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

Airfoil Momentum Coeficient

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   August 10, 2012, 10:33
Question Airfoil Momentum Coeficient
  #1
New Member
 
Amanda Maria Bizzinotto Ferreira
Join Date: Jun 2012
Location: Brazil
Posts: 8
Rep Power: 14
AmandaMBF is on a distinguished road
Send a message via Skype™ to AmandaMBF
Good Morning everyone!

I'm doing the fluid simulation for Eppler 423 arifoil with 40 cm chord using Fluent 12.0. I did the setup accordingly to waht I was told to do however I'm getting a positive momentum coefficient for 0º instead of a negative one.
To do the simulation I'm using a density-based solver, using air as ideal-gas and sutherland viscosity. On Models panel I setup energy as on and Spalart - Allmaras as the turbulence model. The boundary conditions are: for profile: wall default and roughness 0.01; for farfield: farfield - pressure with Gauge pressure 101325 Pa; mach = 0.035; X-Component of Flow direction = 1 and Y-Component of flow direction = 0; Turbulence Specification Method as Intensity and Lenght Scale with Intensity (%) = 0.01 and Lenght Scale (m) = 0.02; Thermal = 270 K.
I'm computing from farfield. And I set the moment center as: X=0.1 and Y=0.025.
Does anybody knows why I'm getting a positive momentum coeficient?
Thanks
AmandaMBF is offline   Reply With Quote

Old   August 11, 2012, 07:03
Default
  #2
Senior Member
 
truffaldino's Avatar
 
Join Date: Jan 2011
Posts: 249
Blog Entries: 5
Rep Power: 17
truffaldino is on a distinguished road
Do not worry: Fluent uses standard right-handed coordinate system, so when the moment is positive aircraft nose is pitching down. On the other hand, Cm in aircraft engeneering is written in the left-handed system.

P.S. Out of topic: Using density-based solver for mach=.035 and Re=3*10^5 is not very good idea: I would rather used the pressure-based one
truffaldino is offline   Reply With Quote

Old   August 11, 2012, 15:37
Smile Thanks
  #3
New Member
 
Amanda Maria Bizzinotto Ferreira
Join Date: Jun 2012
Location: Brazil
Posts: 8
Rep Power: 14
AmandaMBF is on a distinguished road
Send a message via Skype™ to AmandaMBF
Ok! Thank you very much! So I'll use pressure based. I was using this and I changed due to the positive cm, to see if the same problem will happen using density based. Thank you very much!
AmandaMBF is offline   Reply With Quote

Old   August 12, 2012, 03:23
Default momentum coefficient or moment coefficient?
  #4
New Member
 
Michael Freestone
Join Date: Aug 2012
Posts: 5
Rep Power: 14
mikefreestone is on a distinguished road
Do you mean momentum coefficient or (pitching) moment coefficient? if the latter, are you sure that a positive value is incorrect? Why not run a few values of angle of attack, get near the stall, or beyond, and see if the behaviour of the coefficient looks sensible.
mikefreestone is offline   Reply With Quote

Old   August 12, 2012, 22:14
Default Pitching moment coefficient
  #5
New Member
 
Amanda Maria Bizzinotto Ferreira
Join Date: Jun 2012
Location: Brazil
Posts: 8
Rep Power: 14
AmandaMBF is on a distinguished road
Send a message via Skype™ to AmandaMBF
I am having problem with both. About the pitching moment coefficient, I did a 2D simulation to evaluate the cm, cl and cd of the arifoil Eppler 423. My cm was positive for 0° and other low values, negative for values like 7° or 8°, the simulation didn't had a solution for values like 9° or 10°, and it was positive for 11°. So I don't know how to deal with this!
AmandaMBF is offline   Reply With Quote

Old   August 13, 2012, 10:54
Default
  #6
New Member
 
Michael Freestone
Join Date: Aug 2012
Posts: 5
Rep Power: 14
mikefreestone is on a distinguished road
I think it is fairly certain that the sign convention is positive nose down. At low incidences there should be an almost linear relationship between CL andCM, and the slope of this will give you the position of the aerodynamic centre, which should be about 0.25 of the chord aft of the leading edge. You might need to run more incidence values
to get the slope adequately. When the boundary layer starts to separate as the incidence is increased, the aerofoil will probably tend to pitch up (CM becomes negative). At higher incidences still the calculations may well be unreliable.

You can always compare your calculated results at low incidence with some other aerofoil code results. If you get stuck I can run some cases for you using VGK.
mikefreestone is offline   Reply With Quote

Reply

Tags
momentum coeficient


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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Momentum coefficient of airfoil flow Marat Hoshim Main CFD Forum 2 February 8, 2016 17:53
Low Speed Airfoil Mancusi FLUENT 7 April 3, 2014 07:11
Derivation of Momentum Equation in Integral Form Demonwolf Main CFD Forum 2 October 29, 2009 20:53
Modeling Backflow for a 3D Airfoil (Wing of Finite Span) Josh CFX 9 August 18, 2009 12:31
Airfoil boundary condition Frank Main CFD Forum 1 April 21, 2008 19:36


All times are GMT -4. The time now is 03:49.