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the skin friction coefficient in Fluent

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Old   April 13, 2013, 23:27
Question the skin friction coefficient in Fluent
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I simulated the low-reynolds-number flow over airfoils, there is a seperation bubble on the upper surface of the airfoil.
The experiment data shows that the skin friction near the seperation bubble is negative. But the result I calcuate by the Fluent is positive.
I don't konw why this happened for there is already have the backflow in the seperation bubble near the surface.
Is it also a reason for the Fluent can't give a good prediticion of drag?

This question puzzle me for a long time. I need your help!
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Old   April 13, 2013, 23:47
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The numerical result depends on several aspects, such as mesh topology, turbulent model, Yplus, you can check this parameters and try the process again. And you can find some tutorials on the airfoil on the Internet, check how the others settle it.
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Old   April 14, 2013, 20:23
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Skin Friction is a vector quantity. Are you plotting the directional component of skin friction? If you are just plotting "Skin Friction Coefficient", you are plotting the vector magnitude which of course can never be negative.

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Old   April 15, 2013, 08:08
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Our friend "stuart23" says true.

I had the same problem for a turbulent separated flow. To get round this problem you must use the equation of Skin Friction Coefficient given in the help:

file:///C:/Fluent.Inc/fluent6.3.26/help/html/ug/node1219.htm#217810

Good luck.
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