Airfoil aerodynamics

After a bit of a break from the European winter back home in Australia for Christmas, here is the first post for 2008, I thought I would start with airfoil aerodynamics and develop that concept over the next few months. I

For current 'state of the art' turbines, let's say 1.5 MW to 3.5 MW, a typical operational Reynold's numbers of around 1-5 million are typical. An indicative airfoil for the power-production area of the blade (the outer half) is the NACA 63415 as seen below (coordinates taken from the public UIUC coordinates database). The NACA five-digit system says (in relation to chord): a maximum camber (curvature) of 6% at 34/2=17% from the leading edge, with a maximum thickness of 15%. This is a typical low speed airfoil, and quite similar to that seen in the glider industry.

The Danish research institution, Risø, have conducted numerous tests on this airfoil, specifically for wind turbine applications, with a good indicative Reynold's number of 1.6 million, some good test data is available from the Risø-R-1280 'aifoil catalogue'. Using this wind tunnel test data, the lift-curve for the NACA 63415 is shown below; CL-max and CL/CD-max are marked for information. In the context of wind turbine blade design, the CL-max value represents the most lift the section can produce prior to the onset of stall, followed by the post-stall region (which is a lot more uncertain than the graph suggests!). The fairly linear lift-curve slope prior to CL-max, and the 'very roughly' linear stall region, are characteristics utilised by the turbine's controller system to regulate power production (this will be the subject of an upcoming post!).


The CL/CD-max value of around 5 degrees is derived from the lift and drag characteristics as shown on the left here. Here, the maximum CL/CD ratio is around 70, which means the airfoil is generating 70 times moer lift than drag at that particular AoA. This is the optimal relationship between lift and drag, and the most efficient lift generation for an airfoil possible. For fixed-wing aircraft (especially gliders!) this is fundamental to gliding performance (actual glide ratios around this value have been achieved!), however for wind turbines this represents the 'sweet spot' where the most amount of energy can be extracted from the wind resource and therefore the turbine's achieved Cp optimised.

The main task therefore for the detailed blade aerodynamic design, as well as the turbine's control system algorithm design, is therefore to optimise the AoA as close to CL/CD-max as possible . When the blade's AoA is outside of CL/CD max, turbine efficiency (Cp) and therefore energy production drop.

This discussion is limited to 2D only, so neglects 3D effects which further complicate the matter! Some further topics I'll write about include: detailed control systems, influence of dirty blades and icing, and changes in Reynold's number (varying the tip speed, and density altitude).