Convergence Problem in Axisymmetric Periodic Flow

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

 February 8, 2014, 12:12 Convergence Problem in Axisymmetric Periodic Flow #1 New Member   Anj Join Date: Dec 2013 Posts: 8 Rep Power: 5 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 half-width (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 1e-06 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: 1,591 Rep Power: 25 2D axisymmetric domains in fluent must have the x-axis 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. 10e-6 for the residual values may still be too loose as a convergence criterion in this case. Make sure to double-check convergence with some other quantity like wall shear stress. atheresia likes this. Last edited by flotus1; February 9, 2014 at 16:47.

February 9, 2014, 18:03
#3
New Member

Anj
Join Date: Dec 2013
Posts: 8
Rep Power: 5
Quote:
 Originally Posted by flotus1 2D axisymmetric domains in fluent must have the x-axis 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. 10e-6 for the residual values may still be too loose as a convergence criterion in this case. Make sure to double-check convergence with some other quantity like wall shear stress.

I've just tried it in xy plane and it converges now!! Thank you!

I am not sure, however, how to 'double-check 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: 1,591 Rep Power: 25 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: http://www.cfd-online.com/Forums/flu...nvergence.html http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F atheresia likes this.

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Sachin U. Nimbalkar FLUENT 5 December 22, 2016 02:34 GM_XIII OpenFOAM Running, Solving & CFD 14 March 27, 2014 04:56 mep10jl FLUENT 2 July 30, 2013 17:09 diwakaranant SU2 1 March 24, 2013 00:45 Phil FLUENT 15 October 2, 2000 14:56

All times are GMT -4. The time now is 13:16.