Experimental rig for measuring relative flow mounted in the bridle line system (24 March 2017)
Johannes Oehler Researcher Delft University of Technology Faculty of Aerospace Engineering
Wind Energy Research Group Kluyverweg 1 2629 HS Delft The Netherlands
j.d.oehler@tudelft.nl kitepower.tudelft.nl
Experimental Characterization of a Force-Controlled
Flexible Wing Traction Kite
Johannes Oehler1,2, Roland Schmehl1 1Delft University of Technology
2University of Stuttgart
In order to use flexible wing kites effectively for power generation their size must be increased and aerody-namic performance should be systematically improved. Validated models are essential for an efficient design process. The largest uncertainty in current modeling ap-proaches is the apparent wind speed vaexperienced by the kite. In this work we develop an in-flight measure-ment setup for the apparent wind speed vector. An air data boom with a Pitot tube and a pair of wind vanes is used to measure the relative flow below the wing, at suf-ficient distance not to be influenced significantly by the wing itself.
In-flight measurement of apparent wind speed and inflow angles (24 March 2017)
The experimental data supports the current quasi-steady model of a pumping kite power system [1]. In this work
we propose a mechanistic model for the resulting aero-dynamic coefficient cR. A sole dependency of this coef-ficient on the angle of attack, as it is common for rigid airfoils, must be rejected. Instead, we find that the coeffi-cient strongly depends on the wing loading and to a lesser extent also on the power ratio, i.e. the non-dimensional pitch parameter of the wing and the inflow angle, as as-sumed in [2].
We further observe that the force-controlled character-istic of the system majorly affects the flight dynamics. Important findings are that by changing the tether reel-ing velocity to obtain constant traction force the ground station balances wind gusts which thus hardly affect the relative flow vector but the power output. Sudden controller-induced changes in reeling velocity cause in-flow angle variations similar to fluttering observed on rigid airfoils. Further can cRand vanot vary indepen-dently for any force-controlled phase. The presented re-lation of different parameters affecting cRcan be used for flight path optimization as well as kite and control sys-tems design.
References:
[1] van der Vlugt, R., Bley, A.,Noom, M., Schmehl, R.: Quasi-Steady Model of a Pumping Kite Power System. Submitted to Renewable Energy, 2017. arXiv:1705.04133 [cs.SY]
[2] Fechner, U.: A Methodology for the Design of Kite-Power Control Systems. PhD thesis, TU Delft, doi: 10.4233/uuid:85efaf4c-9dce-4111-bc91-7171b9da4b77, 2016
