# CFX Convergence Problem

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 May 17, 2007, 00:23 CFX Convergence Problem #1 Abdul Guest   Posts: n/a Dear Pals, I am simulating a duct in CFX. it is a inverted L shape duct. the inlet is located in the horizontal duct and the outlet is in the vertical duct. the vertical duct is bigger in size when compared to the horizontal. my problem is when the flow is coming out of the horizontal duct, circulations are formed at the two sides of the vertical duct as the velocity of my flow is bit higher (30m/s). so the solution is not converging it is keep on oscillating in 10e-3. i am using k-epsilon turbulence model and even i tried SST k-omega. can anyone help me to resolve this.

 May 17, 2007, 19:11 Re: CFX Convergence Problem #2 alex Guest   Posts: n/a hi abdul! Do u have a mesh adapted to your turbulence model? if i were you i will try to use an adaptated time step at first. Ansys published a very interesting doc concerning fluid flow separation. try to find it. by

 May 17, 2007, 22:57 Re: CFX Convergence Problem #3 Omer Guest   Posts: n/a A few more details about how good your mesh is and about your boundary conditions would give more options.Try refining your mesh a little at the corner where the two ducts meet!

 May 18, 2007, 02:39 Re: CFX Convergence Problem #4 Abdul Guest   Posts: n/a Thank you Omer for your reply. my mesh quality is around 0.35 and the min angle is about 41. i am using pressure inlet and massflow outlet boundary conditions.

 May 18, 2007, 07:06 Re: CFX Convergence Problem #5 Luke Guest   Posts: n/a Try using a very low physical timescale. If ur still having problems with convergence try a local timescale factor. But to make sure you have achieved convergence, swithch back to an auto timescale and if there is no change in your imbalances and mass flow then your case is converged. Your problem could be due to large difference in velocity timescales.

 May 18, 2007, 22:30 Re: CFX Convergence Problem #6 Omer Guest   Posts: n/a Pressure inlets are generally not a good idea. Is there a way you can have massflow or velocity inlet, and pressure outlet? It would really help you with the convergence.

 May 21, 2007, 00:21 Re: CFX Convergence Problem #7 Abdul Guest   Posts: n/a hai omer i tried that conditions too but the result is same. it is oscillating in 10-3.

 May 21, 2007, 23:31 Re: CFX Convergence Problem #8 Omer Guest   Posts: n/a Trying changing your autotimestep to a large physical timestep. It might work...

 May 25, 2007, 08:08 Re: CFX Convergence Problem #9 prabhu Guest   Posts: n/a Hi Omer, Why do you think pressure inlet is not a good option compared to mass flow and velocity b.c.s.

 June 25, 2014, 02:51 #10 Member   Daniel Frederik Join Date: Apr 2014 Location: Switzerland Posts: 38 Rep Power: 3 Sometimes convergence is not achieved even thou the mesh quality is ok. That can be due to transient effects in the flow. Coarsening the mesh can help to make it converge because the sransient effects will not be detected by the mesh. The resukts obtained this way have to be treated with caution. In my opinion the best way would be to do a transient analysis in this case.

 June 30, 2014, 08:42 #11 New Member   Thomas Join Date: May 2014 Posts: 12 Rep Power: 3 I would try different timesteps in a user-function and run every step for lets say 100it. Starting with the largest step (e.g. 0.1) to the smallest (e.g. 1e-6). You can easily see if there is a significant change in the convergency. There is also some features to force the simulation in a steady-state solution but probably it is best to do a transient simulation and do a average over time after.

 June 30, 2014, 08:48 #12 Senior Member     Mr CFD Join Date: Jun 2012 Location: Britain Posts: 291 Rep Power: 6 The original post was made 7 years ago. I'm sure he's achieved a converged solution by now assuming he's not using a timestep of a picosecond.

 June 30, 2014, 08:55 #13 New Member   Thomas Join Date: May 2014 Posts: 12 Rep Power: 3 uhhh...so he should better change to the latest Version in CFX as well - they have better convergency

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