Aero-elastic analysis of a large airborne wind turbine

Aero-elastic analysis of a large airborne wind turbine: Prediction of divergence, control reversal and effectiveness, and flutter of a tethered wing

Wijnja, J. (TU Delft Aerospace Engineering)

van Bussel, G.J.W. (mentor)
Schmehl, R. (mentor)
De Breuker, R. (mentor)

Delft University of Technology

Aerospace Engineering

2013-12-18

The objective of this research is the simulation of aero-elastic behaviour of Makani’s large airborne wind turbine. This tethered wing operates in crosswind motion, and is equipped with on-board wind turbines. The tether-bridle system attaches the energy generating system to the ground station. It is likely that the structure of this, 28m span, carbon fibre, high aspect ratio wing, will deform considerable under aerodynamic loads. In the worst case scenario, static and/or dynamic aero-elastic e_ects cause destructive failure. The aero-elastic simulation program ASWING is used for the analysis. This program uses a fully nonlinear Bernoulli-Euler beam representation for structural modelling in combination with a lifting linerepresentation for aerodynamic modelling. Linearised unsteady analyses are derived for the Eigenmode analysis. Since the tether-bridle system cannot be modelled in the current program version, an additional, ASWING compatible, module is written. The tether is modelled as a spring with user defined characteristics for the spring sti_ness, mass and aerodynamic drag area. The bridle lines are assumed massless and perfectly rigid. The tether and bridle forces are dependent on the wing flexibility, and wing position and orientation. The tether-bridle module is verified against analytical expressions and by using MATLAB. A wind tunnel test validates the dynamic aero-elastic responses. For the Makani wing, divergence, aileron e_ectiveness and reversal, and flutter behaviour is analysed. Divergence and aileron reversal are no critical modes. However, aileron e_ectiveness is critical. The requirements state a minimum aileron e_ectiveness of 75% at 95m=s flight speed. The program calculated this minimum aileron e_ectiveness at 92m=s flight speed. These problems can be resolved by a 10% increase in the wing’s torsional sti_ness or a 10% increase of lift force increment with aileron deflection. The Eigenmode results showed a critical flutter mode at flight speeds higher than 90m=s, whereas the design flutter speed is equal to 120m=s. This susceptibility to flutter can be resolved by (1) a 50% increase in torsional sti_ness, (2) a 50% increase in in-plane-bending sti_ness or (3) a 10cm upstream shift in center of gravity. A 50cm upstream shift of bridle-wing attachment location increases the flutter speed to 110m=s. It was found that the e_ects, of tether aerodynamic drag and tether weight, are negligible for the aero-elastic behaviour. Also, in the analysis for the Makani wing, the tether spring constant does not contribute to the static and dynamic aero-elastic e_ects. The position of the bridle-wing attachments influences the twist angles and tip deflections of the wing. These results are useful in case maximum twist angles and/or wing tip deflections are critical. For the dynamic aero-elastic behaviour the wing-bridle attachment positions can be adjusted to decrease the susceptibility to flutter.

http://resolver.tudelft.nl/uuid:0d778339-a97e-423a-b3a8-64ab7a64417f

2020-07-27

Student theses

master thesis

© 2013 J. Wijnja