Experimental assessment of active flow control techniques for wind turbines with a wind tunnel Demonstrator

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The global goal of the second funding period is to continue the investigations of load alleviations methods on the Berlin Research Turbine (BeRT) and to analyze the influence of local unsteadiness and rotation. During the first period, BeRT was designed, constructed and set up in the 4.2 m x 4.2 m wind turbine test section of the TU Berlin large wind tunnel. All data acquisition and control hardware is installed inside the rotating system (hub). This unique facility is the first German load control research turbine in a wind tunnel with the advantages of high availability and low operational costs which are optimal properties for scientific studies. BeRT also allowed the development of a novel measurement technique where a quantitative tuft flow visualization technique was synchronized with time resolved pressure and vibration measurements. With this technique, an arbitrary measured variable (e.g. pressure) can be linked to an instantaneous surface flow field on the rotor blades. It is possible to capture the complete rotor in one image, with allthree blades equipped with flow tufts and image registration markers. This method is very helpful for analyzing unsteady flow phenomena and loads on wind turbine blades. So far, the project was focused on large-scale inflow distortions such as a velocity shear and yaw misalignment. These unsteady flow conditions are also found on large scale wind turbines. With ever-increasing rotor blade lengths, the local inflow distortions along the span become dominant in the blades load spectrum.In the first period, the unsteady inflow conditions were generated by driving the turbine into a yaw misalignment of up to 30 degrees. In the second period of the project, the inflow conditions complexity will be raised to explore the limitations of the load control concept servo-actuated flaps of PP 2 [Nayeri/Paschereit] and adaptive camber of PP 5 [Tropea]. The inflow distortion will now be generated locally by a velocity deficit (wake) upstream of the rotor plane. As the rotor blade passes through the wake-region, both inflow velocity and angle of attack will be altered simultaneously. This represents a typical distortion or a typical gust to which the respective load control system will react. The wake velocity deficit will be varied to obtain three angle of attack/velocity variations. The load control concepts of PP 2 [Nayeri/Paschereit] and PP 5 [Tropea] will then be compared under similar inflow conditions. The experimental results will be compared to numerical simulations conducted within the context of PP 3 [Lutz/Krämer].In addition to testing the load control concepts under otherwise similar inflow distortions, it is also planned to study the effect of 3D, or rotational, effects on the (unsteady) lift generation. It is known that when the inflow velocity and/or angle of attack of a 2D wing is varied in a harmonic manner, the generated unsteady lift deviates from the respective quasi-steady lift value.