# Turbulence parameters and convergence

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

 October 14, 2013, 11:29 Turbulence parameters and convergence #1 Senior Member   --------- Join Date: Oct 2010 Posts: 303 Rep Power: 10 I have observed that turbulent parameters played a major role in the convergence of a simulation. Initially when I simulated with the default values of intensity ans viscosity ratio (5 and 10) the convergence is bad and when I switched over to use intensity and hydraulic diameter (= 4A/P) there is an improvement in the convergence. Could some one please provide a reasoning to this behavior __________________ Best regards, Santhosh.

 October 14, 2013, 17:25 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,456 Rep Power: 104 Yes, turbulence makes a big difference to fluid flow. I am glad you figured that out In simple 2-eqn turbulence models the turbulence is modelled to increase the local viscosity (and diffusivity for other equations). This means turbulence is assumed to slow a flow down and make it more viscous - this is in effect a reduction in Reynolds number and convergence is easier in low Reynolds number flows compared to high. (Only up to a point, CFX has great troubles converging for very small Re numbers <<1).

 October 15, 2013, 04:46 #3 Senior Member   --------- Join Date: Oct 2010 Posts: 303 Rep Power: 10 Hi Glenn, Thank you very much for the clarification so when a convergence difficulty is observed with a simulation how is it possible to sense at as an effect of improper or unphysical turbulence settings. And if a good convergence is acheived by say reducing the viscosity ratio and increasing the intensity of turbulence then is it implied that such turbulence settings represent the real physics __________________ Best regards, Santhosh.

October 15, 2013, 06:00
#4
Super Moderator

Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 13,456
Rep Power: 104
The decision as to whether to choose a laminar or turbulent model only depends on the Reynolds number and whether the flow is turbulent or not. But the choice of which turbulence model to use for a turbulent flow is complex - all I will say is that the SST turbulence model is the best all-round model and use that one unless you have a very good reason to choose something else.

Quote:
 And if a good convergence is acheived by say reducing the viscosity ratio and increasing the intensity of turbulence
I did not say that! That is not correct.

Quote:
 then is it implied that such turbulence settings represent the real physics
? I have no idea how one implies the other. I think you need to read some background textbooks on turbulence modelling to understand what it is all about. "Turbulence modelling for CFD" by wilcox is good, and most introductory CFD textbooks include a section on turbulence modelling.

 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 tippo CFX 2 May 5, 2009 10:55 KK CFX 0 January 31, 2008 21:46 sek OpenFOAM Running, Solving & CFD 0 September 15, 2006 13:45 amv Phoenics 4 September 21, 2005 06:49 irfan CD-adapco 1 November 19, 2002 05:38

All times are GMT -4. The time now is 22:00.