[General] Detect vortex core (point) in 2D

t.teschner · June 2, 2017, 19:07

I am trying to extract the vortex core(s) of my 2D simulations (for simplicity lets assume it is the lid driven cavity) and have tried the lambda2 method which i implemented via the python calculator. but it seems the method is only valid for 3D(?!).

I was going through some literature and it seems that this is not an easy topic but I was wondering if someone has already extracted 2D vortex center points in paraview and could share the approach taken. In a case, like the lid driven cavity, where I have multiple vortex cores corresponding to a couple of vortices, how can I differentiate between those different vortices?

juliom · June 4, 2017, 16:53

Hi, in fact that's is quite challenging because Q criterion is the second invariant of the velocity gradient tensor. We know that velocity is 3D vector field. Therefore, visualizing it in 2 D yields to a wrong conclusion. Also remember that vorticity does not exist in 2 D flows since the Z complement is zero. A lot of different thoughts isn't ?. Ok, if you see the definition of Lamda 2, you have the the hessian of pressure equals The symmetric plus the asymmetric part of the velocity gradient tensor. Again, the hessian is a matrix 3x3 but you endup with a 2x2 matrix, thus the physics your are going after is lost.
My impression is that you will see spots with high rotation but you are not sure if those are vortices.i haven't read anything related to 2D vortical structures

t.teschner · June 6, 2017, 05:16

thanks julio, your reply actually makes a lot of sense. if i try to detect vortices (in the lid driven cavity), the whole cavity is visualised as basically one coherent vortex and i can not detec individual vortices. it would have been just nice to get the vortex core for comparison reasons (in the ghia et al. paper there are vortex core coordinates given). anyways, thanks again, i will focus on more measurable values

poyilil · June 13, 2017, 03:54

I want to get the bottom corner vortices in a 2D lid driven cavity simulation of OpenFOAM post processed in ParaFOAM. I'm not exactly sure why I'm not getting plots similar to ghia et. al. though I've tried refining my mesh, changing total simulation time, etc. Now I'm doubting if the error is in post processing. Can somebody explain how to get the curve?

juliom · June 13, 2017, 09:35

Have you tried other measurements? instead of using qualitatively plots. For example, have you compared your velocity field with theirs? I think this will be easier and straightforward. Once the velocity field agrees then you can think about the vortices, like I said before, vortices is a 3D phenomena. That is why there is no literature in 2D vortical structures.

June 2, 2017, 19:07	Detect vortex core (point) in 2D	#1
t.teschner Senior Member Tom-Robin Teschner Join Date: Dec 2011 Location: Cranfield, UK Posts: 204 Rep Power: 16	I am trying to extract the vortex core(s) of my 2D simulations (for simplicity lets assume it is the lid driven cavity) and have tried the lambda2 method which i implemented via the python calculator. but it seems the method is only valid for 3D(?!). I was going through some literature and it seems that this is not an easy topic but I was wondering if someone has already extracted 2D vortex center points in paraview and could share the approach taken. In a case, like the lid driven cavity, where I have multiple vortex cores corresponding to a couple of vortices, how can I differentiate between those different vortices?

June 4, 2017, 16:53		#2
juliom Senior Member Julio Mendez Join Date: Apr 2009 Location: Fairburn, GA. USA Posts: 290 Rep Power: 18	Hi, in fact that's is quite challenging because Q criterion is the second invariant of the velocity gradient tensor. We know that velocity is 3D vector field. Therefore, visualizing it in 2 D yields to a wrong conclusion. Also remember that vorticity does not exist in 2 D flows since the Z complement is zero. A lot of different thoughts isn't ?. Ok, if you see the definition of Lamda 2, you have the the hessian of pressure equals The symmetric plus the asymmetric part of the velocity gradient tensor. Again, the hessian is a matrix 3x3 but you endup with a 2x2 matrix, thus the physics your are going after is lost. My impression is that you will see spots with high rotation but you are not sure if those are vortices.i haven't read anything related to 2D vortical structures amolrajan likes this.

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June 6, 2017, 05:16		#3
t.teschner Senior Member Tom-Robin Teschner Join Date: Dec 2011 Location: Cranfield, UK Posts: 204 Rep Power: 16	thanks julio, your reply actually makes a lot of sense. if i try to detect vortices (in the lid driven cavity), the whole cavity is visualised as basically one coherent vortex and i can not detec individual vortices. it would have been just nice to get the vortex core for comparison reasons (in the ghia et al. paper there are vortex core coordinates given). anyways, thanks again, i will focus on more measurable values

June 13, 2017, 03:54		#4
poyilil New Member LAKSHMIPRIYA Join Date: Jun 2017 Posts: 7 Rep Power: 8	I want to get the bottom corner vortices in a 2D lid driven cavity simulation of OpenFOAM post processed in ParaFOAM. I'm not exactly sure why I'm not getting plots similar to ghia et. al. though I've tried refining my mesh, changing total simulation time, etc. Now I'm doubting if the error is in post processing. Can somebody explain how to get the curve?

June 13, 2017, 09:35		#5
juliom Senior Member Julio Mendez Join Date: Apr 2009 Location: Fairburn, GA. USA Posts: 290 Rep Power: 18	Have you tried other measurements? instead of using qualitatively plots. For example, have you compared your velocity field with theirs? I think this will be easier and straightforward. Once the velocity field agrees then you can think about the vortices, like I said before, vortices is a 3D phenomena. That is why there is no literature in 2D vortical structures.