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why the drag in airfoil2D tutorial is negative

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Old   August 31, 2018, 05:57
Default why the drag in airfoil2D tutorial is negative
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Jiadong Wang
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I just run the airfoil2D tutorial and output the forces by function in controlDict.
But the horizontal force is -80.258, which has the opposite direction with the free flow.

Is that reasonable?

Personally, drag of airfoil should be positive and has the same direction with free flow.

I'm quite confused about this. Any suggestion is appreciated.

Thank you.
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Old   September 6, 2018, 11:27
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Drag is a force opposite to the relative motion.
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Old   September 7, 2018, 22:44
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Quote:
Originally Posted by Jeneas View Post
Drag is a force opposite to the relative motion.
Thank you for reply.

So drag is the force opposite to the relative motion of airfoil, or the relative motion of free flow?

Because the airfoil2d tutorial assume airfoil is static, while free flow is from left to right (point to positive direction of x_axis). So I assume the relative motion of airfoil is from right to left (point to negative direction of x_axis), which should lead to the positive direction of drag along x_axis if 'drag is the force opposite to the relative motion of airfoil'.

Please correct me if I'm wrong.

Thank you.
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