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Cd and Cl scaling with velocity

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Old   November 17, 2013, 14:50
Post Cd and Cl scaling with velocity
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Hi,

I recently began using Fluent as part of a final year project part of which is analyzing aerofoil performance. At the moment I am modeling a NACA 0012 using inviscid flow.

I noticed from monitoring the Cd and Cl values (from the force monitors drop down menu, in the monitors section) that they seem to scale with velocity squared. I used exactly the same mesh and angle of attack, only varying the inlet velocity and got the following results:

V = 1 m/s, Cd = 1.2602e-03, Cl = 1.8608e-02
V = 171.6 m/s, Cd = 3.7109e+01, Cl = 5.4793e+02

I would expect the lift and drag forces to scale with velocity squared but not the coefficients of lift and drag.

I feel I am misunderstanding the Cl and Cd monitors and was hoping someone could explain them.

Thanks,
Simon
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Old   November 18, 2013, 14:56
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Andrew Kokemoor
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Have you set the Reference Values properly? All nondimensional quantities are calculated using the quantities specified in Reference Values; in this case, the density, velocity, viscosity, and length/area. You will have to manually set these (including the velocity) for each calculation. If you want to automate this, it may be best to define nondimensional quantities yourself as output parameters in CFD_Post or Workbench.
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Old   November 19, 2013, 04:29
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Ah thank you, absolutely right about the reference values
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airfoil, drag, lift, scaling

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