Is there a way to access the gradient limiter in Fluent ?

CFDYourself · February 9, 2016, 08:38

Hi there,
I've been struggling with something for several days, and I thought it might be worth asking here in-case someone with more experience can help me out.

I'm interested in getting access to the gradient limiter ('psi' in the equation in the link).
http://www.afs.enea.it/project/neptu...th/node369.htm

There should be a value of 'psi' calculated & stored in every cell each iteration. I'm interested in extracting this data and plotting it, so that I can show where psi is close to zero and where psi is close to one in my model. Psi should be stored in every cell, but I can't find any way to access that data directly.

So rather than accessing psi directly, I've been trying to access it using the "gradient"(G) and the "reconstructed gradient"(RG), which are available to access. My thinking is that the RG/G should give me psi, because psi*G = RG; that's what RG is.
I've tried doing this in two ways: one using a UDF and the data access macros C_K_RG(c,t,[0]) C_K_G(c,t,[0]) {or whatever transported variable}, and then saving this result to a user defined memory slot. The other using the "custom field function" in Fluent to do the same calculation. I'm dividing the component vectors because I know if I divide one vector by the other, I'll likely get a vector for psi where I expect a scalar.

The problem I have is the answer I get, in both cases, isn't the answer I want. I expect psi to be a scalar, between 1 and 0. I do get an average value of 0.9ish {good}. But the maximum and minimum values are in the tens-of-thousands {bad}.

Anyway, I'm at an impasse with it. My question is:
(a) is what I've done right or wrong ?
(b) is there an easier way, say, to access psi directly, say through UDF or Scheme ?

Many thanks,
Jim

PS: it's worth noting I am using the "keep temporary solver memory from being freed?" option in Fluent to access the gradient data, I'm using the UDF from the manual to check the gradients I want to access are available, and I seem to be getting similar answers between the UDF and the custom field function.

CFDYourself · February 16, 2016, 06:49

Hi, thought I would bump this thread, as I'm still having this problem.

Are there any rules for bumping on this forum ? I searched but couldn't find anything.

February 9, 2016, 08:38	Is there a way to access the gradient limiter in Fluent ?	#1
CFDYourself Member Jim Join Date: May 2015 Posts: 47 Rep Power: 11	Hi there, I've been struggling with something for several days, and I thought it might be worth asking here in-case someone with more experience can help me out. I'm interested in getting access to the gradient limiter ('psi' in the equation in the link). http://www.afs.enea.it/project/neptu...th/node369.htm There should be a value of 'psi' calculated & stored in every cell each iteration. I'm interested in extracting this data and plotting it, so that I can show where psi is close to zero and where psi is close to one in my model. Psi should be stored in every cell, but I can't find any way to access that data directly. So rather than accessing psi directly, I've been trying to access it using the "gradient"(G) and the "reconstructed gradient"(RG), which are available to access. My thinking is that the RG/G should give me psi, because psiG = RG; that's what RG is. I've tried doing this in two ways: one using a UDF and the data access macros C_K_RG(c,t,[0]) C_K_G(c,t,[0]) {or whatever transported variable}, and then saving this result to a user defined memory slot. The other using the "custom field function" in Fluent to do the same calculation. I'm dividing the component vectors because I know if I divide one vector by the other, I'll likely get a vector for psi where I expect a scalar. The problem I have is the answer I get, in both cases, isn't the answer I want. I expect psi to be a scalar, between 1 and 0. I do get an average value of 0.9ish {good}.* But the maximum and minimum values are in the tens-of-thousands {bad}. Anyway, I'm at an impasse with it. My question is: (a) is what I've done right or wrong ? (b) is there an easier way, say, to access psi directly, say through UDF or Scheme ? Many thanks, Jim PS: it's worth noting I am using the "keep temporary solver memory from being freed?" option in Fluent to access the gradient data, I'm using the UDF from the manual to check the gradients I want to access are available, and I seem to be getting similar answers between the UDF and the custom field function. Last edited by CFDYourself; February 9, 2016 at 09:58.

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February 16, 2016, 06:49		#2
CFDYourself Member Jim Join Date: May 2015 Posts: 47 Rep Power: 11	Hi, thought I would bump this thread, as I'm still having this problem. Are there any rules for bumping on this forum ? I searched but couldn't find anything.