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August 10, 2012, 09:33 
Airfoil Momentum Coeficient

#1 
New Member

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 densitybased solver, using air as idealgas 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; XComponent of Flow direction = 1 and YComponent 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 

August 11, 2012, 06:03 

#2 
Senior Member

Do not worry: Fluent uses standard righthanded 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 lefthanded system.
P.S. Out of topic: Using densitybased solver for mach=.035 and Re=3*10^5 is not very good idea: I would rather used the pressurebased one 

August 11, 2012, 14:37 
Thanks

#3 
New Member

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!


August 12, 2012, 02:23 
momentum coefficient or moment coefficient?

#4 
New Member
Michael Freestone
Join Date: Aug 2012
Posts: 5
Rep Power: 6 
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.


August 12, 2012, 21:14 
Pitching moment coefficient

#5 
New Member

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!


August 13, 2012, 09:54 

#6 
New Member
Michael Freestone
Join Date: Aug 2012
Posts: 5
Rep Power: 6 
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. 

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