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 fruitkiwi March 22, 2012 01:22

Vortex area measurement

Good days to everyone.

Recently has a meeting with on board when present a poster, When i point out that the vortex strength is high because the vortex area is large.

One of my boss (No cfd field) ask can you measure the vortex area?like measure a rectangular use height times width.

I personally think this is impossible to me to measure the vortex area in terms of quatitative,but just want to confirm, can we measure vortex area by calculate using 2*pi*r?

 cdf_user March 22, 2012 01:50

This is not an informed answer.
Your english is not very clear but if you are trying to compare several systems using your vortex areas then you can use non-dimensionalized form of radius of vortex.
The vortex area also fluctuates with time so I am not sure if a simple pi*r*r would work.

 fruitkiwi March 22, 2012 03:14

Hi, cdf_user.

Because in the poster i only show the last time frame, and compare two different geometry at that time point and both geometry show me the different vortex area.but in my picture, there are a lot of vortex line surrounding a vortex ring.
I only look at size of vortex ring, then conclude the vortex strength is larger when vortex ring area is larger.
the 'strength' of a vortex-tube (also called vortex flux) [2] is the integral of the vorticity across a cross-section of the tube following Helmholtz's theorems.

Is my conclusion "vortex strength is larger when vortex ring area is larger." is still valid?

 cdf_user March 22, 2012 03:33

First of all vortex strength is NOT a function of area of the vortex but the curl of the velocity field of the vortex. This is a common sense answer from basic calculus 3 course from curl and divergence chapter.

A detailed answer to your question is in the book Vorticity and Vortex dynamics by J.Z. Wu on page 73, chapter 3 vortex kinematics.

I am a competitive undergraduate so I have limited knowledge about the vorticity topic. Hope this helps.

 fruitkiwi March 22, 2012 03:36

Thanks cdf_user.
You save my days.

Thank you. It solve my problem.

 lostinicem May 18, 2012 04:26

hey,
I also have a little problem with it; I have a 2-D problem and I have to measure the vortex in my geometry ( its a 2D diffuser). Can you give me some hints please?

 cdf_user May 21, 2012 03:46

I am not an expert in this field and I havent even taken the course on vortex dynamics. I refer to the book Vorticity and vortex dynamics by J.Z. Wu for all vorticity related question. The answer to your question is definitely in that book. I currently dont have the book on me since I borrowed it from the library.

 lostinicem June 13, 2012 06:17

thank you cdf user,
well seems i found a way to do it with tecplot; but it dependends on the mesh quality ...

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