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

2D car simulation: k-w convergence problems

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

Reply
 
LinkBack Thread Tools Display Modes
Old   September 21, 2012, 06:25
Default 2D car simulation: k-w convergence problems
  #1
New Member
 
Jelmer
Join Date: Sep 2012
Posts: 2
Rep Power: 0
Jelmer is on a distinguished road
Hi all,

I've been trying to do 2D car simulations with Fluent, but I keep getting the same problems.

I am using the k-w SST model with low-Re correction. We need this to accurately predict the separation behaviour of the flow. The y+ values on the car and ground are both order 1. However, the residuals do not properly converge to low values (> 1e-4 ), and quite often, the velocity field contains strange non-realistic artefacts.

We don't really understand what's going on. With coarser meshes, no low-Re corrections and y+ ~ 100 we do get realistic results (however residuals still are higher than we would like), and drag trends due to changing the carshape are realistic too.

Some things about the mesh:
- We use a partially structured mesh, made in Gambit 2.4.6 (see attachments). It is a sample geometry (4500x1200 LxH) with an offset layer of 10cm, which represents the boundary layer. This layer is meshed structured.

- Around the car, a rectangular box is fitted. The area in between is meshed with a pave scheme (quad pave in this example). The rest of the domain is structured mesh again.

- There could be a problem with the maximum element aspect ratio. The elements on the car/ground are fractions of a millimeter thick, while their length is much larger ( AR > 10^3). I have heard however that high AR can be ok if the gradient in length direction (streamwise direction) is small. In the coarser mesh this AR is much lower.


Any help would be much appreciated! What is causing my simulation to perform badly?

Greetings,
Jelmer
Attached Images
File Type: jpg carmesh1.jpg (91.6 KB, 24 views)
File Type: jpg carmesh2.jpg (94.3 KB, 25 views)
File Type: jpg carmesh3.jpg (94.8 KB, 22 views)
Jelmer is offline   Reply With Quote

Old   September 21, 2012, 13:04
Default
  #2
Far
Super Moderator
 
Sijal
Join Date: Mar 2009
Location: Islamabad
Posts: 4,285
Blog Entries: 6
Rep Power: 43
Far will become famous soon enoughFar will become famous soon enough
Send a message via Skype™ to Far
Quote:
Originally Posted by Jelmer View Post
Hi all,

I've been trying to do 2D car simulations with Fluent, but I keep getting the same problems.

I am using the k-w SST model with low-Re correction. We need this to accurately predict the separation behaviour of the flow. The y+ values on the car and ground are both order 1. However, the residuals do not properly converge to low values (> 1e-4 ), and quite often, the velocity field contains strange non-realistic artefacts.

We don't really understand what's going on. With coarser meshes, no low-Re corrections and y+ ~ 100 we do get realistic results (however residuals still are higher than we would like), and drag trends due to changing the carshape are realistic too.

Some things about the mesh:
- We use a partially structured mesh, made in Gambit 2.4.6 (see attachments). It is a sample geometry (4500x1200 LxH) with an offset layer of 10cm, which represents the boundary layer. This layer is meshed structured.

- Around the car, a rectangular box is fitted. The area in between is meshed with a pave scheme (quad pave in this example). The rest of the domain is structured mesh again.

- There could be a problem with the maximum element aspect ratio. The elements on the car/ground are fractions of a millimeter thick, while their length is much larger ( AR > 10^3). I have heard however that high AR can be ok if the gradient in length direction (streamwise direction) is small. In the coarser mesh this AR is much lower.


Any help would be much appreciated! What is causing my simulation to perform badly?

Greetings,
Jelmer

You need to work on your mesh. Even better try ICEM for high quality structured mesh.
Far is offline   Reply With Quote

Old   September 21, 2012, 14:04
Default
  #3
New Member
 
Jelmer
Join Date: Sep 2012
Posts: 2
Rep Power: 0
Jelmer is on a distinguished road
Dear Far,

Thank you for your response. What kind of improvements would you suggest on the mesh? I can reduce the aspect ratio, but then the amount of cells will be enormous ( now its already ~100k )
Jelmer 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 for transient/steady state simulation Gab CFX 36 August 20, 2012 12:55
Problems with convergence with an easy system franzdrs Main CFD Forum 0 June 15, 2009 18:17
Problems on H2/air CFX simulation xulixian OpenFOAM Running, Solving & CFD 2 April 14, 2009 15:00
How stop simulation if convergence is not reached? Andreas FLUENT 0 October 16, 2007 12:35
Boundary layer simulation - convergence problems Gavin Tabor Main CFD Forum 3 July 2, 2004 05:16


All times are GMT -4. The time now is 21:49.