Computational Fluid Dynamic Analysis and Optimisation of a Multi-Element Airfoil for an Airborne Wind Energy System
Creators: name: De Fezza, Gianluca
name: Barber, Sarah
Publisher: name: Zenodo
Year: 2021
Subjects: Airborne Wind Energy
Airfoils
CFD
Fluid-Structure Interaction
Optimisation

description: Airborne wind energy systems have the potential to play an important role in the today’s energy transition (Cherubini et al. 2015). The Swiss start-up Skypull SA is an ambitious developer of such airborne concepts (Skypull 2018). Its drone has a double wing design to maximise lift with reduced wingspan. Each wing is made of a multi-element airfoil as shown in Fig. 1(a). In this work, the aerodynamic performance of this multi-element airfoil is analysed using steady-state 2D simulations using the Computational Fluid Dynamics (CFD) code OpenFOAM (OpenFOAM 2020). Then the geometry is optimised to maximise the Loyd’s Power Ratio (LPR), C<sub>L</sub><sup>3</sup> /C<sub>D</sub><sup>2</sup> (C<sub>L</sub> = lift coefficient, C<sub>D</sub> = drag coefficient) (Loyd 1980). Following a grid dependency study, a hexahedral grid consisting of 50’000 cells is chosen, which gives a y-plus value of 30 with the nutUSpalding wall function. A range of turbulence models are tested for a Reynolds number of 1.5e6 and angles of attack (AoA) ranging from 0° to 30°. The Spalart-Allmaras model (Spalart-Allmaras Turbulence Model, 2019) is chosen because there are no significant differences in the results between different turbulence models and the one-equation is comparatively more efficient. The resulting LPR vs. AoA is shown in Fig. 2(a). The coefficients are calculated using the chord length c marked on Fig. 1(a). The optimal AoA to maximise LPR is 17°. Having found the ideal AoA, a geometric optimisation is carried out. For this, the following geometric parameters are altered as marked on Fig. 1(a): front and rear flap chord length c<sub>flap1</sub> and c<sub>flap2</sub> (from 60%-140% of original size), strut chord length c<sub>strut</sub> (20%-140%), angle of flaps α<sub>flap1</sub> and α<sub>flap2</sub> (+/-10°), vertical distance x between main airfoil and strut (65%-265%). Changes in chord length are always accompanied by equal changes in airfoil thickness in order to preserve the airfoil shape. At the same time, the distance between the leading and trailing edges of neighbouring airfoils are preserved. The geometric optimisation limits are provided by Skypull based on structural and manufacturing constraints. The results show that the flap scaling and angles do not have a great impact on the LPR: changes of ±40% in flaps chord length and ±10° in flaps angle only affect LPR by ±5% and ±13.7%, respectively (Fig.2(b)). However, these two plots indicate that the behaviour of LPR is not entirely linear, and local maxima do exist. Increasing x increases LPR by 7.5%. The largest improvement is found by reducing c<sub>strut</sub>: an 80% reduction brings an improvement in LPR of 39.1%. Altering the chord length of the flaps (-40%) and the strut (-60%) at the same time results in the largest improvement in LPR of 43.4%. This represents the new, optimised geometry shown in Fig. 1(b). The relative effects of the geometry changes on the LPR are summarised in Fig. 2(c). Further work includes creating an automated optimisation strategy allowing the altering of all parameters at the same time, in order to ensure that no local maxima have been missed, as well as integrating the results into Skypull's mechanical system model.
description: {"references": ["A. Cherubini, A. Papini, R. Vertechy and M. Fontana, \"Airborne wind energy systems: A review of the technologies,\" Renewable and Sustainable Energy Reviews, vol. 51, pp. 1461- 1476, 2015.", "\"Skypull's technology,\" 2018. [Online]. Available: https://www.skypull.technology/tech.", "\"OpenFOAM,\" 2020. [Online]. Available: https://openfoam.org/.", "M. L. Loyd, \"Crosswind kite power (for large-scale wind power production),\" Journal of energy, pp. 106-111, 1980.", "\"Spalart\u2013allmaras turbulence model,\" 2019. [Online]. Available: https://en.wikipedia.org/wiki/Spalart\u2013Allmaras_turbulence_model."]}