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February 8, 2014, 12:12 
Convergence Problem in Axisymmetric Periodic Flow

#1 
New Member
Anj
Join Date: Dec 2013
Posts: 8
Rep Power: 4 
Hi everyone,
I am modelling a periodic flow for a 3D pipe, using an axisymmetric model. The flow is to be in the Z direction, with halfwidth (or radius) of 3 m. The Reynolds number is 34,132 and the working fluid is air. This gives my inlet velocity of 0.081 m/s. I set the turbulence intensity as 4.3% and turbulent length scale of 0.42. I have a problem, however, in obtaining convergence. My current convergence criterion is 1e06 for all the variables. With the initialization values below: My convergence plot is: I have tried to run a full 3D pipe periodic flow as well. For my initial mesh, I managed to obtain convergence, however, as I refine it further (by increasing the bias factor from 10 to 20), now it won't converge. Not sure what's going on! So any comments/feedbacks will be greatly appreciated! Thanks so much in advance for your help! Last edited by atheresia; February 8, 2014 at 12:16. Reason: Forgot to attach the images! 

February 9, 2014, 05:13 

#2 
Senior Member
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,282
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2D axisymmetric domains in fluent must have the xaxis as symmetry axis.
Additionally, for translational periodic flows with a very short streamwise extent, it is not unusual to take 10e5 or even more iterations to converge. 10e6 for the residual values may still be too loose as a convergence criterion in this case. Make sure to doublecheck convergence with some other quantity like wall shear stress. Last edited by flotus1; February 9, 2014 at 16:47. 

February 9, 2014, 18:03 

#3  
New Member
Anj
Join Date: Dec 2013
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Rep Power: 4 
Quote:
I've just tried it in xy plane and it converges now!! Thank you! I am not sure, however, how to 'doublecheck convergence' with other quantity like wall shear stress.. I'm still new to CFD so I would really appreciate it if you would share some guidelines/resources on this? :) Many thanks!! 

February 10, 2014, 04:00 

#4 
Senior Member
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,282
Rep Power: 22 
The basic idea is that you should never judge convergence based on the value of some residuals alone.
They are a good indicator, but sooner or later you will run into trouble if you rely on them too much. The absolute minimum for a simulation like yours is to set up a monitor point to monitor the average wall shear stress at the pipe wall. These two links have lots of valuable information on this topic: Convergence http://www.cfdonline.com/Wiki/Ansys..._inaccurate.3F 

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