Comment on wes-2021-155

Abstract

Multi-element airfoils can be used to create high lift, and have previously been investigated for various application such as in commercial airplanes during take-off and landing and in the rear end of Formula One cars. Due to the high lift, they are also expected to have a high potential for application to airborne wind energy (AWE), as confirmed by recent studies. The goal of this work is to investigate and optimise a multi-element airfoil for application to AWE, in order to further the understanding and improve the knowledge base of this high-potential research area. This is done by applying the Computational Fluid Dynamics code OpenFOAM to a multi-element airfoil from the literature (the "baseline"), set up for steady-state 2D simulations with a finite volume mesh generated with snappyHex mesh. Following a grid dependency study and a feasibility study using simulation data from the literature, the angle of attack with the best performance in terms of E2CL (E = glide ratio, CL = lift coefficient) is identified. The maximum E2CL is found to be approximately seven times larger than that of a typical single-element AWE airfoil, at an angle of attack of 17°. Having found the ideal angle of attack, a geometric optimisation is carried out by altering the relative sizes and angles of the separate airfoil elements, first successively and then using promising combinations. The limits of these changes are set by the structural and manufacturing limitations given by the designers. The results show that E2CL can be increased by up to 46.6 % compared to the baseline design. Despite the increased structural and manufacturing challenges, multi-element airfoils are therefore promising for AWE system applications, although further studies on 3D effects and drone-tether interactions, as well as wind tunnel measurements for an improved confidence in the results, are needed.