Airborne Wind Energy (AWE) systems are tethered aircraft for wind energy harvesting that, since not constrained by a tower like conventional wind turbines, can operate at high altitudes with access to a better wind resource. This work presents a comprehensive review of the current knowledge and state of the art of the aerodynamics of AWE systems. Aerodynamics, which affects power generation, flight physics, control, structure, and safety, among others, is the most transversal area for AWE technology. It is a rich field of experimental and theoretical research due to its significant impact on performance. The review starts organizing actual AWE prototypes, some of them reaching the 100 kW range, according to some selected dimensionless parameters strongly related with their aerodynamics including the Reynolds and Mach numbers, the aspect ratio, the maximum lift-to-weight ratio and aerodynamic efficiency, the reduced frequency, and the sweep and dihedral angles of the wing. AWE machines with different electrical generation solutions (on the ground and onboard), links to the ground (tethered and rotary machines), aircraft (non-rigid or soft, hybrid and fixed wing), and control (aerodynamic surfaces, hanging control pod, ground-based, etc.) are considered and the implication of each architecture on the aerodynamics is discussed. After such a fundamental introduction, the work reviews the current state of AWE numerical and experimental aerodynamics, detailing the modeling methods and key findings. The numerical models are categorized into fast, low- to mid-fidelity methods based on potential flow, and high-fidelity computational fluid dynamics methods like Reynolds-averaged Navier–Stokes and Large-Eddy Simulations. Most numerical studies aim to understand local phenomena by examining the flow and pressure fields over wings, and/or to calculate the aerodynamic force and moment coefficients of 2D airfoils or entire wings. On the experimental side, the significant progress characterizing different types of aircraft in wind tunnels, water channels and in-flight during typical AWE trajectories is summarized. Special attention is paid to the experimental setups and on-board instruments that have been used for the in-situ measurements of aerodynamic variables, as well as the estimation theory and applications of the experimental data to construct aerodynamic models. Furthermore, this paper analyzes the effective application of current numerical and experimental aerodynamic knowledge and models in related areas such as dynamics and control, and fluid–structure interaction. The paper concludes with a critical assessment of the current state of knowledge, highlighting the main open questions, challenges, and opportunities in the field of AWE aerodynamics.
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