UDS value go infinitie!

arash.mp · February 28, 2019, 10:51

Dear all,
i have a problem in using UDS!
i have a pipe and i want to solve a scalar! in 2D simulation I have no problem! But in 3D geometry I have problem! I solve the flow at first, then turn off the flow equation and solve only for UDS! My mesh is very low at first layer! (It is because of adjacency of pipe with another pipe which is in scale of Micrometer!)
the geometry in a simple one-eight of a cylinder with symmetry B.C at sides!
the problem comes when I change the B.C of scalar! at first i use "specified value" at inlet and wall of pipe and "specified value" at outlet!
so far everything is ok! but when I change the scalar B.C of outlet to "Specified flux of 0" at outlet, even at first iteration my solution diverges! values become negative and very large!
is that come from mesh or something else???
if i didn't explain well, please tell me to tell more details!
Thanks in advance.

LuckyTran · February 28, 2019, 11:09

Why you are putting specified flux of 0 at an outlet?

You're saying, that in 2D, you also did specified flux of 0 at the outlet and it did not blow up?

arash.mp · February 28, 2019, 15:09

Quote:

Originally Posted by LuckyTran

Why you are putting specified flux of 0 at an outlet?

You're saying, that in 2D, you also did specified flux of 0 at the outlet and it did not blow up?

in the article im validating, the BC of UDS at outlet is (dC/dx=0 and C is concentration of a particle)! so if i cant set BC at outlet to specifed flux, which BC i can use for (dC/dx=0)??
in 2D simulation i set that and all of my answers where correct!
can u help me please???
and one more strange thing is that when i use a coarser mesh in 3D simulation there isnt any problem :|

schwaral · September 1, 2020, 13:11

Could you solve the problem with minus infinity? I have the same problem when I use bigger cells.

February 28, 2019, 10:51	UDS value go infinitie!	#1
arash.mp New Member Join Date: Dec 2018 Posts: 17 Rep Power: 7	Dear all, i have a problem in using UDS! i have a pipe and i want to solve a scalar! in 2D simulation I have no problem! But in 3D geometry I have problem! I solve the flow at first, then turn off the flow equation and solve only for UDS! My mesh is very low at first layer! (It is because of adjacency of pipe with another pipe which is in scale of Micrometer!) the geometry in a simple one-eight of a cylinder with symmetry B.C at sides! the problem comes when I change the B.C of scalar! at first i use "specified value" at inlet and wall of pipe and "specified value" at outlet! so far everything is ok! but when I change the scalar B.C of outlet to "Specified flux of 0" at outlet, even at first iteration my solution diverges! values become negative and very large! is that come from mesh or something else??? if i didn't explain well, please tell me to tell more details! Thanks in advance.

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February 28, 2019, 11:09		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,675 Rep Power: 66	Why you are putting specified flux of 0 at an outlet? You're saying, that in 2D, you also did specified flux of 0 at the outlet and it did not blow up?

September 1, 2020, 13:11		#4
schwaral Member Deutschland Join Date: Jun 2020 Location: https://t.me/pump_upp Posts: 43 Rep Power: 5	Could you solve the problem with minus infinity? I have the same problem when I use bigger cells.