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 Kevin November 1, 2006 14:42

Lift and Drga coefficients for an airfoil

I am having some difficulty obtaining the correct lift and drag coefficients for a simple NACA 0015 blade profile. The blade has a chord length of 0.4 m and my domain size is 5 m x 8 m. The blaede is located in the front half of the domain. I am using an SST turbulence model. My boundary layer is 4 mm thick and contains 20 layers. The problem i am having is that the simulations are predicting hte location of stall at a higher angle of attack then the experimental values. As such, at every angle of attack before the location of stall my lift coeffience from teh numerical results is significantly lower then the experimental values. Does anyone have any suggestions as to what i can do to correct this problem?

Kevin

 Glenn Horrocks November 1, 2006 16:03

Re: Lift and Drga coefficients for an airfoil

Hi,

What is the Reynolds number?

It is likely you will need the transitional turbulence model for this. It will also be sensitive to upstream turbulence and airfoil surface smoothness.

Regards, Glenn Horrocks

 Kevin November 1, 2006 16:13

Re: Lift and Drag coefficients for an airfoil

The reynolds number is 160000 and ive treated the blade as a smooth no slip surfaces. Ive tested the model with 1, 5, and 10 % turbulence intensity without much change in results. Finally, im not exactly sure what teh transitional turbulence model is. if you could explain i would appreciate it.

Thanks.

Kevin

 Michael B November 2, 2006 05:05

Re: Lift and Drag coefficients for an airfoil

The transitional model is the numerical model that predicts the transition from laminar to turbulent flow, hence the name. It is a quite new feature in Ansys CFX. Here's a link to a flyer.

http://www.ansys.com/assets/tech-bri...sition-mod.pdf

 Glenn Horrocks November 2, 2006 16:48

Re: Lift and Drag coefficients for an airfoil

Hi Kevin,

Further to what Michael has said, in the mid to low Reynolds number range you are working in, most airfoils have a turbulent transition point mid way along the foil. This means the front half of the foil is laminar and the rear half is turbulent. Whether the foil separates or not is strongly dependant on getting the transition point right as laminar flows separate easily but turbulent flows are more tenacious.

The tranisition point will be strongly influenced by upstream turbulence levels, surface finish, boundary effects (if present) and other 3D effects.

Therefore for this Re range you almost certainly need the transitional turbulence model. This will possibly lead to other interesting features like a laminar separation followed by transition and re-attachment to the surface, causing a small flow separation bubble.

Foils in this Re range are tricky to do accurate simulations.

Glenn Horrocks

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