Aerodynamic insights -- Dutch thesis

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Energy production from wind turbines worldwide could be improved by around 9% if the airflow over the rotor blades could be better controlled, according to the results of a doctoral thesis completed by Gustave Corten at the University of Utrecht, Netherlands. In his examination of wind turbine rotor blade flow behaviour Corten has particularly studied where separation of the flow occurs.

Separated flow on a blade is generally undesirable as it reduces the power output, but in certain circumstances, particularly with stall-controlled rotors in higher winds, it plays a vital role in limiting the power output. Although separated flow can sometimes be influenced by roughness strips or by vortex generators, accurate placing of these aids is essential if the correct result is to be achieved.

Corten's aim was to determine whether the various predictive tools currently available accurately forecast the location of separation. His principle tool was "stall flags" -- light hinged flaps patented by the Netherlands Energy Research Foundation (ECN). The flags are positioned so that flow separation causes them to hinge upwards from the aerofoil surface, thus exposing a light-reflecting surface and enabling this to be captured on film. In this way the zones of flow separation can be mapped.

The value of the measurement technique was demonstrated during tests carried out on a 43 metre rotor. Initially the power output was 30% too high, making it ncessary to set the wind turbine's cut-out speed well below the wind strength it was designed to capture enegy from. The investigations showed that some areas of the blade were not stalling as intended. By repositioning vortex generators on the blade, the stalled zone was increased and the power curve was then much closer to its design values.

Overall, the energy production of the rotor assembly increased by 8%. The author appears to imply that similar improvements can be expected on most wind turbines, but he does not seem to provide evidence to substantiate this claim.

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