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-   -   Cd and Cl scaling with velocity (http://www.cfd-online.com/Forums/fluent/126437-cd-cl-scaling-velocity.html)

scoobysimon November 17, 2013 14:50

Cd and Cl scaling with velocity
 
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

Kokemoor November 18, 2013 14:56

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.

scoobysimon November 19, 2013 04:29

Ah thank you, absolutely right about the reference values


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