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August 16, 2005, 00:43 
Convergence problem(please help)

#1 
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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. ke 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.. 

August 16, 2005, 11:22 
Re: Convergence problem(please help)

#2 
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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 ( ke 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. 

August 17, 2005, 00:03 
Re: Convergence problem(please help)

#3 
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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 3e5. But while solving, before convergence, i had got it as 2e5.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 

August 18, 2005, 13:42 
Re: Convergence problem(please help)

#4 
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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. 

August 19, 2005, 00:23 
Re: Convergence problem(please help)

#5 
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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... 

August 21, 2005, 13:39 
Re: Convergence problem(please help)

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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. 

August 22, 2005, 02:26 
Re: Convergence problem(please help)

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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...


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