Wind turbines normally produce a swishing sound due to directivity of broadband trailing edge noise from the blades. The sound level variation of this 'Normal Amplitude Modulation' (NAM) is usually only a few decibels. However, in some cases periods of increased swish or thumping are reported. The source mechanism for this phenomenon, here denoted as 'Other Amplitude Modulation' (OAM), is presently not clear. In order to identify potential causes for OAM, a simulation study is performed into the effects of wind shear on the noise from a modern large wind turbine. The results show that, as long as the flow over the blades is attached, wind shear has practically no effect on amplitude modulation. However, strong wind shear can lead to local stall during the upper part of the revolution. This can yield noise characteristics which are very similar to those of OAM. Thus, it can be concluded that local stall is a plausible explanation for OAM. A sensitivity study indicates that the occurrence of wind shear induced amplitude modulation mainly depends on the size of the stall region and the stall noise behaviour of the airfoils. Apart from wind shear, other causes for a non-uniform inflow, such as yaw or topography, may also lead to local stall and OAM.
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