CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > FLUENT

Convergence sensitivity to inlet turbulence quantities?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   October 25, 2010, 10:00
Default Convergence sensitivity to inlet turbulence quantities?
  #1
Member
 
Rain
Join Date: Apr 2009
Posts: 36
Rep Power: 7
gravis is on a distinguished road
Hi all,

I wonder if anyone has experienced convergence problems connected to inlet boundary conditions for turbulence quantities in Fluent or CFX (or any CFD code).

Description of the issue:

I am trying to do a comparison between Fluent and CFX for a species mixing problem without reactions. The fluid is air with equal properties, which enters the domain through several inlets.

I started out from a converged steady state solution from CFX, in which the code had automatically computed the turbulence quantities at the inlet. Then I manually set constant values of k and epsilon at the inlets to agree with the values of this initial solution. I ran the simulation with these new (almost identical) boundary conditions and again it converged fine.

Then I went over to Fluent and set the same boundary conditions and used the same numerics as much as possible. When I ran the simulation it did not converge. Then I approximated the turbulence intensity using Eq. (7.3-1) in the Fluent 12 User Guide:

I=0.16 \left(Re_{D_H}\right)^{-1/8},

and the turbulence length scale:

l=0.07L.

I set these values as inlet values and ran the simulation in Fluent again, and the convergence was good.

However, when I set the same turbulence quantities in CFX I do not get a converged steady state solution. I ended up with running the CFX simulation in transient mode (for which the solution stabilizes after some initial (numerical?) transients) and comparing to the converged steady state Fluent simulation. Of course, it is time-consuming to run transient simulations so I would like to avoid that.



Does anyone have experience to share on this subject?


To clarify: with "converged solution", in addition to low residuals, I mean that the velocity and species fields are stable and are not changing with more iterations.
gravis is offline   Reply With Quote

Reply

Thread Tools
Display Modes

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 Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Pressure Inlet BC Convergence OxIan FLUENT 0 May 26, 2010 09:11
High turbulence intensity wenfengxie FLUENT 1 April 16, 2010 12:00
CONTINuity convergence in ke TURBulence model CFDstudent Main CFD Forum 0 May 1, 2009 01:09
turbulence modeling questions llowen Main CFD Forum 3 September 11, 1998 04:24


All times are GMT -4. The time now is 04:15.