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Old   June 29, 2004, 11:53
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  #1
John
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Hello When I calculate the drag coefficient of airfoil I think I am getting a real high value. The lift/drag ratio I am getting is about 3.5. Is that a reasonable value? Please help me because i think I have tried everything and my supervisor will go crazy
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Old   June 30, 2004, 14:01
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4xF
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That may happen in the zone where the angle of attack is small (small lift force) but the drag is nearly constant. BTW, is your solution converged? Monitor drag and lift over the number of iterations and look at the value for the drag. If it is not plateauing, then your solution is not converged with respect to that value. Also, using UD or any kind of blending with difference schemes of order less than 1 will bias your solution and you will get a bad drag prediction (because of numerical viscosity). Furthermore, you have to check (when using a turbulence model) that you fulfill the requirements of the turbulence model for the near wall modelling, i.e. 12<Y+<50 for the use of wall functions with High-Re models, Y+ < 1 for Low-Re models. For details on how to use the Low-Re/High-Re models, have a look into the methodology guide.
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Old   June 30, 2004, 15:05
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Jörn Beilke
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It also might be a big difference between a steady and a transient calculation. If you want to be on the safe side a transient calculation is required.

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Old   July 1, 2004, 04:16
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4xF
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I totally agree, especially when you are approaching stall and the boundary layer detaches and vortices are created!
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