Designing for Low Aerodynamic Noise from Wind Turbines
Aerodynamic Sources of Sound Emission
The wind hitting an object at a speed can produce a sound. For example, wind hitting leaves or against water will generate random and varied high frequency sound, which is often called white noise.
The wind blowing against a solid surface can produce vibration within the object. This can happen with parts of a building or a vehicle. These surfaces then emit sound in turn. In fact, a surprising amount of the noise generated by aircraft come not from the engines, but the vibration of the flight surfaces (wings in a aeroplane or the rotors in a helicopter). Different materials vibrate at different frequencies and therefore produce different noise.
Wind hitting a sharp edge may produce a pure tone. This principle, in fact, is how woodwind instruments work
Rotor Blade Sound Emission and the Fifth Power Law
Rotor blades will make a swishing sound when operating at low speeds. This noise is slight and you are only likely to hear it if you stand close to the wind turbine operating in relatively low wind speeds.
Rotor blades work by braking the wind. Some of the kinetic energy of the wind is transferred to the blade, which turns the blade and slows the wind. It is this process which can generate white noise. Because the surfaces of rotor blades must be very smooth (for aerodynamic reasons), the surface itself will vibrate at a high frequency and will itself emit some (although very little) of the noise. Much of the noise generated will come from the trailing (back) edge of the rotor blades as the air is stirred by the leading edge of the blades. By carefully designing the shape of the trailing edge and by carefully balancing the rotor blades during manufacture and installation this can be reduced to an absolute minimum. These techniques are now routine throughout the wind turbine industry.
Everything else being equal, the volume of sound will increase on a scale of the fifth power of the blade speed relative to the air. This is one of the reasons why modern wind turbines have larger rotor diameters and much lower rotational speed.
As a rotor blade rotates past a wind turbine tower, the flow of air over the blade is disturbed and the noise volume and characteristics change. The overall effect is a noise that beats at a frequency related to the rotor speed. Amplitude modulation is just another way of describing this beat. Good blade design helps reduce this phenomenon down to low levels.
Rotor Blade Tip Design
Since the blades rotate, the tips are actually travelling through the air substantially faster than the base of the blade. Because of this, the profile of the blade varies along its length and great care must be taken in the design, especially at the rotor tip. By comparing rotor blades made at different times, you can visibly see the subtle changes in the geometry of the bade and how it has developed over time. This is down to more research being done all of the time.
Research is constantly being carried out as manufacturers seek greater performance from their turbine. The rotor tips are important as the principles of leverage mean that the torque (rotational moment) applied to the rotor increases as you move along the blade, with the most at the tip. In addition to this, the way the air flows around the tip of the blades is far more complex, even when compared to how the air flows over any other part of rotor.
Research on Quieter Blades
Research into rotor blades which produce less noise is ongoing, but this is actually a performance issue rather than an impact issue. The noise generated by modern wind turbines is minor. Most of the benefit from research into quieter blades is in the reduction of vibration. Vibration reduces efficiency of the rotor and generator, as well as adding wear and tear to the materials. Lower noise levels would also allow increased rotational speeds of the rotor which would improve the energy output. Faster rotational speeds are currently possible, but the added noise would be a cause for concern.