convergence problem in a long pipe

feizaghaee · February 4, 2010, 17:04

hi freinds
i'm trying to simulate a long pipe (180 m length) with 1.5 m diameter that has a 90 degree curve in one third of length from begining.inlet boundry conditions are velocity of .8 m/s and static pressure. outlet boundry condition is velocity of .8 m/s normal to the boundry. after about 2000, residules don't change and they approximately became a smooth line parallel to each other. i use hexa mesh that each cell along the pipe has 1 meter length. i use multiphase option. does anyone know what can i do to obtain convergence.

ckleanth · February 4, 2010, 17:57

do you need to model the problem using cfd? you didnt say what you want to model anyway.

feizaghaee · February 4, 2010, 18:37

i want to model the water and air flow.

feizaghaee · February 5, 2010, 10:49

i work with CFX 11.0.

ckleanth · February 5, 2010, 11:22

I will reiterate; you did not say what you want to model, no information on the physical or how you modeled it. if you want an answer please post an apropriate question.

ghorrocks · February 7, 2010, 16:20

For general comments on convergence see here:
http://www.cfd-online.com/Wiki/Ansys...gence_criteria

AliTr · February 8, 2010, 17:45

1m mesh length is large if you are modeling multiphase Air/water, you haven't told the other mesh dimensions but relatively they should be large as well. you need smaller mesh size for better resolved water surface, in most of the Air/Water models, this is the reason for flat line convergence.
you can also try smaller physical timescale while reducing the mesh size.

feizaghaee · February 16, 2010, 08:05

i solved it by changing the time scale. tanks a lot everyone

February 4, 2010, 17:57		#2
ckleanth Senior Member George Join Date: Mar 2009 Location: Birmingham, UK Posts: 257 Rep Power: 18	do you need to model the problem using cfd? you didnt say what you want to model anyway. __________________ Top 4 tips 1. Knowledge is everything and Ignorance is dangerous. 2. Understand your limitations and try to eliminate them. 3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window. 4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials

February 5, 2010, 11:22		#5
ckleanth Senior Member George Join Date: Mar 2009 Location: Birmingham, UK Posts: 257 Rep Power: 18	I will reiterate; you did not say what you want to model, no information on the physical or how you modeled it. if you want an answer please post an apropriate question. __________________ Top 4 tips 1. Knowledge is everything and Ignorance is dangerous. 2. Understand your limitations and try to eliminate them. 3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window. 4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials

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February 4, 2010, 18:37		#3
feizaghaee Senior Member moein Join Date: Dec 2009 Posts: 132 Rep Power: 16	i want to model the water and air flow.

February 5, 2010, 10:49		#4
feizaghaee Senior Member moein Join Date: Dec 2009 Posts: 132 Rep Power: 16	i work with CFX 11.0.

February 7, 2010, 16:20		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	For general comments on convergence see here: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

February 8, 2010, 17:45		#7
AliTr Member Ali Torbaty Join Date: Jul 2009 Location: Sydney, Australia Posts: 72 Rep Power: 16	1m mesh length is large if you are modeling multiphase Air/water, you haven't told the other mesh dimensions but relatively they should be large as well. you need smaller mesh size for better resolved water surface, in most of the Air/Water models, this is the reason for flat line convergence. you can also try smaller physical timescale while reducing the mesh size.

February 16, 2010, 08:05		#8
feizaghaee Senior Member moein Join Date: Dec 2009 Posts: 132 Rep Power: 16	i solved it by changing the time scale. tanks a lot everyone