The wings of eagles, and many smaller birds, are serrated and on closer examination resemble a number of fingers rather than a single wing. The design has the effect of breaking up the tip vortices into a number of smaller vortices and improves the efficiency of flight, whilst at the same time reducing noise. A Joule project lead by the University of Stuttgart in Germany aims to investigate these mechanisms with a view to building the concept in to the tip of a wind turbine blade.
Initial studies have shown that the work needs to concentrate on the blade tip, where most of the noise is generated, but that the complex three-dimensional nature of air flow in the vicinity of blade tips may modify conclusions reached as a result of wind tunnel tests. A number of tip layouts have been examined and three different designs will be tested on a wind turbine with two blades and a 16 metre rotor.
In a separate project in the Netherlands, paid for by Dutch energy agency NOVEM, rotor blade manufacturers Rotorline and Aerpac, and wind turbine manufacturer Nedwind, tests are being carried out on a 52 metre diameter wind turbine. Its blades have been provided with a serrated trailing edge along five metres of the blade near the tip.
Initial results are promising, with a 5 dBA reduction of noise in the 250-2000 Hz band, although noise levels at high frequencies increased. It was concluded that the five metre length was sufficient and the overall noise reduction for the turbine is expected to be around 2-3 dBA. The Netherlands Energy Research Foundation (ECN) is involved both this and the Stuttgart project, indicating that there will be some correlation.