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-   -   Airfoil-values of cd/cl are too high-please help! (http://www.cfd-online.com/Forums/fluent/45495-airfoil-values-cd-cl-too-high-please-help.html)

matthias July 24, 2007 10:13

Airfoil-values of cd/cl are too high-please help!
 
Hi, i try to recompute the polare of an airfoil in 2d (a laminar profile), that was testet in a windtunnel (Reynoldsnumber approximately 1400000). i tried it with different turbulence modells (spalart-allmaras,k-w standart,k-w sst/transition) but every time my values for cl and cd where too high (in the range of alpha 0-6 cl was correct, for higher angles it was about 1.1-1.2 times higher then at the experiment, cd was about 1.8-2 times higher for all angles of attack (e.g for 0 cd(exp.):0.006/cd(fluent):0.011). is there perhaps anybody who can help me to advance my values?

my boundary conditions are (SI-values)-> turb intensity:0.01, turb lengthskale:0.01, roughness:0.000005, velocity inlet, pressure outlet, incompressible flow, y+max:1.25, steady-state, the length of the control volume is about 100xchordline, the reference values for cl and cd are computed from the velocity inlet, about 195000 nodes at the grid, hybrid grid.

thanks in advance matthias

Joe July 24, 2007 10:20

Re: Airfoil-values of cd/cl are too high-please he
 
For higher angles of attack: http://www.fulton.asu.edu/~squires/p...ions/dles5.pdf

red lemon July 24, 2007 15:39

Re: Airfoil-values of cd/cl are too high-please he
 
Refence values used for calculation of cl and cd are are report?refence values. check these are correct for your case otherwise caluclate the coefficients directly (eg using cff)

matthias July 24, 2007 16:37

Re: Airfoil-values of cd/cl are too high-please he
 
well joe thank you for the hint with DES. i will think about it. the problem is, that i have just a few days left to solving this problem and i think it will be very difficult to simulate the problem in 3D and to set the right boundary conditions. the other problem is that my pc has a limited capacity (memory,cpu). so i think it could be hard to do this with DES. do you think it is possible to get reasonable results with an RANS-modell (my angle of attack won't cross a value of 12)

matthias July 24, 2007 16:44

Re: Airfoil-values of cd/cl are too high-please he
 
hi red lemon, what do you mean by report reference values?

red lemon July 25, 2007 14:56

Re: Airfoil-values of cd/cl are too high-please he
 
reference values are used to calculate the coefficients cd and cl. If not correctly set (modified from default) they they will be incorrectly reported so set them realistically (such as free speed)

matthias July 26, 2007 06:59

Re: Airfoil-values of cd/cl are too high-please he
 
hi red lemon i think the reference values are ok. for computing cl and cd fluent should need the free speed of the fluid and the reference length (chord line) or the reference area these values are correct. i don't know how fluent computes the forces (shear forces and pressure forces) but i think i have no possibility to change these values.


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