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Old   July 20, 2010, 08:12
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Hello,

I was analysing a delta wing ( Span 22 meters, Length 40 meters, Sweep ange 65 degree, and using double delta, notched wing ) in fluent for getting its Cd. But the value of Cd I am getting is around 28 which is very much higher than the actual (actual value of Cd < 1.00 ). Whats happening ? where is the problem ?

Please help.

Note: I was using K-epsilon model for getting turbulance, and a moving wall condition with a velocity of 2 mach and pressure of 12032 pascals.
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Old   July 20, 2010, 09:04
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First check the Reference values panel, and be sure you are using the correct values for calculating your Cd (you should have the correct area, density and speed in order to get the non-dimensional drag).

If the above was OK check your mesh.

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Old   July 22, 2010, 06:02
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Hello Dohander,

Thanks for your comment. But what do you mean by reference values panel ? Also, I was not calculating Cd, instead just given the option to plott Cd curve in fluent.

I think meshing is correct. Because defferent meshings gives almost same value.

please help.

Waiting for your reply.
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Old   July 22, 2010, 06:33
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fluent calculates forces over the surface.
To find nondimensional coefficient, fluent divides this force to reference values.
You have to define reference velocity, density, surface area, etc. correctly.
If you don't define, it is normal to get cd values of 28, or even 100.
Go to define>reference values.
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Old   July 22, 2010, 10:12
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Quote:
Originally Posted by chitharenjan View Post
Hello Dohander,

Thanks for your comment. But what do you mean by reference values panel ? Also, I was not calculating Cd, instead just given the option to plott Cd curve in fluent.

I think meshing is correct. Because defferent meshings gives almost same value.

please help.

Waiting for your reply.
Same advice as kdrbrk .

In order to check your mesh just see what is your min and max y+ on the wing surface, your min y+ should be greater then 50 for a fast calculation or lower then 1 for a fine resolution of the flow near the wall. However you should not a use a mesh that has y+ between the above values, for e.g. if you have an y+ ~ 10 you are in trouble and your Cd will be off even if you have the correct settings for the reference values.

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