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P Smith October 25, 1999 05:08

Fluent forces -lift/drag interpretation
I am currently looking at a simple 2D model of the cross section of a wing using Fluent v5. Having run the model using a ke model until it converges, could someone please summarise how the CL and CD results are calculated, ie in a 2D model as it cannot calculate it through the span of the wing so does it calculate it on the cross section?

Similarly how (if at all) is the total lift and drag calculated - is this the "Pressure Force" and "Viscous Force" as shown in the "Report Forces" drop down menu?

Thanks in advance.

O. Breitfeld October 25, 1999 07:23

Re: Fluent forces -lift/drag interpretation

for horizontal mean flow you can take the x and y components of the pressure and viscous forces. Devision by the dynamic pressure and the depth of you 2D model will lead to the CL and CD provided that the forces due to pressure and viscousity are the total forces on the patch. If not you have to integrate the local pressure and wall shear stress over the aerofoil surface. Could you please drop me a line about your results since I had a similar problem using a structured grid with CFX4.2 and experienced some difficulties to obtain Cl and Cd correctely.

Regards Oliver

Sung-Eun Kim October 26, 1999 15:00

Re: Fluent forces -lift/drag interpretation
Dear client,

Sorry for your inconvenience.

Here's what I understand regarding your queries. For conveninece' sake, I broke your queries into three parts and numbered them. Sorry, John, for using your "trademark" numbering system ;)

(1) How Cd and Cl are computed in FLUENT 5 ? - Force coefficients in the "Report/Forces" panel in FLUENT 5 are computed by deviding (non-dimensionalizing) total lift and drag forces, which are the resultant forces (i.e., integrated along the airfoil surface)acting on airfoils, by 0.5*rho*S*U**2, where rho, S and U are reference density, area and velocity scale, respectively. You can specify these reference values (reference area and velocity)in the "Report/Refernce Values" panel. To make it consistent with the convention in adopted airfoil aerodynamics, you should set S as product of chord-length and unit-span (1.0 in whatever unit you employ) and U as freestream velocity. rho can be taken from the freestream density (2) Regarding how to interpret 2D forces - You may well regard 2D forces reported in FLUENT 5 as computed for unit span, i.e., forces per unit delpth, whatever unit you're using (3) How total drag and lift are computed ? - As is obvious from the table reported on your monitor and you can confirm using a hand calculator, they are merely sums of the pressure (normal) forces and shear (tangential) forces at the airfoil surface.

If you have any further questions, please contact your support engineer.

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