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

Convergence in Nat conv problem - simple geometry

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 11, 2012, 17:23
Default Convergence in Nat conv problem - simple geometry
  #1
New Member
 
Indu
Join Date: Apr 2012
Posts: 1
Rep Power: 0
gradstudent2430 is on a distinguished road
I am simulating natural (Rayleigh Benard convection) in a rectangular 2D cavity, heated at the bottom, colder at the top (~5K delta T), with a Rayleigh number of ~10e6. I cannot get convergence with the steady state solver for this Rayleigh number. I know that flow is laminar at this Ra from several books.

As suggested in earlier discussions I did a stepwise solve, by increasing g from 0.098 to 0.98 to 9.8 (to modify Rayleigh number) and using each solution as the initialization for the next. Things converge upto Ra~10e5. After that the residuals diverge/oscillate - the velocity and contunuity specially, with the net heat flux actually being lower for same delta T at higher rayleigh number (less convection)!!

Relaxation factors:
Momentum - 0.15
Energy - 0.5

Boussinesq model, Presto for pressure, SIMPLE scheme. I entered in starting density and thermal exp coeff. in the material model

MESH:
Regular mesh with the rectangular area (25mm W, 15mm H) split into 40x100 cells.

What else can I do? I tried starting with a transient solution with a time step of 0.0006 s and 1000 iterations and going on to steady stater from there but same divergence issue. It looks like this is a very simple problem for fluent, why is it tricky?

Thanks
gradstudent2430 is offline   Reply With Quote

Old   April 19, 2012, 05:23
Default
  #2
New Member
 
Yogesh Patil
Join Date: Nov 2009
Posts: 20
Rep Power: 7
foamcfd is on a distinguished road
Hi Indu,

Generally this type of problems are very tough to converge with steady state solver. You said that you also tried with transient solver. How did you calculate the time step?

What I can suggest you to use linear variation of density instead of boussinisque approximation. This is something strange as later is very simple to converge but give a try.

Thanks
foamcfd 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
convergence problem in the turbomachinery problem. u k jha CFX 1 September 7, 2010 18:41
Problem in Initializing Blocking for 2d geometry nitin Dewangan CFX 6 October 24, 2008 01:03
CONVERGENCE PROBLEM - oil boiler MM FLUENT 1 February 15, 2007 06:24
Cooling simple 3D geometry problem Luk FLUENT 4 May 12, 2006 09:47
convergence problem with SIMPLER NURAY KAYAKOL Main CFD Forum 1 February 24, 1999 14:43


All times are GMT -4. The time now is 07:56.