Abstract

This research work focuses on the optimization of airfoils and wings for the use in airborne wind energy applications, especially crosswind kite power systems. The thesis is divided into three major parts. Each part includes an optimization framework which is composed of four blocks: an airfoil or wing geometry builder, an aerodynamic solver, an optimizer, and a post-processor. These blocks interact with each other to solve the problem of maximizing/minimizing an aerodynamic objective function while at the same time maximizing/minimizing a structural objective function such as airfoil thickness or wing aspect ratio. It is thus inevitable to have a multi-objective framework that includes both objective functions.

The first part addresses airfoil optimization by minimizing the inverse aerodynamic efficiency and the negative maximum thickness ratio, which are conflicting objectives. Results with and without induced drag (from a finite aspect ratio wing) and tether drag effects show that optimal airfoils accounting for induced drag have a cusped trailing edge, while those considering tether drag have a flap-like trailing edge, both improving aerodynamic performance.

The second part focuses on wing planform optimization, aiming to minimize the inverse aerodynamic efficiency and aspect ratio simultaneously. The optimization framework is validated by minimizing the drag-to-lift ratio, resulting in wings with elliptic planforms. The planforms optimized for aerodynamic efficiency do not differ significantly from elliptic shapes.


The third part of the thesis deals with the optimization of box-wing airfoils. Box-wings consist of two wings connected by vertical fins at their tips, forming a box-like shape when viewed from the front. The analysis is performed for an infinite aspect ratio, i.e., two-dimensional configuration. The inverse of the total aerodynamic efficiency and the negative of the combined maximum thickness ratio of the two airfoils are minimized simultaneously. The numerical results indicate that the optimal configurations feature a thick lower (or forward) airfoil and a thin upper (or aft) airfoil. This suggests that the lower airfoil primarily serves a structural role, while the upper airfoil plays a crucial aerodynamic role.