NASA-Langley Wind Turbine Noise Research

Author:  Swinbanks, Malcolm

to the Michigan WERZB Board, Case no. U-15899

NASA-Langley Wind-Turbine Research, 1980-1990

Following my letter to the MPSC, I returned to the UK over the Christmas break, which enabled me to access papers from conferences I had attended in the 1980’s. Specifically, NASA-Langley, probably the foremost aeroacoustic research organization in the world, carried out and published extensive research on wind-turbine noise, starting with their first computer predictive codes in 1980, and continuing through 1990.

During this period, NASA & NASA sub-contractors identified almost all of the specific issues relating to wind-turbine noise, that now are being re-learned the hard way, by bitter experience.

Specific Issues

NASA calculated the sound levels generated by ideal turbine blades operating in clean airflow, and identified how inevitably, inflow conditions associated with atmospheric turbulence could result in unsteady blade loadings, thus increasing these sound levels. (1982 [1]). Moreover, they extended the work to consider the effect of a wind gradient, whereby the incident wind velocity varies with height across the face of the turbine, and showed that substantially higher noise levels can then be generated. (198, [2]).

Finally, they measured in practice the low-frequency threshold of hearing under laboratory conditions, for persons subjected to impulsive wind-turbine noise, and showed this could be almost 20dB lower (i.e. more sensitive) than the conventionally accepted noise threshold for less obtrusive sounds (1982 [3]).

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3 responses to “NASA-Langley Wind Turbine Noise Research

  1. This is HUGELY important.
    Another potential factor is whether, at some wind speeds, natural vortex-shedding from the Towers, augmented and re-enforced by blade-passing interaction, could radiate narrow-band tonal infrasound at 90 degrees to the wind vector.

  2. Ron,
    I believe you are right, and that there was a publication demonstrating this theoretically recently.

    The other thing is that wind shear also impacts the mini-tornado generated by the rotation: In wind tunnel experiments simulating the idealized conditions, the helical wake is cylindrical extending downwind perpendicular to the rotor. In simulated high wind shear, it bends downward. Imagine a slinkie, so spacing at the bottom and the top is no longer even and “receptors” below will be more greatly exposed.

    Wind shear also bends sound waves downward.

    It’s been clear for years that the calculated sound power reported by manufacturers is for neutral atmosphere, restricted wind shear, turbulence and terrain conditions, not to mention vear. Vestas warns about deviations from those conditions at particular planned sites; the IoA published about it (albeit with the wrong solutions) and last Dec. EWEA had a workshop devoted to the consequences, yet it seems largely ignored. Wind shear is still treated as a static function of “terrain” rather than the dynamic and hard to evaluate function that it is. Almost a decade after van den Berg’s seminal studies, the ramifications for the impact on human perception of wind turbine acoustic immissions is ignored, and the fundamental principle that deviations from uniform in-flow rise in proportion to rotor swept area has eluded wind developers. I see some proponents now explaining noise problems as resulting from operation in “non-design” conditions. Exactly. Part of environmental assessment should be establishing that operation will be restricted to design conditions and restrictions to be enforced to prohibit operation outside of them or plans to mitigate non-design conditions immediately rather than after delayed, prolonged acoustic measurement and years of intolerable nuisance to neighbors.

    The other thing is that while some in the industry are now admitting to the “van den Berg effect” for light winds on summer nights (when curtailment costs less), they refuse to acknowledge that a great many suffer most during strong winds in other seasons. I note that in the Netherlands, wind shear is less during strong, winter winds. By tradition, manufacturers have not provided information about the acoustic immissions at higher wind speeds, at least under real life conditions when it is hard to measure. They prefer to blame it all on the wind itself.

    • Kathy,
      You wrote, in part, “… and that there was a publication demonstrating this theoretically recently….”
      Do you know a specific reference to that work? I’d be interested.
      I intend to make an order-of-magnitude quantitative estimate of the sound power radiated from a typical tower. Not too difficult…just haven’t done I yet. I’ll express it not in dB, but rather in Watts, so the Public will be able to grasp it, e.g. in relation to audio systems, rock concerts etc.

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