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Old   January 31, 2006, 09:37
Default Error in drag
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When I calculate invicid subsonic flow field around airfoil I got 35 counts (Cd=0.035). It result was achieved on different smooth meshes. Node numbers was increased 2X, 4X and result was change to 30 counts (achieved minimum). Who can comment this effect?
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Old   January 31, 2006, 13:51
Default Re: Error in drag
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First, you say that you calculated INVISCID flow around the airfoil. This means that you actually calculated "form" drag, am I right? For the complete "drag" you need to activate viscosity. The interesting phenomena is that although both form and viscous coefficients are sometimes wrong, their sum is quite close to the experimental results! So try, you might obtain surprising results.

Secondly, using a dense mesh around the foil (viscous type mesh) in inviscid flow, is very wrong, because Euler formulation tends to increase numerical error in those regions with mesh clustered to the wall. So, increasing mesh density near the wall of the airfoil is actually worsening the numerical solution!!! This is of course not true with Navier-Stokes formulation (if y+ is appropriate to wall treatment used).

Be careful, Razvan
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Old   January 31, 2006, 14:00
Default Re: Error in drag
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Hi Razvan! could you add some details to what you said about grid-density and Euler formulation? I'm very interested to this topic...Luca
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Old   February 1, 2006, 05:01
Default Re: Error in drag
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Excuse me! I made mistake i. e. 35 counts = Cd = 0.0035
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