User-defined k or epsilon?

tstorm · October 12, 2009, 13:40

Has anyone written successful UDF source equations for turbulence kinetic energy (k) or turbulence dissipation rate (epsilon)?

I've tried writing mine based on the k-e equations here (http://www.cfd-online.com/Wiki/Standard_k-epsilon_model) but velocity profiles in the boundary layer don't match the Blaussius curve.

Thanks!

jalarron · October 12, 2009, 20:52

Currently I'm implementing a splitting technique for k-e model, so I have to add two k-e extra equations. I've done it successfully.
Are you trying to modify existing Fluent k-e equations? or you are trying to add k-e from scratch?
Cheers,
Javier

tstorm · October 12, 2009, 22:13

I'm trying to start from scratch. I'm making several v2-f models, which use the standard k and e equations.
Would you mind posting your k and e source code? I'd appreciate it.
Thanks!

jalarron · October 12, 2009, 23:47

Which viscous model are you using for your implementation from scratch of k-e? laminar model + 2 UDS equation for k-e?
About the source code, I have to check first with my supervisor, just for IP reasons. I think it shouldn't be a problem, because is a method already published and I'm just implementing it in FLUENT.
Currently I haven't tried to start from scratch but it is one option that we were thinking because patching the in-built k-e it's a bit tricky specially in the near-wall region.
Cheers,
Javier

tstorm · October 13, 2009, 01:11

I'm actually using the k-e model with turbulence equations turned off. I hadn't considered running it with the laminar model. I might try that tomorrow, it may solve some problems.

Just to give you some background, the v2-f model is based on the k-e model, and uses the same k and e equations, with some minor differences. The constants are calibrated differently - in particular, Ce2 is a function of turbulent quantities, and isn't constant. Also, the model is integratable to solid walls and doesn't use damping functions or wall functions. So, FLUENT's k and e equations won't work.

I hope you can share your code - I can't imagine that the k and e sources would be proprietary, but I understand if you can't share them.

Thanks!

tstorm · October 13, 2009, 11:35

I just remembered why I haven't used the laminar model with 4 UDSs (k, e, v2, f). I need to use a user-defined turbulent viscosity, which is made easy when you use the k-e model with turbulence equations turned off.

I'm assuming if you plan to start from scratch with k and e UDSs you'll need to store turbulent viscosity as well. Had you figured out how you were going to hook that to the solver?

jalarron · October 13, 2009, 21:21

I haven't tried anything with the laminar option, but I think that you could store the turbulent viscosity in a UDM variable, and updated in every iteration.
I'm sending you a document of what I did through a private message. OK?
Cheers,
Javier

tstorm · October 14, 2009, 11:55

You could store mu_t as a UDM, but I don't see how you'd hook it to the solver in the laminar model. The laminar model should have mu_t = 0. If you figure that out I'd love to hear how it turns out.

October 12, 2009, 13:40	User-defined k or epsilon?	#1
tstorm Member Join Date: Apr 2009 Posts: 78 Rep Power: 17	Has anyone written successful UDF source equations for turbulence kinetic energy (k) or turbulence dissipation rate (epsilon)? I've tried writing mine based on the k-e equations here (http://www.cfd-online.com/Wiki/Standard_k-epsilon_model) but velocity profiles in the boundary layer don't match the Blaussius curve. Thanks!

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October 12, 2009, 20:52		#2
jalarron New Member Javier Larrondo Join Date: Jun 2009 Posts: 17 Blog Entries: 1 Rep Power: 16	Currently I'm implementing a splitting technique for k-e model, so I have to add two k-e extra equations. I've done it successfully. Are you trying to modify existing Fluent k-e equations? or you are trying to add k-e from scratch? Cheers, Javier

October 12, 2009, 22:13		#3
tstorm Member Join Date: Apr 2009 Posts: 78 Rep Power: 17	I'm trying to start from scratch. I'm making several v2-f models, which use the standard k and e equations. Would you mind posting your k and e source code? I'd appreciate it. Thanks!

October 12, 2009, 23:47		#4
jalarron New Member Javier Larrondo Join Date: Jun 2009 Posts: 17 Blog Entries: 1 Rep Power: 16	Which viscous model are you using for your implementation from scratch of k-e? laminar model + 2 UDS equation for k-e? About the source code, I have to check first with my supervisor, just for IP reasons. I think it shouldn't be a problem, because is a method already published and I'm just implementing it in FLUENT. Currently I haven't tried to start from scratch but it is one option that we were thinking because patching the in-built k-e it's a bit tricky specially in the near-wall region. Cheers, Javier

October 13, 2009, 01:11		#5
tstorm Member Join Date: Apr 2009 Posts: 78 Rep Power: 17	I'm actually using the k-e model with turbulence equations turned off. I hadn't considered running it with the laminar model. I might try that tomorrow, it may solve some problems. Just to give you some background, the v2-f model is based on the k-e model, and uses the same k and e equations, with some minor differences. The constants are calibrated differently - in particular, Ce2 is a function of turbulent quantities, and isn't constant. Also, the model is integratable to solid walls and doesn't use damping functions or wall functions. So, FLUENT's k and e equations won't work. I hope you can share your code - I can't imagine that the k and e sources would be proprietary, but I understand if you can't share them. Thanks!

October 13, 2009, 11:35		#6
tstorm Member Join Date: Apr 2009 Posts: 78 Rep Power: 17	I just remembered why I haven't used the laminar model with 4 UDSs (k, e, v2, f). I need to use a user-defined turbulent viscosity, which is made easy when you use the k-e model with turbulence equations turned off. I'm assuming if you plan to start from scratch with k and e UDSs you'll need to store turbulent viscosity as well. Had you figured out how you were going to hook that to the solver?

October 13, 2009, 21:21		#7
jalarron New Member Javier Larrondo Join Date: Jun 2009 Posts: 17 Blog Entries: 1 Rep Power: 16	I haven't tried anything with the laminar option, but I think that you could store the turbulent viscosity in a UDM variable, and updated in every iteration. I'm sending you a document of what I did through a private message. OK? Cheers, Javier

October 14, 2009, 11:55		#8
tstorm Member Join Date: Apr 2009 Posts: 78 Rep Power: 17	You could store mu_t as a UDM, but I don't see how you'd hook it to the solver in the laminar model. The laminar model should have mu_t = 0. If you figure that out I'd love to hear how it turns out.