In Q-criterion method, is the value of Q representative of vortex strength?

SBanerjee · December 14, 2019, 06:11

It is known that connected regions of Q>0 represents vortices. Therefore can we think the magnitude of Q above 0 is representative of the vortex strength?

sbaffini · December 14, 2019, 06:26

I know this doesn't seem to help but, stick with me for a moment: what do you think it is vortex strength in the first place?

sbaffini · December 14, 2019, 06:32

Or, even better, what do you think a vortex is?

SBanerjee · December 14, 2019, 06:39

Quote:

Originally Posted by sbaffini

I know this doesn't seem to help but, stick with me for a moment: what do you think it is vortex strength in the first place?

Strength of vortex is its intensity, i.e., the vigor of the swirl

sbaffini · December 14, 2019, 06:59

But you are just using synonyms for the exact same wording. There is nothing mathematical or physical in that concept.

What I wanted to put forward is that, as a matter of fact, we don't have a single, clear, physical definition of what a vortex is.

What we have, instead, is an ensemble of features we think should be present in what we call vortex.

Q is just one of them. Quoting Lesieur: "Q>0 is a necessary condition for the existence of thin, convex low-pressure tubes [...] which are generally associated with coherent vortices".

In particular, note that nothing is said on its magnitude. However, the higher is Q, the larger the local pressure Laplacian, which is a measure of the local pressure difference.

FMDenaro · December 14, 2019, 07:25

I think we should come back to the basic definition of vortex tube as detailed by Chorin & Marsden in their textbook. Intensity of vorticity multiplied by the area section is constant.

December 14, 2019, 06:11	In Q-criterion method, is the value of Q representative of vortex strength?	#1
SBanerjee New Member S Banerjee Join Date: Jul 2019 Posts: 4 Rep Power: 7	It is known that connected regions of Q>0 represents vortices. Therefore can we think the magnitude of Q above 0 is representative of the vortex strength?

December 14, 2019, 06:59		#5
sbaffini Senior Member Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 2,195 Blog Entries: 29 Rep Power: 39	But you are just using synonyms for the exact same wording. There is nothing mathematical or physical in that concept. What I wanted to put forward is that, as a matter of fact, we don't have a single, clear, physical definition of what a vortex is. What we have, instead, is an ensemble of features we think should be present in what we call vortex. Q is just one of them. Quoting Lesieur: "Q>0 is a necessary condition for the existence of thin, convex low-pressure tubes [...] which are generally associated with coherent vortices". In particular, note that nothing is said on its magnitude. However, the higher is Q, the larger the local pressure Laplacian, which is a measure of the local pressure difference. FMDenaro likes this.

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December 14, 2019, 06:26		#2
sbaffini Senior Member Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 2,195 Blog Entries: 29 Rep Power: 39	I know this doesn't seem to help but, stick with me for a moment: what do you think it is vortex strength in the first place?

December 14, 2019, 06:32		#3
sbaffini Senior Member Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 2,195 Blog Entries: 29 Rep Power: 39	Or, even better, what do you think a vortex is?

December 14, 2019, 07:25		#6
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,896 Rep Power: 73	I think we should come back to the basic definition of vortex tube as detailed by Chorin & Marsden in their textbook. Intensity of vorticity multiplied by the area section is constant.