CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > FLUENT

Convergence problem(please help)

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   August 16, 2005, 00:43
Default Convergence problem(please help)
  #1
Shiju G.Thomas
Guest
 
Posts: n/a
Sir,

I am simulating flow distribution in a simple circular header.One inlet pipe in vertical direction and 11 outlet pipes in horizontal direction.I have a 3d model with tet meshes.Velocity inlet and pressure outlet are specified.The problem is that as i go on iterating, the solution is continuosly changing.The net mass flow also seem to be fluctuating.it comes to a low of 2e(-5) and then shoots up. k-e turbulent model is used.Turbulent quantities are the first ones to go up in the residuals.Following that, continuity also moves up.Then all the problem starts....Please help..
  Reply With Quote

Old   August 16, 2005, 11:22
Default Re: Convergence problem(please help)
  #2
Aly
Guest
 
Posts: n/a
what do you mean by " solution is continuosly changing" elborate more....

if it goes up then its diverging ......... could be many factors!

start by checking your 3d grid quality, then y+ values ( k-e turbulent model), then try lowering relaxtion factors, take a systematic approach i.e. change the r.f one at a time. Use a low accuracy algorithm to start the solution.

  Reply With Quote

Old   August 17, 2005, 00:03
Default Re: Convergence problem(please help)
  #3
Shiju G.Thomas
Guest
 
Posts: n/a
Sir,

Thanks for ur response.My purpose is to measure the distribution of flow to 11 outlet pipes from one inlet pipe through a header.What i meant by saying solution continuosly changing was that the flow distribution through outlet pipes(flow rates) was changing.On ur response i tried a 2d domain to check y+ and grid quality.What i find is after 1600 iterations the residuals stop varying and i get a solution.The mass imbalance now is 3e-5. But while solving, before convergence, i had got it as 2e-5.Can i trust the final one as real solution.

I read that using under relaxation factors final solution will not change theoretically.Is this always true.Did any one have a different experience on this.. hoping response from cfd wizards
  Reply With Quote

Old   August 18, 2005, 13:42
Default Re: Convergence problem(please help)
  #4
Masood
Guest
 
Posts: n/a
hi Thomas i think ur main problem is that u r using velocity Boundary Condition at inlet with Pressure B.C. at exit. not every B.C. can be used with each other. try giving the velocity outlet condition at exit if u can claculate velocity by hand (flow incompressible) or try to give pressure B.C. at inlet. or give outflow condition at exit with velocity B.C. at inlet.

wish u good luck.
  Reply With Quote

Old   August 19, 2005, 00:23
Default Re: Convergence problem(please help)
  #5
Shiju G.Thomas
Guest
 
Posts: n/a
Thanks very much for ur response masood.. What u said was true. It worked well when i gave velocity outlet and pressure inlet.But my problem is to find the variation in flow rate through 11 outlet pipes connected to a single inlet through a circular header.In the case where i know experimental results i can try velocity outlets and it works well.But in the other cases what can i do?

In the case of outflow and velocity inlet, it converges very well. But because it assumes fully developed flow at outlets, i am getting a uniform flow rates through all the outlets which is not true...

I really think some of u wil be able to help...
  Reply With Quote

Old   August 21, 2005, 13:39
Default Re: Convergence problem(please help)
  #6
Masood
Guest
 
Posts: n/a
hummmm outflow is used with more than one outlet by defining the portion of inflow coming out from the each out let. which have range of (0 - 1) means 0 to 100% of inflow. u didnt mentioned that u r using compressible or incompressible. flow. if incompressible, and i also assumes that u r solving for steady state solution. so outflow can be used. as it is easy to apply. just as u want to study only the behaviour in the header so dont worry if outflow require fully developed at out let. just extend the outlets from headers that much that where fluid is fully developed and no eddies are present at that section. so reverse flow at outlet will not occur. and study the region of ur interest. and u mentioned uniform flow rates, do u mean same quantity or not varying with time flow rates(that should be in case of unsteady case). also if u want to steady flow distribution try apply pressure boundary condition at inlet and at outlet, which is possible . as that what actually happen pressure diffence cause flow. try it m8.

wish u good luck this time.

  Reply With Quote

Old   August 22, 2005, 02:26
Default Re: Convergence problem(please help)
  #7
shiju G.thomas
Guest
 
Posts: n/a
Thanks for giving valuable suggestions..... Mine is an incompressible flow.Steady state.What i meant by uniform flow rates through outlets was same value of mass flow rate through all outlets.When i give outflow this is happening which is not the actual result..I will try the diferent options given by u...
  Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Convergence Centurion2011 FLUENT 26 November 24, 2015 15:46
Force can not converge colopolo CFX 13 October 4, 2011 22:03
Convergence of CFX field in FSI analysis nasdak CFX 2 June 29, 2009 01:17
increasing mesh quality is leading to poor convergence tippo CFX 2 May 5, 2009 10:55
Defect correction and convergence ganesh Main CFD Forum 4 June 30, 2006 14:20


All times are GMT -4. The time now is 02:32.