Q1. You mention the NASA wind turbine research of the 1980s. Is that relevant to the type of wind turbines used today?
Research into very large (multi-megawatt) wind turbines began at NASA in 1975. Much of this work was undertaken by very competent aero-acousticians, drawing on experience gained in the context of propeller and jet-engine development, and which has successfully resulted in substantial improvements in aero-engine noise. They identified at an early stage why the existing “downwind rotor” turbines were so noisy, and in 1979 commenced theoretical and practical evaluation of the first very large “upwind-rotor” turbine, the 2.5 MW “MOD-2”. In this context, in 1981 they confirmed the predicted reduced noise characteristics, while also investigating the adverse power generation and noise effects associated with close spacings between wind-turbines. They subsequently identified additional circumstances under which the low-frequency and infrasound generation of such upwind-rotor turbines could be compromised, and performed important studies on the human perception of low-frequency noise and infrasound. The latter investigations initially concentrated on the noise characteristics of the earlier downwind-rotor turbines, but the underlying physics governing hearing perception relate also to the upwind-rotor configuration.
Over the intervening 25-35 years, the basic physics of aerodynamic noise generation has not changed, the adverse effects of unduly close-spaced wind-turbine interaction remain the same, and the characteristics of human hearing have not changed. These aspects all continue to have immediate relevance to modern wind-turbine installations, yet this research has often been dismissed as old-fashioned and irrelevant by the wind-development community.
Q2. How do wind turbines produce infrasound and is this hazardous to humans if they cannot hear it?
The infrasound is generated by the aerodynamic lift forces on the blades, which are necessary to provide the driving torque to rotate the blades and generate electrical power. Newton’s law requires that there are corresponding forces of reaction on the air passing over the blades. Although these forces may be comparatively “steady”, the constantly changing position of the blades means that the resultant force pattern acting on the surrounding air is also changing so that, inevitably, infrasound is generated. Additional factors, such as the difference between blades encountering slow moving air at (continue reading here)…