Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 120

Interaction of turbulent eddies in the blade boundary layer with the trailing edge [i.e. (i) in the previous section] is usually regarded as the most important noise source, and efforts are continuing to design blades to minimise it. Modelling techniques have been developed to predict the aero‐acoustic radiation from this source; see Brooks et al. (1989) and Zhu et al. (2005). Two main methods of reducing the intensity have been considered:

1 Blade profile design to reduce the thickness of the suction surface boundary layer (which, being the thicker of the two, therefore has the larger turbulent eddy scales as well as the greater source layer thickness) at the trailing edge. Some progress has been achieved in reducing this boundary layer's thickness by reducing the strength of the suction pressure on the suction surface while compensating to maintain overall lift and particularly lift/drag ratio by increasing the positive pressure downstream of stagnation on the pressure surface. This method, perhaps because of the constraints involved, has yielded moderate noise reductions of up to about 2 dB. Families of low noise aerofoils have been designed, such as the DTU‐LNxxx series shown in Figure 3.78a–c; see also Wang et al. (2015).

2 Making the trailing edge serrated (see Figure 3.79) or by adding flexible ‘brushes’ to it. This concept is based on Howe's (1991) analysis of the reduction in radiation efficiency of a trailing edge as a result of making it serrated in plan. Although Howe's theory doesn't give a very good prediction of the actual sound power reduction that is achieved, nonetheless the technique has been shown to give useful noise reductions of more than 3 db. Serrations of this type appear to be possible without significantly affecting the section lift or drag. They may be part of the outer blade design or have been sometimes in the form of an add‐on to existing blades. A good description is given in Zhu et al. (2016).