# Convergence

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November 14, 2014, 10:24
#21
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Manu Poolakkal
Join Date: Oct 2014
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for convergence, is it necessary to display "solution converged!" or something like that in fluent?
my simulation is for a double pipe counter flow heat exchanger (3d). even after 10000 iteration fluent is not displaying anything like "solution converged !" but the results seem to be not changing. what should i do? how can i check whether the solution is converged or not?

 November 18, 2014, 05:17 Convergence in gas quenching process #22 New Member   Join Date: Oct 2014 Posts: 17 Rep Power: 7 Hi Centurion 2011, I got a big problem in convergence criterion for my simulation through Ansys Fluent. I have almost tried all steps that you have suggested in your post about convergence. Let me expalin my problem first. Inorder to run a transient simulation, initial converged inputs are essential. So, I ran a steady state pressure based simulation. Convection and conduction processes takes place in quenching. So,obviously energy and turbulent models were switched on. I used pressure based coupled solver with pseudo transient method to speed up the convergence. The discretization schemes were (default), Pressure- Standard Momentum- second order upwind Energy- second order upwind I have tried Higher order term relaxation with default (0.75) relaxation factor and also 0.25 (in another simulation). The simulation ran for 5000 iterations with pseudo time step of 1e-05. The result was not so convincing i.e., the solution was not converged. At the end of the simulation, my conservative terms (residuals)were of the order continuity e-03 momentum e-05 in x direction e-04 in y direction e-05 in z-direction respectively energy e-17 mass imbalance e+02 These residuals keeps on oscillating and I couldn't standardise the solution.I have also cross-checked the mesh. It has decent orthogonal quality and skewness. Minimum orthogonal quality is 0.20 Max. Skewness is 0.85 Max aspect ration is 45 but, these values are accepted as the geometry of the CAD model is relatively complex. And my question here is , should I try for more iterations or should I further reduce relaxation terms? If there are any other suggestions, it is always welcome. And please correct me, if the steps that I mentioned here are inappropriate. I also expect inputs from everyone.

 February 18, 2015, 08:17 e #23 Member     Join Date: Jul 2011 Posts: 74 Blog Entries: 1 Rep Power: 14 sorry, wasn't around for quite some time, didnt see questions __________________ I'M NOT A GYNECOLOGIST BUT I'LL TAKE A LOOK.

 February 18, 2015, 15:12 #24 New Member   Join Date: Oct 2014 Posts: 17 Rep Power: 7 got my problem solved long back

May 9, 2015, 09:02
#25
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Imran Ansari
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Location: Aligarh Muslim University
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When using NITA in the fraction Step method. As CL And CD achived to their correct value and as time advances there is no changes in these values . but continuity residual varies between 0.1 to 0.001 . Is we can say solution converged or not . is solution correct or not??

 November 19, 2015, 04:51 limitation of pressure-based and density-based solvers Fluent V15 #26 New Member   HALOUANE Yacine Join Date: Jun 2015 Posts: 2 Rep Power: 0 Hello everyone, I know that the pressure based solver is better for slow incompressible flow and the density-based solver is recommended for supersonic flow and shock waves simulation. I'm wondering about the limitation of each solver? when or where can I use them or cannot use them? I prefer to give the reference of your answers and thank's a lot. good luck.

 November 24, 2015, 15:46 problem #27 New Member   frank Join Date: Oct 2014 Posts: 5 Rep Power: 7 Hi there my case is about oil-WATER two phase flow. I couldn't achieve convergence because my continuity residuals are about 10^-2. what do you think about this problem? is this problem because of courant number(about 0.02) or momentum under relaxation(0.5) or multiphase flow coupled scheme that I used?

 February 7, 2016, 12:58 continuity equation convergence #28 Senior Member   Join Date: Oct 2014 Posts: 124 Rep Power: 7 Hello centurion2011. I am simulating an open channel with some hydraulic structures inside it. Steady state, VOF for the free surface. All the variables (xvelocity, y velocity, z velocity, k, epsilon, VOF of the secondary phase) converge with residual of 10^(-4), whereas the continuity equation residuals remain around 10^(-1), although the flow does not change any longer during iterations (10000 iterations!). Do you know why? Thank you

 August 27, 2016, 04:08 Convergence #29 New Member   asma hosseini Join Date: Jul 2016 Posts: 2 Rep Power: 0 Hello I'm working a nano channel flow with electrophorse. I'm told get a plot of EOF velocity every 2000 iterations and when 2 plots had no changes, my solution is converged. I have calculated over 51000 iterations but plots change always. What should I do?

 August 27, 2016, 04:19 #30 New Member   Imran Ansari Join Date: Jun 2014 Location: Aligarh Muslim University Posts: 3 Rep Power: 7 it will depend on type of problem steady/transient >you should run more than 150 non dimensional time. and see the pattern it will be of oscillatory nature. and reduce your time step size.

October 3, 2016, 05:45
Same convergence results - problem solved?
#31
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 Originally Posted by Ema40 Hello centurion2011. I am simulating an open channel with some hydraulic structures inside it. Steady state, VOF for the free surface. All the variables (xvelocity, y velocity, z velocity, k, epsilon, VOF of the secondary phase) converge with residual of 10^(-4), whereas the continuity equation residuals remain around 10^(-1), although the flow does not change any longer during iterations (10000 iterations!). Do you know why? Thank you

Hi Ema40,

I'm having the same results for what it seems a similar problem to yours. Did you find out the reason why continuity equation residuals remain 10^(-1)?

 October 25, 2016, 05:54 #32 New Member   Gujarat Join Date: Oct 2016 Posts: 6 Rep Power: 5 hi..i am trying to solve double pipe heat exchanger using inserts in CFD. i used- double precision, energy eq-on model-turbulent(2 eq,stnd wall fun) solution method:-SIMPLe(PRESSURE-VELOCITY BASED)-second order my energy residules are not converges during solution.so ans is not converged. boundary conditions:- hot inlet:-mass flow rate-1.3 kg/s,inlet temp-413 k hot outlet-pressure based cold inlet:-mass flowrate-1.3, temp-298 k cold outlet:-pressure base any one have solution?plz guide me. thanks in advance. Last edited by mahipal parmar; November 1, 2016 at 02:04.

 October 28, 2016, 19:18 #33 Member   Radwanma Join Date: Oct 2014 Posts: 30 Rep Power: 7 Hi All, Could you confirm that the form of ct = Turbine_Torque / (0.5*density*pi*R^3*wind_speed^2) is correct to get coefficient of torque directly from fluent, please? Thanks,

 November 8, 2016, 16:08 #34 Member     Join Date: Jul 2011 Posts: 74 Blog Entries: 1 Rep Power: 14 You are all asking specific questions. This blog entry was written as a guideline only. Please post your questions on other subforums here __________________ I'M NOT A GYNECOLOGIST BUT I'LL TAKE A LOOK.

 March 13, 2017, 11:29 #35 Member   kaouachi anouar Join Date: Jul 2016 Posts: 64 Rep Power: 5 hello thank you for your help please im doing simulation in fluent for study the flow overe stepped spillways but the results its stay bed please i want to send you my projet to chek can you help me please ?

March 23, 2017, 02:17
#36
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Grama Srivatsa
Join Date: Jun 2016
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 Originally Posted by A CFD free user Hi friend, Obtaining a value of e-2 or even e-4 for residuals doesn't necessarily mean that your simulation is converged. However, a value of e-3 is acceptable for continuity, for other equations, it depends on your problem. For instance, in my recently work, a value of residual about e-8 is not enough to get convergence for a user_defined scalar! So, my suggestion is first, keep doing iteration until you don't observe any big change or oscillation in residuals. Secondly, in some cases, you need to monitor some effective parameters in your model beside the usual residuals monitoring. For example, depending on your work, you can monitor some surface or volume characteristics of flow. Only one more thing, in some cases you may not receive a very low residuals but the values will not change any more longer. In that case, your simulation is converged too. So, don't worry and be patient when you grip into a CFD problem. Hope it helps.
Hello,

I am working on a flow simulation values over a blunt body with a spike, after running around 5000 iterations, the residuals(continuity, Energy, X-velocity and Y-velocity) are oscillating at E-3 and E-4 but the drag coefficient is constant, can I say my results have converged?

 April 20, 2017, 13:20 #37 Member   kaouachi anouar Join Date: Jul 2016 Posts: 64 Rep Power: 5 hello for I'm working for simulation for the flow over stepped spillways 10^-3 is enough ? regards

 June 16, 2017, 21:10 #38 New Member   BlackHeartInertia Join Date: May 2017 Posts: 29 Rep Power: 4 I check mass imbalance and it was 1e-5 and I saved project. then it changes to 1e-3.. why? What should i do? Sent from my SM-G570M using CFD Online Forum mobile app

June 19, 2017, 07:30
#39
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 Originally Posted by BlackHeartInertia I check mass imbalance and it was 1e-5 and I saved project. then it changes to 1e-3.. why? What should i do? Sent from my SM-G570M using CFD Online Forum mobile app
So you saved the case and the date and continued to iterate? Or did you read the saved files after closing the Fluent window? If it's the latter, then there's nothing weird about because those bumps come every time one reads an older case and continues the calculation. I have thought -not 100% sure about this- that the bumb in residuals is because the data is not saved in maximal size, i.e. in order to save disk space, Fluent chooses to neglect some decimals from cell values. After reading the case again, the program once again starts to use the defined amount of decimals, which causes a shock to the solution.

June 19, 2017, 09:16
#40
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BlackHeartInertia
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 Originally Posted by divergence So you saved the case and the date and continued to iterate? Or did you read the saved files after closing the Fluent window? If it's the latter, then there's nothing weird about because those bumps come every time one reads an older case and continues the calculation. I have thought -not 100% sure about this- that the bumb in residuals is because the data is not saved in maximal size, i.e. in order to save disk space, Fluent chooses to neglect some decimals from cell values. After reading the case again, the program once again starts to use the defined amount of decimals, which causes a shock to the solution.
It doesnt change after reading the case it change when i continue iterations. Now the mass imbalance is 1exp-7 but if I continue iterations maybe it can change to a different value like 0.001 (very different) so I dont know what it means or what to do. Should I continue iterations or stop there.

(In the case of residuals they are stable but dont decrease from arround 1 exp-2, and convergence monitors are stable)

 Tags convergence, fluent, mesh