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This article presents the consolidated results of a comprehensive validation campaign of a pneumatic active flap system (AFS) developed within the scope of the Induflap2 project [1] in a collaboration effort between Siemens Gamesa Renewable Energy A/S (SGRE), the Technical University of Denmark (DTU), and Rehau GmbH. The validation results presented herein include wind tunnel measurements, measurements in a rotating test rig under free atmospheric conditions, as well as the validation at full scale on a 4MW wind turbine with 130m rotor diameter (SWT-4.0-130). This article is of a summarizing character and the reader is referred to further literature where applicable. During the course of the project, several revisions of the AFS were developed and tested at different levels of fidelity. Along with the different variations of the AFS, the impact of variations of the geometric and material related design parameters were studied. Two of the latest revisions of the AFS were tested at full scale on a wind turbine. The measurements presented herein focus solely on the latest revision of the AFS (internally referred to as the FT008rev10 concept). The different tests and measurements were performed in the period from 2016 to 2019. During the full scale measurements, a wind speed dependent load impact between 5% and 10% was measured for the blade root flapwise bending moments, demonstrating thus a large potential for turbine loading adjustments to perform active load control on modern wind turbines. 1. State of the art Active devices for load control of wind turbine rotors have been extensively studied in the past decades and it is not the purpose of this article to give an exhaustive review of literature in this field. For this purpose, the reader is referred to [2, 3, 4], which in combination, give a very good overview of the variety of concepts for wind turbine active and passive load control including active flaps, microtabs, as well as diverse mechanisms for boundary layer (BL) control such as BL suction and blowing, plasma actuators, or synthetic jets. Most of the research groups active within active flap systems for rotor blades (including the authors) participated with short technical and scientific contributions to the technical expert meeting nr. 87 of the IEA Task 11 [5] (only available to members), making this document a good compendium of the current state of affairs with regards to research and development within the Smart Blades community. The amount of literature and studies available for system level full scale experimental validation of active control strategies and devices is rather limited. Recent tests were performed by Sandia Laboratories [6, 7] within the scope of the SMART Rotor Project on a 9m long blade, where the outer 20% of the blade span included a hinged flap section covering the aft 20% of The Science of Making Torque from Wind (TORQUE 2020) Journal of Physics: Conference Series 1618 (2020) 052024 IOP Publishing doi:10.1088/1742-6596/1618/5/052024 2 the chord. The flap was actuated in a mechanical fashion with means of motors embedded in the structure of the blade. The characterization of the system was performed mainly via on-off actuation of the flap as well as sinusoidal actuation. A summary of the results of the project is given in [8]. A further full scale test was documented during the period from 2010 to 2014 through a series of publications [9, 10] describing the test of a flap system as well as the controller strategy [11, 12] of a Vestas V27-225 kW (13m long blade) equipped with a 70cm long trailing edge flap. During these tests, the turbine was operated with and without flap control in intervals of 2 minutes. Recently, a shape conforming active flap (morphing trailing edge) developed within the framework of the EU INNWind project was tested under atmospheric conditions on the rotating test rig at the Risoe Campus of DTU [13, 14, 15]. Parts of the text matrix included flap actuation steps, periodic feed-forward control based on azimuth position, as well as feed-forward control based on inflow angle. The flap step actuation is performed with cycles of 10s over a test period of 5 minutes, repeating this procedure for different pitch angles of the flap. From the flap steps, a range of variation of lift coefficient delta ranging from +0.2 down to -0.25 was measured. Azimuth and inflow based feed forward control showed an average reduction of the standard deviation of the bending moment at the base of the rotating rig boom of 12% and 11%, respectively. Previous work on morphing flaps carried out within the framework of the Induflap1 project [1] includes testing under atmospheric conditions in the rotating test rig as well [16, 17]. The bending moment measurements using this flap (of 15% chord coverage) as described in [16] showed that a 5 deg flap actuation has a similar response as 1 deg blade pitch actuation, which is an interesting proxy for comparison of flap control authority. The work reported in [17] mentions a flap actuation in the range of -5 to +5 deg corresponding roughly to a pitch actuation of 3 deg for the full blade section. At a smaller scale and under controlled wind tunnel conditions, the work described in [18] and in [19, 20] demonstrate the use of active trailing edge flaps on a 3m and 3.5m diameter model wind turbine model at the TU Berlin and the University of Waterloo, respectively. The investigations of Samara [20] focus mainly on the characterization of the flap system, benchmarking it against the pitch system of the turbine. For this purpose, an active flap segment of 20% chord and 22% span coverage was installed on the model test blades. The work concludes that the control authority of the active flap system and the blade pitch are similar. A close look at the results reveals that a flap angle actuation in the -20 to +20 deg is similar in terms of load impact to a pitch actuation in the range from 0 to 9 deg. In recent years, experimental, computational and theoretical work related to active trailing edge flaps has been carried out by different groups and at different levels of fidelity. The reader is referred to the final report of the Innwind.eu project [21], in particular deliverables 2.23 and 2.33. In parts of this project, the active flap technology was explored both from an aerodynamic as well as from a control perspective with the focus of LCOE reduction. It is shown that the combination of individual flap control strategies with individual pitch control can lead to significant blade flapwise fatigue load reduction with some penalty on tower loading. The potential of AEP increase by flap scheduling at low wind speeds is also discussed and estimated to be approx. 0.5% (calculated at a mean wind speed of 10m/s). Similar results are presented in [22], where the AEP potential is estimated between 0.5% and 1.0% depending on the type of rotor design (cp optimized vs. low induction rotors). Older references to active flow control in wind turbines date back to the early 1980’s, where plain hydraulically activated hinged flaps and ailerons were studied both for load and power control within the frame of the MOD-5A research and development program [23]. These active flow control technologies were mostly based on a direct knowledge transfer from the aeronautical industry. The Science of Making Torque from Wind (TORQUE 2020) Journal of Physics: Conference Series 1618 (2020) 052024 IOP Publishing doi:10.1088/1742-6596/1618/5/052024 3 2. Objectives The main objective of the Induflap2 project is to design and subsequently perform a comprehensive characterization of an active flap system. The design of the active flap system shall comply with industrial requirements and be characterized at full scale level striving for a high technology readiness level. Furthermore, it has also been an objective of the project to perform the characterization of the AFS in a stepwise approach, increasing the fidelity level of validation in a systematic manner starting with tests under controlled conditions in a wind tunnel, continuing to testing under atmospheric flow in a rotating test rig and finally on a full scale wind turbine. An important task accompanying these measurements is the simulation of the sub-systems with tools such a CFD, as well as aeroelastic simulations of the full system. 3. Test methodology and selected results This section describes the measurement setup for the AFS at different levels of fidelity and gives some exemplary results for each of the characterization steps listed below. As mentioned in the previous section, an important objective of the project is to develop the active flap technology in a systematic step-wise approach, where each step brings important aspects for the full system characterization. The logical sequence for this is as shown below. • Wind tunnel measurement • Rotating test rig measurements • Full-scale validation The main purpose of the wind tunnel measurements is to provide input for aeroelastic models in terms of aerodynamic polars at different flap settings, but also to test the AFS behaviour as sub-system from a fluid-structure interaction perspective, i.e. how ”aerodynamically stiff” the system is. The AFS as designed within this project is not equipped with position feed-back control to counteract any deflection changes due to external loading. Therefore, although the flap position is driven mainly by the input pneumatic pressure to the system, there will also be a weak coupling to the external aerodynamic loading. A wind tunnel test is well suited for this purpose. The next step in validation is to move away from a controlled flow environment and step closer to free atmospheric conditions. This is achieved by testing the system on an airfoil section mounted on an outdoor rotating test rig as will be described below. Besides the aerodynamic testing of the AFS (measurement of pressure distributions, Cn, Ct, etc.), the rotating

Consolidated results of the laboratory and full scale field validation of an active flap system