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

Negative coefficient of drag

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

Reply
 
LinkBack Thread Tools Display Modes
Old   February 3, 2014, 05:59
Default Negative coefficient of drag
  #1
New Member
 
Nick Prendergast
Join Date: Dec 2013
Posts: 27
Rep Power: 3
neprendo is on a distinguished road
Hey guys.

No matter what I do I can't seem to get stop the coefficient of drag becoming negative when plotted using the monitors in fluent v.14. I'm modelling a 2-D NACA 0012 aerofoil in a pressure far field at a velocity of 10m/s and angle of attack of 9 degrees using a steady pressure based solver. Mesh quality is 0.83 and refined around the aerofoil. The URF are 0.1 for pressure and momentum and default for the others. So far I have been adjusting the boundary conditions at the outlet to reflect what is seen at the inlet and playing around with the URF.
Any help on the topic would be appreciated as I'm kinda at a lose of what to try next.
The prj file I used in Icem is attached and I'll happily attach anything else that is requested.

Thanks in advance
Nick
Attached Files
File Type: zip NACA0012.zip (4.0 KB, 3 views)
neprendo is offline   Reply With Quote

Old   February 4, 2014, 05:22
Default
  #2
Senior Member
 
flotus1's Avatar
 
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,098
Rep Power: 19
flotus1 will become famous soon enoughflotus1 will become famous soon enough
Quote:
Originally Posted by neprendo View Post
I'll happily attach anything else that is requested.
How about sketches or screenshots of the computational domain where we can see the boundary conditions, the coordinate system and the mesh.

Quote:
I have been adjusting the boundary conditions at the outlet to reflect what is seen at the inlet
I dont fully understand that. Are you using a pressure far field boundary condition or not?
flotus1 is offline   Reply With Quote

Old   February 4, 2014, 10:53
Default
  #3
New Member
 
Nick Prendergast
Join Date: Dec 2013
Posts: 27
Rep Power: 3
neprendo is on a distinguished road
Hi,

I think I have attached what you asked for (over 2 posts). Sorry if some of the screen shots are a little crap, I struggled with getting some of them within the file size limit. The word doc has a labelled diagram of the domain without the mesh and the coordinate system with the origin at the leading edge of the aerofoil.

In regard to the boundary conditions, I was initially using a pressure far field at the outlet, but that was forcing me to have the fluid as an ideal gas (for density) and was defining a mach number of 0.6 and component directions for the flow out. I thought that by matching the mach number and component directions of the flow at the outlet to the inlet it might help. It didn't really help so I changed the outlet boundary condition to a pressure outlet.

Nick
Attached Images
File Type: jpg inlet BC at wall.jpg (25.7 KB, 5 views)
File Type: jpg mesh at the aerofoil.jpg (64.2 KB, 4 views)
File Type: jpg mesh in farfield.jpg (74.6 KB, 4 views)
File Type: jpg Pressure outlet BC.jpg (21.8 KB, 4 views)
Attached Files
File Type: docx computational domain.docx (44.4 KB, 3 views)
neprendo is offline   Reply With Quote

Old   February 4, 2014, 10:54
Default
  #4
New Member
 
Nick Prendergast
Join Date: Dec 2013
Posts: 27
Rep Power: 3
neprendo is on a distinguished road
these are the last 2 screen shots

thanks
Nick
Attached Images
File Type: jpg Pressure far field BC at outlet.jpg (27.9 KB, 4 views)
File Type: jpg wall BC at crvs.jpg (29.2 KB, 4 views)
neprendo is offline   Reply With Quote

Old   February 5, 2014, 04:23
Default
  #5
Senior Member
 
flotus1's Avatar
 
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,098
Rep Power: 19
flotus1 will become famous soon enoughflotus1 will become famous soon enough
There are some minor issues with the setup, but none of them could explain a negative drag if you are using velocity inlet and pressure outlet.
Is there any particular reason why you used under-relaxation factors of 0.1 for pressure and momentum? Unless there is something seriously wrong with the setup, the default values of 0.3 and 0.7 respectively should work much better.
I hate to bring this up again and again, but as we dont know what your background in CFD is, did you make sure that the solution we are talking about is converged?
These two links should help you with this.
Once you covered this topic, we can have a closer look at the simulation.

http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

Convergence

BTW: please dont post images as microsoft word documents. You can always save them as an image file. This way, even people without microsoft software can have a look at them.
flotus1 is offline   Reply With Quote

Old   February 5, 2014, 05:07
Default
  #6
New Member
 
Nick Prendergast
Join Date: Dec 2013
Posts: 27
Rep Power: 3
neprendo is on a distinguished road
Thank you for your help. My background in CFD is pretty limited, I'm a engineering masters student. This my first major project using it. I'll check out those links and try sort the problem. And thanks for the heads up about word docs.

Nick
neprendo 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
non-orthogonal faces and incorrect orientation? nennbs OpenFOAM Native Meshers: blockMesh 7 April 17, 2013 05:42
error message with modeling a cube with a hold at the center hsingtzu OpenFOAM Native Meshers: blockMesh 2 March 14, 2012 10:56
mesh airfoil NACA0012 anand_30 OpenFOAM Meshing & Mesh Conversion 12 December 12, 2011 05:16
BlockMesh FOAM warning gaottino OpenFOAM Native Meshers: blockMesh 7 July 19, 2010 14:11
Axisymmetrical mesh Rasmus Gjesing (Gjesing) OpenFOAM Native Meshers: blockMesh 10 April 2, 2007 14:00


All times are GMT -4. The time now is 08:12.