*2.1. Numerical Tools*

The aerodynamic design process described in the previous sections was conducted using open-source, industry-standard computational tools. It is worth pointing out again that these methods are indeed not novel and well-known to the scientific community. Additionally, they have been extensively validated on a many study cases, both of small and utility-scale wind turbines, assessing their suitability for the scope. However, while approaching the industrial design of a new SWT, the authors found that detailed guidelines on how to consciously, organically, and in an integrated fashion use these methods were missing. On these bases, this study would like to represent a support to the industrial and scientific community in an overall attempt of improving the future design of this class of machines. A brief overview of the tools used in the study is provided in this paragraph.

Lift and drag airfoil characteristics were obtained with XFoil [17]. The tool is based on an inviscid panel method, and it has been used in the design of a vast amount of airfoils for all sorts of engineering uses, including the families used herein [18]. Even though a recent study pointed out that this method may have some issues in case of low Reynolds numbers and high angles of attack [19], its use in horizontal-axis rotors from the present power output and above can be considered as a solid choice, especially for the first design of blades, when multiple design variations need to be compared quickly.

For the present study, the characteristics were calculated using 200 panels per airfoil and setting a trailing edge gap of 2%. A Reynolds number of 1 × <sup>10</sup><sup>6</sup> was used. The boundary layer transition was calculated with the Ncrit-based shear layer transition method [20], and a value of Ncrit = 9 was used. The Reynolds number matched the final operating *R*e number fairly well, ranging between 0.8 × <sup>10</sup><sup>6</sup> and 1.3 × 106 depending on operating conditions, and was therefore considered acceptable; however, if this is not the case, a few design iterations might be required to ensure that lift and drag polars were suitable for the test case. The full-blade aerodynamic design was conducted in OpenFAST [21]. This open-source modular tool was developed by the National Renewable Energy Laboratory (NREL) and can model the full response of wind turbines, accounting for a wide variety of effects such as aerodynamics, elastodynamics, control-dynamics, and, for offshore installations, hydrodynamics. The code has been widely adopted, validated, and used in the design of multiple, industry-standard, reference wind turbines [22,23]. In the present study, only the aerodynamic and control-dynamics perspectives were explored in detail. The aerodynamic module AeroDyn [24] allows for the simulation of dynamic inflow conditions in the presence of atmospheric turbulence. Blade element momentum (BEM)-based aerodynamics also include corrections for wind shear, yaw misalignment, tip and hub losses, and tower-shadow effects. Dynamic stall is treated with the Beddoes–Leishman dynamic stall model included in AeroDyn. This correction is especially relevant for a stall-operated turbine operating in turbulent conditions. The turbine controller was integrated through the ServoDyn module. For the pitch-controlled turbine, an external routine was used, as detailed in the next section.
