Airfoil Design and Testing

A number of airfoils have been designed to supply Dutch wind turbine blade manufacturers with well documented airfoils dedicated to wind turbine applications.

The relative thickness of the airfoils ranges from 18% (DU 96-W-180) for the tip to 30% (DU 97-W-300) for inboard stations of the blade. Drivers for the design process were among others high lift-to-drag ratio and roughness insensitivity.

Wind tunnel tests

The characteristics of new DU airfoil designs are experimentally verified in the Delft University 1.25 x 1.80 m low-speed low-turbulence wind tunnel. (see image figure 1)

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This tunnel has a contraction ratio of 17.6 , giving a maximum test section velocity of 120 m/s. The turbulence intensity ranges from .015% at 10 m/s to .07 % at 75 m/s.

Standard wind tunnel tests to establish airfoil performance consist of measuring the surface and wake pressure distributions around a 0.6 m chord pressure tapped model. Lift, drag and moment coefficients are derived at Reynolds numbers up to 3.5x106 . Zigzag tape of 0.35 mm thickness is used as a standard procedure to investigate the airfoil performance with fixed transition at the 5% chord station and increased turbulent boundary layer thickness.

Airfoils DU 96-W-180 and DU 97-W-300 have also been tested at angles-of-attack from -180 to +180 degrees with a six-component balance system using 0.90m span models with 0.25m chord.

Aerodynamic devices

Aerodynamic devices such as stall strips, vortex generators or even Gurney flaps are being employed to tune rotor performance. During the wind tunnel tests the effect of these devices on the performance of the appropriate airfoils have been addressed: stall strips or trip wires on DU 93-W210, vortex generators at several chord locations on DU 93-W-210, DU 91-W2-250 and DU 97-W-300. To study the effect of a diverging trailing edge Gurney flaps and wedges of several heights and height-to-depth ratios have been applied on DU 93-W-210 and DU 91-W2-250.
Figure 2 shows the effect of zigzag tape on the performance of DU 97-W-300 equipped with vortex generators.

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