about Taylor-Green Vortex problem in CFX

cuteapathy · March 25, 2012, 07:15

Hi everyone,

I am trying to compare cfx and openfoam by solving taylor-green vortex problem on a cube in both of them. I applied the same initial conditions in both of them. For boundary conditions, in openfoam, I applied both of pressure and velocity boundary conditions on my boundaries. In cfx, I applied pressure on three of boundaries and velocity on the other three ones. But, I got totally different results and figures from them. I am sure that I got correct results and figures in openfoam, but I do not know why it is wrong in cfx. I am wondering whether anyone tried this problem before and could kindly give me some advice about how to fix it. Thanks for any response.

ghorrocks · March 25, 2012, 07:32

Why does the Taylor-Green vortex need flow boundary conditions at all? It is the decay of a vortex so why not put the outer boundary far away and simply use walls?

cuteapathy · March 25, 2012, 10:07

Dear Glenn,

My advisor asks me to apply both of bondary conditions on six faces. I did it in openfoam and it worked. In cfx, do you mean I could use three walls instead of three outlets and remain to apply velocity on the other three inlets? Thank you for your response.

ghorrocks · March 25, 2012, 19:05

Do you know how many times on the forum the reply to "why are you doing something bizarre?" is that my supervisor told me to do so......

I am no expert on the Taylor-Green vortex, but from what I have just read it is simply the decay of a vortex. There are no flows in or out of the domain. To accurately model this you will need your boundaries far enough away to not affect the dissipation. So the domain which makes sense to me is one with slip walls quite a way away from the central vortex. I cannot see how inlet or outlet boundaries are required here.

Also note there is an analytical solution for this flow so you have precise answers to compare against.

If you insist on using inlet/outlet/opening boundaries can you post an image of what you are modelling as in that case I am clearly missing something.

cuteapathy · March 26, 2012, 07:44

Dear Glenn,

When I check some online information again, I think you are right. I am gonna try your suggestions. Thank you for your detailed advice. I appreciate your help.

March 25, 2012, 07:15	about Taylor-Green Vortex problem in CFX	#1
cuteapathy Member Join Date: Jan 2012 Posts: 36 Rep Power: 14	Hi everyone, I am trying to compare cfx and openfoam by solving taylor-green vortex problem on a cube in both of them. I applied the same initial conditions in both of them. For boundary conditions, in openfoam, I applied both of pressure and velocity boundary conditions on my boundaries. In cfx, I applied pressure on three of boundaries and velocity on the other three ones. But, I got totally different results and figures from them. I am sure that I got correct results and figures in openfoam, but I do not know why it is wrong in cfx. I am wondering whether anyone tried this problem before and could kindly give me some advice about how to fix it. Thanks for any response.

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March 25, 2012, 07:32		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Why does the Taylor-Green vortex need flow boundary conditions at all? It is the decay of a vortex so why not put the outer boundary far away and simply use walls?

March 25, 2012, 10:07		#3
cuteapathy Member Join Date: Jan 2012 Posts: 36 Rep Power: 14	Dear Glenn, My advisor asks me to apply both of bondary conditions on six faces. I did it in openfoam and it worked. In cfx, do you mean I could use three walls instead of three outlets and remain to apply velocity on the other three inlets? Thank you for your response.

March 25, 2012, 19:05		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Do you know how many times on the forum the reply to "why are you doing something bizarre?" is that my supervisor told me to do so...... I am no expert on the Taylor-Green vortex, but from what I have just read it is simply the decay of a vortex. There are no flows in or out of the domain. To accurately model this you will need your boundaries far enough away to not affect the dissipation. So the domain which makes sense to me is one with slip walls quite a way away from the central vortex. I cannot see how inlet or outlet boundaries are required here. Also note there is an analytical solution for this flow so you have precise answers to compare against. If you insist on using inlet/outlet/opening boundaries can you post an image of what you are modelling as in that case I am clearly missing something.

March 26, 2012, 07:44		#5
cuteapathy Member Join Date: Jan 2012 Posts: 36 Rep Power: 14	Dear Glenn, When I check some online information again, I think you are right. I am gonna try your suggestions. Thank you for your detailed advice. I appreciate your help.