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

fluent convergence problem

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

Like Tree2Likes
  • 1 Post By zombiaska
  • 1 Post By ghost82

Reply
 
LinkBack Thread Tools Display Modes
Old   March 12, 2014, 13:59
Question fluent convergence problem
  #1
New Member
 
Join Date: Feb 2014
Posts: 8
Rep Power: 3
itzme is on a distinguished road
hi guys,

I am having difficulty with convergence,especially continuity residuals.
My model and boundary conditions are shown below.Air flows in a swirling pattern and creates negative pressure in the central region.
the nozzle is 1.5 mm and the gap between skirt and work-piece is 0.3 mm.
I want to find the pressure distribution on the work-piece..
The peak velocity is around 100 m/s only and so pressure based solver is sufficient...right?

The mesh created has a max skewness of .7 with average skewness of .2
Is the mesh too coarse.do i have to increase the no. of elements where thickness is very less.( I am modelling the fluid domain only and considered an outer envelope to apply pressure boundary condition)

How to attain faster convergence for this case.
should I use simplec or coupled ??
do i need to modify the under-relaxation factors ?
also,how to reduce back-flow ?

Any help is appreciated...
thanks in advance...
Attached Images
File Type: png 111.PNG (78.2 KB, 32 views)
File Type: png 222.PNG (22.4 KB, 28 views)
File Type: jpg Capture.jpg (100.5 KB, 29 views)
itzme is offline   Reply With Quote

Old   March 13, 2014, 03:24
Default
  #2
Member
 
Simas
Join Date: Mar 2012
Posts: 37
Rep Power: 5
zombiaska is on a distinguished road
Quote:
Originally Posted by itzme View Post
do i need to modify the under-relaxation factors ?
Yes you can do this higher under-relaxation factors will accelerate. If iterations grahps go up it means you have too high relaxation factors and need to lower it.
itzme likes this.
zombiaska is offline   Reply With Quote

Old   March 13, 2014, 08:31
Default
  #3
Senior Member
 
RodriguezFatz's Avatar
 
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,097
Rep Power: 16
RodriguezFatz will become famous soon enough
itzme,
1) What turbulence model do you use?
2) Can you show the residuals?
3) Numerics ("Solution Methods") are?
__________________
The skeleton ran out of shampoo in the shower.
RodriguezFatz is offline   Reply With Quote

Old   March 13, 2014, 08:46
Default
  #4
New Member
 
Join Date: Feb 2014
Posts: 8
Rep Power: 3
itzme is on a distinguished road
Quote:
Originally Posted by RodriguezFatz View Post
itzme,
1) What turbulence model do you use?
2) Can you show the residuals?
3) Numerics ("Solution Methods") are?
Thanks for the reply

K-e RNG model
I dont currently have the residuals picture...everything except continuity converges...
Simplec... Initially with first order and then to second order...
itzme is offline   Reply With Quote

Old   March 13, 2014, 10:34
Default
  #5
Senior Member
 
RodriguezFatz's Avatar
 
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,097
Rep Power: 16
RodriguezFatz will become famous soon enough
Di you try to run the whole case with much lower massflow? Try e-5 inlet massflow instead of e-4, and tell us what happens.
__________________
The skeleton ran out of shampoo in the shower.
RodriguezFatz is offline   Reply With Quote

Old   March 13, 2014, 10:51
Default
  #6
New Member
 
Join Date: Feb 2014
Posts: 8
Rep Power: 3
itzme is on a distinguished road
Quote:
Originally Posted by RodriguezFatz View Post
Di you try to run the whole case with much lower massflow? Try e-5 inlet massflow instead of e-4, and tell us what happens.
Well...I managed to achieve convergence... but the pressure distribution on the plate is not accurate...
Will improving mesh or increasing no. of elements yield correct results??

Thanks in advance
itzme is offline   Reply With Quote

Old   March 13, 2014, 10:52
Default
  #7
Senior Member
 
RodriguezFatz's Avatar
 
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,097
Rep Power: 16
RodriguezFatz will become famous soon enough
You mean convergence for the e-5 case?
Can you post a picture of the y+ value on your surfaces?
__________________
The skeleton ran out of shampoo in the shower.
RodriguezFatz is offline   Reply With Quote

Old   March 13, 2014, 11:15
Default
  #8
New Member
 
Join Date: Feb 2014
Posts: 8
Rep Power: 3
itzme is on a distinguished road
Quote:
Originally Posted by RodriguezFatz View Post
You mean convergence for the e-5 case?
Can you post a picture of the y+ value on your surfaces?
No...for the e-4 case itself...and I am not using any type of inflation in the mesh....is y+ value significant for accuracy and convergence of solution....
(sorry, I am only a beginner in cfd analysis)
Thanks
itzme is offline   Reply With Quote

Old   March 13, 2014, 11:18
Default
  #9
Senior Member
 
RodriguezFatz's Avatar
 
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,097
Rep Power: 16
RodriguezFatz will become famous soon enough
y+ is the normalized size of your cells, perpendicular to the surface, so its always plotted on the surface of your domain. If it is too large, the boundary conditions of the turbulence model become garbage.

What did you change to achieve convergence?

To your question: Of course results can get better if you make the grid even smaller. But you already have 1.3 mio cells for such a simple geometry. Do you have ICEM for meshing or just the ansys meshing?
__________________
The skeleton ran out of shampoo in the shower.
RodriguezFatz is offline   Reply With Quote

Old   March 13, 2014, 11:32
Default
  #10
New Member
 
Join Date: Feb 2014
Posts: 8
Rep Power: 3
itzme is on a distinguished road
Quote:
Originally Posted by RodriguezFatz View Post
y+ is the normalized size of your cells, perpendicular to the surface, so its always plotted on the surface of your domain. If it is too large, the boundary conditions of the turbulence model become garbage.

What did you change to achieve convergence?

To your question: Of course results can get better if you make the grid even smaller. But you already have 1.3 mio cells for such a simple geometry. Do you have ICEM for meshing or just the ansys meshing?
Thanks for the info
I refined mesh a bit and changed the relaxation parameters.
I am not familiar with icem meshing.
itzme is offline   Reply With Quote

Old   March 14, 2014, 07:11
Default
  #11
Senior Member
 
RodriguezFatz's Avatar
 
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,097
Rep Power: 16
RodriguezFatz will become famous soon enough
You should have a look at the y+ anyway. Maybe post the pictures!
__________________
The skeleton ran out of shampoo in the shower.
RodriguezFatz is offline   Reply With Quote

Old   March 16, 2014, 00:23
Default
  #12
New Member
 
Join Date: Feb 2014
Posts: 8
Rep Power: 3
itzme is on a distinguished road
Is it possible to do a 2-D analysis for this case ???
itzme is offline   Reply With Quote

Old   March 16, 2014, 06:49
Default
  #13
Senior Member
 
ghost82's Avatar
 
Daniele
Join Date: Oct 2010
Location: Italy
Posts: 908
Rep Power: 15
ghost82 will become famous soon enough
Quote:
Originally Posted by itzme View Post
Is it possible to do a 2-D analysis for this case ???
you mean axissymmetric; anyway no, because there is a tangential nozzle.
itzme likes this.
ghost82 is offline   Reply With Quote

Old   March 17, 2014, 10:11
Default weird CP distribution near trailing edge for NACA0012
  #14
New Member
 
shubham jain
Join Date: Dec 2013
Posts: 21
Rep Power: 3
shubham jain is on a distinguished road
i am simulating naca0012 with Gurney flap
mesh using ICEM CFD
simulation using FLUENT, k-e RNG, Re=3e06
I have a problem in CLEAN Airfoil (without any gurney flap) in its Static pressure distribution (CP).

1. CP values of pressure and suction surface intersect each other near the trailing edge for every angle of attack.

2. Also there are many riggles (wavy nature of CP) in the CP distribution

Though the pathlines seem to be fine
can anybody explain this weird CP distribution and how can i rectify it???

thanks in advance
Attached Images
File Type: png cp distribution cfd forum pic 1.PNG (11.3 KB, 6 views)
File Type: png cp distribution cfd forum pic 2.PNG (17.8 KB, 6 views)
File Type: jpg cp distribution cfd forum pic 3.jpg (72.4 KB, 5 views)
File Type: jpg cp distribution cfd forum pic 4.jpg (76.6 KB, 4 views)
shubham jain 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 Centurion2011 FLUENT 24 May 9, 2015 08:02
serial/parallel problem in force saving convergence history behest FLUENT 11 July 22, 2014 03:49
Problem in using parallel process in fluent 14 Tleja FLUENT 3 September 13, 2013 10:54
Fluent - license problem. Marcin FLUENT 1 March 4, 2005 12:41
convergence problem limseokmin FLUENT 3 November 14, 2004 13:43


All times are GMT -4. The time now is 20:59.