Wind turbine blade characteristics

[CPP]

I'm about to start a project in which I want to study the effects of making modifications to a wind turbine blade. I am going to be able to use various external flow modeling software our aerospace labs. I am a mechanical engineer and am used to using solidworks and I understand that that can be used for some external flow. My real question is here:

-I have access to a wind tunnel and rapid prototype machine. I wish to model and place in the wind tunnel a wind turbine blade using the S809 airfoil. There is just a major point I am missing: how do I go about determining the lift/drag coefficients within the wind tunnel? I am far removed from the process of determining these and was hoping someone could point me in a general direction.

thanks.

 

[LURCH]

Upon what will your turbine blade model be mounted? Can the mount apparatus be fited with force-measuring devices?

 

[FredGarvin]

Yup. You need to measure the forces on the blade in the tunnel.

 

[CPP]

I'll get a chance to see the tunnel some time soon and then I can better answer those questions.

Regards,
Kane

 

[mgb_phys]

If the blades rotation speed is slow compared to the wind speed would it be enough to mount the blade fixed in the airflow and just measure the torque at the root as it tries to turn?
It would be much convenient to build and instrument than a rotating blade set.

 

[Graviman]

So you want to measure the characteristics of just the S809 or a partial rotor assy? I'll assume you mean assy for now. You'd be better off with the complete blade set, but could use an offset counterbalance for the centrifugal and aerodynamic forces. Trouble is what offset to use because you don't know the aerodynamic forces...

You will need to measure thrust and bending moment to get an idea of lift and lift centre. Same for drag. Lift/drag centre usually works out about 75% of radius for a straight blade, because the aerodynamic and centrifugal forces increase with radius squared (since proportional to v^2, and v increases with radius).

My advice would be optimise your design with an Excel based blade element model first, before you do your CFD or testing. This will get the design as close to right as practical. The general rules are that an "ideal" aerofoil is tapered with chord proportional to 1/Radius, and that this should be superimposed with an elliptical distribution for tip vortex losses. Most practical designs compromise the root choord, since the wind is free...

You can estimate the lift for the S809 section from:
http://www.engapplets.vt.edu/fluids/vpm/index.html

Actually, i'd be curious to develop my own blade element program to handle the windmill brake state of a helicopter rotor. ;)

Mart