Master Thesis Measuring apparent flow vector on a flexible wing kite Johannes Oehler Supervisor: Prof. Dr. P.W. Cheng Dr.–Ing. R. Schmehl University of Stuttgart Stuttgart Wind Energy (SWE) @ Institute of Aircraft Design Prof. Dr. P.W. Cheng Delft University of Technology Wind Energy Section @ Institute for Aerodynamics, Wind Energy, Flight Performance and Propulsion (AWEP) Prof. Dr. G. van Bussel May 2017 Abstract In order to use flexible wing kites effectively in power generation, their size must be increased and aerodynamic performance should be systematically improved. To this end, validated models to predict the behavior of future kite designs are essential. The biggest uncertainty in current modeling approaches appears in the air flow experienced by the kite. This thesis develops a measurement setup for the apparent flow vector. An air data boom with a Pitot-static tube and two wind vanes is used to sense the relative flow directly below the kite. The acquired data is validated and shows that high-quality in-situ measurements of relative flow information are possible using the presented airborne setup. Experimental data supports the current quasi-steady model for a pump- ing kite power system [22]. The thesis suggests a mechanistic model for the resulting aerodynamic coefficient of the airborne kite system cR. A sole dependency of this aerodynamic coefficient on the angle of attack, as it is customary for rigid airfoils, must be rejected. Instead, the coefficient is found to be heavily dependent on the wing loading and to a lesser extent on the power ratio i.e. the non-dimensional measure for the active pitch control of the wing. The experimental data shows that the kite’s angle of inflow changes during flight even when the wing is not actively pitched. This effect is mainly induced by gravitation. Further it is observed that the oscillation of the reeling velocity results in a high frequency flutter of the wing. The relation for cR presented in this work can be used for flight path optimization as well as kite design. III Abstract Um Kitesysteme mit verformbarem Tragflügel effektiv zur Stromerzeugung zu nutzen, sind zum einen deutlich grössere Tragflächen als bei den aktuell genutzten Surfkite-Sportgeräten erforderlich, zum anderen sollte die aerodynamische Güte der Fluggeräte verbessert werden. Validierte Modelle um das Verhalten von neuen Kitekonzepten vorherzusagen sind notwendig. Die grösste Unsicherheit bei der Modellierung ist derzeit die relative Strömung, die der Kite erfährt. In dieser Arbeit wird ein Messaufbau für den relativen Strömungsvektor entwickelt. Ein Air Data Boom mit einem Pitot-Static-System und zwei Windfahnen misst die relative Strömung unter dem Kite. Die aufgezeichneten Daten werden validiert und zeigen, dass es mit der entworfenen Messapparatur möglich ist, eine genaue in-situ Messung des relativen Strömungsvektors durchzuführen. Die experimentell ermittelten Daten stützen das Modell eines Winden- ergiekonverters mit einem zyklisch fliegenden Kite aus [22]. In der Arbeit wird ein mechanistisches Modell für den resultierenden aerodynamischen Koeffizienten des fliegenden Systems cR vorgestellt. Dieser Koeffizient ist bei verformbaren Kites, anders als bei Fluggeräten mit starren Flügeln, nicht allein vom Anstellwinkel abhängig. Der Koeffizient hängt in diesem Fall stark von der Tragflächenbelastung ab und weniger vom Leistungsverhältnis PR, das ein dimensionsloses Mass für die aktive Pitch-Einstellung des Kites darstellt. Die Messungen zeigen, dass sich der Anströmwinkel des Kites auch ohne aktives Pitchen aufgrund der Schwerkraft ändert. Ausserdem wird ein hochfrequentes Flattern des Tragflügels beobachtet, das durch die Schwankungen der Kiteabspulgeschwindigkeit vreel verursacht wird. Die entwickelte Berechnungsmethode für cR kann zur Optimierung der Flugbahn oder des Kitedesigns verwendet werden. IV Acknowledgements I am very grateful that I could work and write my thesis on the interesting and challenging topic of airborne wind energy. I would like to thank my supervisors Professor Po Wen Cheng and Dr. Roland Schmehl for offering me this opportunity. It was motivating for me to dive into this completely new subject and still being able to use and profit of what I had learned during my studies of aerospace engineering in Stuttgart. Thank you Roland and all members of the kite power research group in Delft for your helpful comments and discussions. I am hoping to continue some of them. Without the team of Kitepower this thesis would never have been suc- cessful. Thank you all for supporting my work and helping me with the physical building of crucial parts of my thesis. Special thanks to Joep, Bert and Bruno who contributed a lot to build a useful and working measurement setup. Thank you all my fellow master students from room 6.08! Without you my stay in Delft and my thesis would not have been the same. Obviously. Thank you Bas for all your moral and IT support, thank you Clara and Pranav for helping me in any possible situation and being my friends. Thank you all others for your support and the many nice days and evenings we spent together! Thank you C. V. L. & M., I was unspeakably lucky to meet you and live with you. Thank you for putting me upright again when I wasn’t and for making the time we had so brilliant. Last but not least I would like to thank my brother Daniel and all my family as well as some others for their backing throughout my studies and life and for making the person I am out of me. V Erklärung des Kandidaten Ich versichere, dass diese Masterarbeit selbständig von mir verfasst wurde - abgesehen von der Mitwirkung der genannten Betreuer - und dass nur die angegebenen Quellen und Hilfsmittel verwendet wurden. ——————— —————————————— Datum Johannes Oehler VI "Es genügt nicht, die Welt zu verändern. Das tun wir ohnehin. Und weitgehend geschieht das sogar ohne unser Zutun. Wir haben diese Veränderung auch zu interpretieren. Und zwar, um diese zu verändern. Damit sich die Welt nicht weiter ohne uns verändere. Und nicht schliesslich in eine Welt ohne uns." G. Anders VII Contents 1 Introduction1 1.1 Motivation...............................1 1.2 Use of kites for energy conversion..................2 1.3 Kite development...........................2 2 Problem description and state of the art3 2.1 Description of the pumping kite concept..............3 2.2 Coordinate systems for tethered flight................4 2.3 Flight path, forces and velocities for tethered flight........6 2.4 Attempts to measure air speed.................... 10 3 Goal of the thesis and methodology 13 3.1 Accurate definition.......................... 13 3.2 Research methodology........................ 13 4 Sensor choice and test setup 15 4.1 Requirements............................. 15 4.2 Concepts for flow measurement................... 16 4.2.1 Mechanical and Sonic Anemometers............. 17 4.2.2 Multi-hole probe....................... 18 4.2.3 Air data boom with mechanical wind vanes........ 19 4.3 Sensor position............................ 21 4.3.1 Sensors mounted on the kite................. 21 4.3.2 Sensors mounted in the bridles................ 21 4.3.3 Perturbation of the air flow................. 22 5 Construction and calibration 25 5.1 Mounting and structure of the measurement assembly....... 25 5.2 Pitot tube............................... 27 5.3 Angle measurement.......................... 29 5.4 Data transfer and sensor platform.................. 31 5.5 Calibration and expected measurement error............ 33 5.5.1 Airspeed magnitude..................... 33 5.5.2 Wind vanes.......................... 40 IX Contents 6 Flight data 43 6.1 Airspeed data validation....................... 44 6.2 Calculation of the aerodynamic coefficient............. 50 6.3 Inflow angles............................. 54 6.4 Prediction of cR ............................ 61 7 Conclusion and outlook 65 7.1 Conclusion............................... 65 7.2 Outlook and recommendations.................... 66 Bibliography 79 X Nomenclature Latin Symbols altkite [m] Altitude of the kite from GPS signal altGS [m] Altitude of the ground station altp [m] Altitude of the kite from measured pressure altref [m] Reference altitude of the kite altT [m] Altitude of the kite from measured temperature alt0 [m] Altitude of the kite before launch cD,plate [−] Drag coefficient of the wind vane cR [−] Resultant aerodynamic coefficient airborne system cR∗ [−] Calculated aerodynamic coefficient airborne system cR,hydra,22kt [−] Resultant aerodynamic coefficient of the Hydra kite at 22kt wind speed cR,hydra,29kt [−] Resultant aerodynamic coefficient of the Hydra kite at 29kt wind speed downGP S [m] Position of the kite in NED coordinate system eastGP S [m] Position of the kite in NED coordinate system Fa [N] Resultant aerodynamic force of the airborne system Fa,max [N] Maximum aerodynamic force of the airborne system Ft [N] Tether force at the upper end of the main tether Ftg [N] Tether force at the ground station g [m/s2] Acceleration of gravity XI Contents K [m] Kite position vector in NED coordinates kdown [m] Kite position below the ground station keast [m] Kite position east of the ground station knorth [m] Kite position north of the ground station k∆p [−] Scaling factor for measured differential pressure k∆p,not−aligned [−] Alternative scaling factor for differential pressure L [m] Tether length L [−] Lift-to-drag ratio of the kite D latGS [°] Latitude of the ground station latkite [°] Latitude of the kite longGS [°] Longitude of the ground station longkite [°] Longitude of the kite Mstiction