10 and the Q=10000 what's the difference between the vortices with Q=10 and Q=10000?

Thanks

FMDenaro · February 9, 2021, 14:04

Quote:

Originally Posted by Moreza7

Hello,

According to the definition, the Q-criterion defines vortices as areas where the vorticity magnitude is greater than the magnitude of the rate of strain.

Why does Q-Criterion show the vortices? Why should Q>0 show a vortex? From a mathematical point of view, it's just a number!

Also, suppose that we plot Q=10 and the Q=10000 what's the difference between the vortices with Q=10 and Q=10000?

Thanks

Have you already studied this paper?
https://www.researchgate.net/publica..._JFM_285_69-94

Moreza7 · February 9, 2021, 14:33

Quote:

Originally Posted by FMDenaro

Have you already studied this paper?
https://www.researchgate.net/publica..._JFM_285_69-94

Thank you very much.
The first question is answered now. But what about the second question?
What's the difference between the vortices with Q=10 and Q=1000? I think when I plot the Q=10, the vortices with Q=1000 are also plotted. Also, sometimes I get the same structures with Q=10 and Q=1000! How can a vortex have two Qs?! It's confusing.

FMDenaro · February 9, 2021, 14:56

Quote:

Originally Posted by Moreza7

Thank you very much.
The first question is answered now. But what about the second question?
What's the difference between the vortices with Q=10 and Q=1000? I think when I plot the Q=10, the vortices with Q=1000 are also plotted. Also, sometimes I get the same structures with Q=10 and Q=1000! How can a vortex have two Qs?! It's confusing.

The chosen value Q enters as a source term of the pressure equations, it defines the level of the pressure lines. That means you are considering different sources terms in the Poisson equation for the pressure. The vortex core is associated to minima in the pressure.

aero_head · February 9, 2021, 15:51

Thank you Moreza for the question and thank you professor for the paper and the information! Helped me understand this concept better as well.

FMDenaro · February 9, 2021, 16:39

I suggest also this disseration
https://ntrs.nasa.gov/citations/19910017151

gnwt4a · February 10, 2021, 06:57

the clearest flow interpretation (which still stands) of the (im)balance between the strain & vorticity terms in the poisson equation for the pressure, was originally proposed by Bradshaw&Koh in a short note that appeared in the phys of fluids of 1981. this is where u start. the later elaborations of this are computationally useful.
--

aero_head · February 10, 2021, 13:06

Quote:

Originally Posted by gnwt4a

the clearest flow interpretation (which still stands) of the (im)balance between the strain & vorticity terms in the poisson equation for the pressure, was originally proposed by Bradshaw&Koh in a short note that appeared in the phys of fluids of 1981. this is where u start. the later elaborations of this are computationally useful.
--

Hello JJ,

Do you have a link to this paper you refer to, or even just the full title?

gnwt4a · February 10, 2021, 21:46

https://doi.org/10.1063/1.863442
--

aero_head · February 10, 2021, 22:07

Quote:

Originally Posted by gnwt4a

https://doi.org/10.1063/1.863442
--

Ah, excellent! Thank you very much.

February 9, 2021, 13:12	What's the physical meaning of Q-Criterion?	#1
Moreza7 Senior Member Join Date: Jan 2018 Posts: 121 Rep Power: 8	Hello, According to the definition, the Q-criterion defines vortices as areas where the vorticity magnitude is greater than the magnitude of the rate of strain. Why does Q-Criterion show the vortices? Why should Q>0 show a vortex? From a mathematical point of view, it's just a number! Also, suppose that we plot Q=10 and the Q=10000 what's the difference between the vortices with Q=10 and Q=10000? Thanks rfavalli, Zubeldia, DiegoQuan and 1 others like this.

February 9, 2021, 15:51		#5
aero_head Senior Member Kira Join Date: Nov 2020 Location: Canada Posts: 435 Rep Power: 9	Thank you Moreza for the question and thank you professor for the paper and the information! Helped me understand this concept better as well. FMDenaro and Moreza7 like this.

February 9, 2021, 16:39		#6
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,882 Rep Power: 73	I suggest also this disseration https://ntrs.nasa.gov/citations/19910017151 rfavalli, Moreza7 and aero_head like this.

February 10, 2021, 06:57		#7
gnwt4a Member EM Join Date: Sep 2019 Posts: 59 Rep Power: 7	the clearest flow interpretation (which still stands) of the (im)balance between the strain & vorticity terms in the poisson equation for the pressure, was originally proposed by Bradshaw&Koh in a short note that appeared in the phys of fluids of 1981. this is where u start. the later elaborations of this are computationally useful. -- sbaffini and aero_head like this.

February 10, 2021, 21:46		#9
gnwt4a Member EM Join Date: Sep 2019 Posts: 59 Rep Power: 7	https://doi.org/10.1063/1.863442 -- ilhado, rfavalli and aero_head like this.

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