Self-noise of Airfoils Under Stalled Conditions

In recent years there have been reports of a transient impulsive noise signature being produced sporadically by wind turbines. Impulsive noise, where the noise level periodically increases and decreases at a rapid rate, is of concern to industry as turbines producing this noise restricts the growth potential of wind farms. By developing noise control techniques in order to mitigate the production of impulsive noise, it is easier for wind farm operators to comply with noise regulations, removing a significant barrier to growth and reducing the impact on the comfort of nearby residents. One of the likely candidates for the source of impulsive wind turbine noise is stall of the turbine blade. While it is well understood that an increase in turbulence near a hard surface results in an increase sound production, the sound generated by airfoils under stall conditions is not well researched. The purpose of this thesis is to investigate the sound generated by simple airfoils under stall conditions in order to further the understanding of this noise. In recent publications on the noise produced by stalling airfoils, the noise has been divided into two categories. For much of the stall regime the noise is referred to as “deep stall" noise, where the airfoil sheds large vortices at a specific frequency. This is then contrasted with a “light stall" noise regime, where the noise produced is more broadband and the source mechanism is less well understood. The primary focus of this thesis is to understand of the effect of airfoil profile on this “light stall" noise. The data presented in this thesis show that as the airfoils enter a stalled state, a low frequency dipolar noise appears. The production of this noise corresponds to amplitude increases in the turbulent wake spectra seen in literature and this correlation between noise production and wake spectra was subsequently confirmed by studying the wake velocity spectra. It was found that there was significant coherence between the wake velocity and sound signals, indicating that the source of the noise produced at “light stall" is due to vorticity generated in the fully-separated boundary layer as the airfoil enters a stalled state. The primary effect of the airfoil profile on the noise generated at stall is in the rate at which it increases with respect to angle of attack. A NACA 0021 airfoil was found to have a much sharper increase in noise level with respect to angle of attack as it reaches the stall angle, compared with the thinner NACA 0012 profile and a flat plate. This can be related to the rate of change in lift force and the aforementioned changes in wake spectrum. A sharper increase in noise level with angle of attack is significant because it will lead to a more impulsive amplitude-modulation of the sound signal if the angle of attack of the airfoil is varying periodically with time. As wind turbines can experience stall due to unsteady inflow, this represents more evidence that stall is the source of the impulsive noise observed in the field. Subsequently, an investigation was conducted on the effect of strong vortices, shed by airfoils undergoing dynamic stall, on the directivity of stall noise. A vortex generator was used to produce isolated vortices with a similar time-varying profile to a decaying dynamic stall vortex in order to study these effects in isolation. The effect of this changing vortex on the directivity of sound was measured and compared with a quasi-steady model derived from the literature, which indicated that there is no significant difference between quasisteady modelling and the observed effects under the conditions that wind turbine airfoils can be expected to encounter. Using a quasi-steady approach the effect of refraction through shed dynamic stall vortices on airfoil noise can be modelled, and applied to wind turbines. Estimates indicate that large horizontal axis wind turbines are capable of producing dynamic vortices strong enough to induce significant scattering, however these vortices are produced on the inboard sections of the blade, and the dynamic stall is unlikely to occur on the outboard sections where the majority of the blade noise is generated. Overall, the current research indicates that noise produced as an airfoil enters stall (the “light stall" regime) is a strong candidate for the source of impulsive wind turbine noise. In addition, the occurrence of dynamic stall may be causing short-term changes in noise directivity due to refraction of the sound and this is worth considering in future wind turbine noise modelling efforts.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 201