[sbro]

Hi all,

I'm running a 2D thin airfoil simulation in FLUENT which returns force coefficients for lift and drag at 6 degrees AoA with a chord length=1m of CL=0.60 and a Cd=0.018.

I've re-run this case (same AoA) with a scaled model (1/4c) and the same Reynolds number (model scaled by a factor of 1/4, therefore free-stream/inlet velocity has been doubled and the kinematic viscosity halved to maintain consistent Re) expecting to return equivalent values for the force coefficients but this hasn't been the case; solution returned CL~0.15 and a Cd~0.0045 which is a quarter of the original solution.

As the Re is the same and the mesh has been scaled with the airfoil surface (giving same Y+) I don't understand the reasoning for this or if i've simply made a mistake configuring the sim?

Any advisement would be greatly appreciated! Thanks.

[flotus1]

Did you adjust the reference length to match your scaled model? From the coefficients you obtained, it seems like you forgot to do that.

[sbro]

Originally Posted by flotus1

Did you adjust the reference length to match your scaled model? From the coefficients you obtained, it seems like you forgot to do that.

Thanks for the reply.

You're correct that I forgot to adjust the reference length but, having re-run the simulation now, I'm still obtaining the same result(?)

[flotus1]

I should have been less specific. What I actually meant was "reference values" instead of reference length.
Lift and drag coefficients are calculated using the reference area.
Anyway, that's just post-processing. Changing reference values does not change the solution.

[sbro]

Originally Posted by flotus1

I should have been less specific. What I actually meant was "reference values" instead of reference length.
Lift and drag coefficients are calculated using the reference area.
Anyway, that's just post-processing. Changing reference values does not change the solution.

So, by scaling the geometry by a factor of 1/4 (in X & Y), am I correct in thinking the reference area should be amended by (1/4)^2?

For this case where I am keeping Re consistent, am I correct in thinking I should expect the same force coefficients (lift / drag)?

Thanks again for your help!