A numerical study of the fluid dynamic noise generated by a model-scale wind turbine

In this thesis, we present an acoustic characterization of a model-scale wind turbine, with and without the presence of the tower, using large eddy simulation and the acoustic analogy. The analysis is representative of medium-sized turbines with low tip Mach number (∼ 0.10). In the case of the isolated rotor, the fluid dynamic analysis revealed: a turbulent boundary layer over the blades, together with a trailing edge vortex sheet; a complex near-wake structure, including tip and root vortices; an intermediate wake with vortex instabilities triggering leap-frogging and vortex grouping mechanisms; a far wake characterized by fully developed turbulence. The presence of the tower appears to induce a more intricate wake, exhibiting analogous primary characteristics to the isolated rotor and a nonlinear interaction between the blade’s features and the tower’s vortex shedding. Moreover, the tower induces an inherent asymmetry within the fluid flow, expediting the transition from the near to the far wake. Two primary noise generation mechanisms were identified. The unsteady pressure field over the turbine surface generates tonal noise at the blade passing frequency and a high-frequency broadband noise, associated with the trailing edge vortex sheet (linear-noise contribution). The turbulent wake generates broadband low-frequency noise, driven by the complex fluid-dynamic processes outlined above (non-linear noise contribution). The linear part of the noise was found to dominate over the non-linear one in the acoustic far field, while the non-linear noise may play a role in the acoustic near field. The tower appears to have a significant impact on the rotor plane, particularly in the low-frequency range. In particular, the tower promotes a more evident dipole shape instead of a monopole in the rotor plane. The analysis of the acoustic decay rates reveals that the linear and non-linear terms in the near field decay according to an r−(n+1) law within the rotor plane, where n is the number of blades, consistent with recent findings on the acoustics of rotating sources. In the case of the complete wind turbine, the same results are obtained in the streamwise and vertical directions. However, in the y-direction, a more rapid transition from the decay rate of r−4 to r−1 is evident. This is in line with the findings concerning the linear terms, which once again highlight the non-marginal role played by the tower in the rotor plane. The presence of the reflection plane, representative of a realistic operating environment, significantly alters the acoustic field. In the x–y plane, the acoustic pressure fluctuations are nearly doubled (increase of around 5 − 6 dB in the SPL) relative to the infinite-medium case, while preserving the dominant direction of propagation. In the rotor plane, the reflected waves amplify the dipole characteristics of the linear source and induce a more pronounced quadrupole pattern in the non-linear component.

In this thesis, we present an acoustic characterization of a model-scale wind turbine, with and without the presence of the tower, using large eddy simulation and the acoustic analogy. The analysis is representative of medium-sized turbines with low tip Mach number (∼ 0.10). In the case of the isolated rotor, the fluid dynamic analysis revealed: a turbulent boundary layer over the blades, together with a trailing edge vortex sheet; a complex near-wake structure, including tip and root vortices; an intermediate wake with vortex instabilities triggering leap-frogging and vortex grouping mechanisms; a far wake characterized by fully developed turbulence. The presence of the tower appears to induce a more intricate wake, exhibiting analogous primary characteristics to the isolated rotor and a nonlinear interaction between the blade’s features and the tower’s vortex shedding. Moreover, the tower induces an inherent asymmetry within the fluid flow, expediting the transition from the near to the far wake. Two primary noise generation mechanisms were identified. The unsteady pressure field over the turbine surface generates tonal noise at the blade passing frequency and a high-frequency broadband noise, associated with the trailing edge vortex sheet (linear-noise contribution). The turbulent wake generates broadband low-frequency noise, driven by the complex fluid-dynamic processes outlined above (non-linear noise contribution). The linear part of the noise was found to dominate over the non-linear one in the acoustic far field, while the non-linear noise may play a role in the acoustic near field. The tower appears to have a significant impact on the rotor plane, particularly in the low-frequency range. In particular, the tower promotes a more evident dipole shape instead of a monopole in the rotor plane. The analysis of the acoustic decay rates reveals that the linear and non-linear terms in the near field decay according to an r−(n+1) law within the rotor plane, where n is the number of blades, consistent with recent findings on the acoustics of rotating sources. In the case of the complete wind turbine, the same results are obtained in the streamwise and vertical directions. However, in the y-direction, a more rapid transition from the decay rate of r−4 to r−1 is evident. This is in line with the findings concerning the linear terms, which once again highlight the non-marginal role played by the tower in the rotor plane. The presence of the reflection plane, representative of a realistic operating environment, significantly alters the acoustic field. In the x–y plane, the acoustic pressure fluctuations are nearly doubled (increase of around 5 − 6 dB in the SPL) relative to the infinite-medium case, while preserving the dominant direction of propagation. In the rotor plane, the reflected waves amplify the dipole characteristics of the linear source and induce a more pronounced quadrupole pattern in the non-linear component.