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Negative Fuselage Drag on a Tandem Wing?

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Old   April 9, 2012, 16:08
Default Negative Fuselage Drag on a Tandem Wing?
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Mike
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Hey guys,

So, I am modeling external flow over a tandem wing aircraft in Star-ccm+. The domain size looks sufficient, the volume mesh looks good, the solution has converged and has been run for 3500 iterations, and I have double checked the force vector and force monitor orientations.

I have divided the reports into component and total values for lift and drag, and the overall total values for lift and drag look good. The other sub-components lift and drag values (front wing, rear wing, and tail) all look good except the fuse, which has a negative drag value. The shape of the fuse is basically a revolved airfoil.

When I run the fuse just by itself, it has a positive drag value and reasonable down force.

So, my question is: in conjunction with the other aircraft components for a tandem configuration is it possible to have a negative drag value for the fuse component?

Thanks!

Mike
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Old   April 11, 2012, 14:32
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Ryan Coe
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That seems extremely unlikely if you're modelling shear forces. Is this an inviscid simulation?
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Old   April 11, 2012, 14:54
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I'm using the turbulent viscous regime (k omega GammaReTheta), and the ideal gas model (standard properties of air).
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Old   April 28, 2012, 08:20
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Well, likely or not, at least it is possible! But when you inspect your flow field, you should either be able to find a reason, or it should be judged to be an unphysical behaviour.
Check pressure gradients for example. Divide the force in pressure and shear force and compare them (shear force must cause a positive drag value while the pressure force can also cause a negative one). Maybe divide your body in several patches and check them one by one which patch causes the negative drag value. Once you've identified the cause, you can judge whether that's reasonable or not.
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