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Andrew T. Myers, PhD, PE, Assistant Professor Vahid Valamanesh, Graduate Student Department of Civil and Environmental Engineering Northeastern University The Influence of Aerodynamic Damping in the Seismic Response of HAWTs

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Presentation Outline Motivation Dimensions of utility-scale HAWTs Vulnerability to earthquakes Derivation of aerodynamic damping Fore-aft direction Side-to-side direction Numerical example – 1.5 MW NREL baseline turbine Conclusions

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Installed wind capacity map as of Jan 2011 United States National Seismic Hazard Map Motivation: Exposure of HAWTs to Earthquakes

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Approximate dimensions of a utility-scale HAWT First Period ~ 3 s Dimensions and Period of HAWTs

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No redundancy in the support structure Slender hollow sections (D/t as high as 280) Farms consisting of many nearly identical structures Large directional affect due to aerodynamic damping Side-to-sideFore-aft Vulnerability to Earthquakes

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Aerodynamic Damping of HAWTs in the Fore-Aft Direction Forces based on blade element momentum theory (BEM) Flexibility of rotor is omitted Wind direction is along fore-aft direction Steady wind First mode of vibration is considered

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Aerodynamic Damping of HAWTs in the Side-to-Side Direction

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Numerical Example – 1.5 MW Baseline Turbine by NREL Power output1.5 MW Hub Height84 m Rotor Diameter70 m Number of Blades3 Max Rotational Speed20 rpm Cut in wind speed5 m/s Cut out wind speed25 m/s Nacelle Mass51 Ton Hub Mass15 Ton Tower Mass123 Ton Rotor Mass11 Ton Active Pitch ControlYes [Base image from Nuta, 2010]

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Numerical Example – 1.5 MW Baseline Turbine by NREL Aerodynamic damping in the fore-aft direction with =20 rpm and =7.5

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Numerical Example – 1.5 MW Baseline Turbine by NREL Aerodynamic damping in the side-to-side direction with =20 rpm and =7.5

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Aerodynamic damping in the fore-aft direction with =7.5 (left) and =20 rpm (right) Numerical Example – 1.5 MW Baseline Turbine by NREL

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Aerodynamic damping in the side-to-side direction with =7.5 (left) and =20 rpm (right)

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FAST Derivation Numerical Example – 1.5 MW Baseline Turbine by NREL Validation with FAST in the fore-aft direction with =7.5 and =20 rpm

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Numerical Example – 1.5 MW Baseline Turbine by NREL Effect of aerodynamic damping on the seismic response with =20 rpm

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Conclusions Aerodynamic damping of operational wind turbines strongly depends on wind speed. For the considered example (1.5 MW turbine, = 20 rpm, = 7.5˚, wind speed between cut-in and cut-out): The fore-aft aerodynamic damping varies between 2.6% and 6.4% The side-to-side aerodynamic damping varies between -0.1% and 0.9% For this same operational case, the derivative of the lift coefficient with respect to the angle of attack is the most influential parameter in aerodynamic damping in the fore-aft direction The blade pitch angle and rotational speed also influence the aerodynamic damping in both the fore-aft and side-to-side directions The directional effect strongly influences the seismic response, with median spectral drift predicted to be as much as 70% larger in the side-to-side direction than in the fore-aft direction

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