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Old   March 29, 2011, 05:08
Default Stagnation point position error
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Jonas Schulze
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Hi

I'm computing a 2D-Airfoil in ANSYS CFX and have some trouble with the stagnation point. At an angle of attack of zero I'm expecting it to be right on the leading edge. But as you can see in the pictures below, it is positioned slightly on the suction side. I tried everything I can think of to get it right, but it just won't.

Now a few words to my mesh and my boundaries. It is an ICEM hexa-mesh generated with blocking. The control voume around the airfoil is rectangular with 10 chord lengths distance from the leading edge to the inlet, 12 from chord to top and bottom of the CV and 40 chord lengths from the trailing edge to the outlet. You can also see two pictures of the mesh below. I espacially gave the nose of the airfoil a higher resolution to prevent the error I got now.
The boundaries I'm using in CFX are a velocity inlet, translational periodicity at bottom an top, symmetry at both sides and an entrainment at the outlet. I already tried to use other boundaries, like entrainments at bottom and top, it didn't work either. The k-ome SST model is used for turbulence, I have an value of y+ below 1 at the wall.


Now does anyone have an idea what I am doing wrong?

Greetings and many thanks for your help!
Jonas
Attached Images
File Type: jpg mesh_test_9_steady_0aoa_sp1.jpg (38.8 KB, 17 views)
File Type: jpg mesh_test_9_steady_0aoa_sp2.jpg (38.9 KB, 12 views)
File Type: jpg profil1.jpg (96.6 KB, 16 views)
File Type: jpg profil2.jpg (81.7 KB, 11 views)
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Old   March 29, 2011, 08:07
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siw
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Would it not be the case that the stagnation point would only be always on the tip of the leading edge at zero angle of attack for a symmetrical aerofoil.

Since your aerofoil is un-symmetrical it would not have to be right on the LE tip since the more complex un-symmetrical aerofoil shape is having an un-symmetrical influence on the flowfield just upstream.

Last edited by siw; March 29, 2011 at 08:16. Reason: Typo
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Old   March 30, 2011, 19:10
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Glenn Horrocks
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Stuart is right, the stagnation does not lie at the front tip for an asymetric airfoil. Have a look at the velocity vectors. That might explain what is going on a bit better.
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Old   March 31, 2011, 06:01
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Jonas Schulze
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Hi,

thanks for the helpful answers so far. the explanation why the stagnation point is not located at LE tip seems clear now. Still one question remains. why is the stagnation point in our simulation is on the suction side of the airfoil for zero angle of attack.

If one considers simple model from potenial flow theory with distributed sources and sinks distributed and a vortex sheet on the camber line, wouldn't the stagnation point lie on the pressure side when the airfoil generates lift?

thank you very much for your answers in advance

cheers
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