Wind Energy and the Turbulent Nature of the Atmospheric Boundary Layer

Wind turbines operate in the atmospheric boundary layer, where they are exposed to the turbulent atmospheric flows. As the response time of wind turbine is typically in the range of seconds, they are affected by the small scale intermittent properties of the turbulent wind. Consequently, basic features which are known for small-scale homogeneous isotropic turbulence, and in particular the well-known intermittency problem, have an important impact on the wind energy conversion process. We report on basic research results concerning the small-scale intermittent properties of atmospheric flows and their impact on the wind energy conversion process. The analysis of wind data shows strongly intermittent statistics of wind fluctuations. To achieve numerical modeling a data-driven superposition model is proposed. For the experimental reproduction and adjustment of intermittent flows a so-called active grid setup is presented. Its ability is shown to generate reproducible properties of atmospheric flows on the smaller scales of the laboratory conditions of a wind tunnel. As an application example the response dynamics of different anemometer types are tested. To achieve a proper understanding of the impact of intermittent turbulent inflow properties on wind turbines we present methods of numerical and stochastic modeling, and compare the results to measurement data. As a summarizing result we find that atmospheric turbulence imposes its intermittent features on the complete wind energy conversion process. Intermittent turbulence features are not only present in atmospheric wind, but are also dominant in the loads on the turbine, i.e. rotor torque and thrust, and in the electrical power output signal. We conclude that profound knowledge of turbulent statistics and the application of suitable numerical as well as experimental methods are necessary to grasp these unique features (...)Comment: Accepted by the Journal of Turbulence on May 17, 201

DU00-W-212 airfoil polars, mimicked DanAero inflow

Averaged lift and drag coeffficients of a DU00-W-212 profile in turbulent inflow generated with an active grid at Reynolds numbers 500,000 and 900,000. The inflow pattern was mimicked from measurements a the blade with a 5-hole pressure probe performed in teh DanAero project. Data is obtained with a three-component load cell and via integration of 48 scanned pressure tabs along the chord. Standard wind tunnel corrections according to Allen & Vincenti are applied. Data sets 203, 204, 210, 211: flow tripped on the surface at 1.5% chord on upper airfoil side, 10% chord on lower airfoil side Data sets 203, 211, 225, 230: measured starting at positive angles of attack (AOA) to negative AOAs Data sets 204, 210, 224, 239: measured starting at negative angles of attack (AOA) to positive AOAs The experiment was performed within in the EU-funded project AVATAR (www.eera-avatar.eu)

DU00-W-212 airfoil polars, sinusoidal inflow

<p>Averaged lift and drag coeffficients of a DU00-W-212 profile in sinusoidally varying inflow (turbulence level approx. 5%) generated with an active grid at Reynolds numbers 500,000 and 900,000.</p> <p>Data is obtained with a three-component load cell and via integration of 48 scanned pressure tabs along the chord.<br> Standard wind tunnel corrections according to Allen & Vincenti are applied.</p> <p>Data sets 182, 183, 186, 188: flow tripped on the surface at 1.5% chord on upper airfoil side, 10% chord on lower airfoil side<br> Data sets 183, 188, 242, 245: measured starting at positive angles of attack (AOA) to negative AOAs<br> Data sets 182, 186, 243, 244: measured starting at negative angles of attack (AOA) to positive AOAs</p> <p>The experiment was performed within in the EU-funded project AVATAR (www.eera-avatar.eu).</p