*2.1. Set-Up*

This research was conducted in the facilities of the Energy Department of the University of Oviedo (Viesques University Campus at Gijón, Spain). In particular, a subsonic open-loop wind tunnel of 13.75 m in length and powered by a 30 kW axial fan with a diameter of 1.2 m was used for this research. It has a nozzle with a 1:12 area ratio, which provides a squared test section of 0.68 × 0.68 m2 and allows wind velocities up to 35.5 m/s. A characteristic turbulence intensity of 0.7% for an averaged integral length scale of 0.1 m was obtained at the nozzle discharge. Although its typical configuration is arranged in an atmospheric, fully opened test section, an additional enlargement of the nozzle sidewalls was made to guarantee planar flow over the tested prototypes (discussed later in detail). A sketch of the wind tunnel is shown in Figure 1a. The test wind velocity is measured from the pressure difference in the nozzle, with a 1 in the H2O ± 0.25% differential pressure sensor. Note that, given the contraction ratio, velocities in the settling chamber were considered negligible.

**Figure 1.** (**a**) Sketch of the wind tunnel (Courtesy of Angie L. Ramírez Celis). (**b**) Aerodynamic balance mounted on the mechanical orientation system. (**c**) DU06-W-200 airfoil prototype in the test section.

The custom-built aerodynamic balance under study is a strain gauge force balance with 3 components, which provides lift and drag forces and pitching moment. This balance was originally conceived to measure unsteady forces and torques on small-scale single-axis solar trackers, and successfully employed for recent aeroelastic investigations in our research group [15,16]. Precisely, the balance was designed to provide a larger range in one direction with respect to the other, which is also a very useful feature for testing airfoils where lift forces are much greater than drag forces. The balance, which rotates with the prototype to be tested, is composed of a floating axis supported by two symmetrically placed load cells, which are attached to a frame supported by the third load cell. The assembly is designed so that forces and moments outside of the measuring plane are minimized. Two different balances were built for measuring ranges within 0.75 kg and 5 kg, respectively (ranges of the single load cell direction), although the design can be easily scaled to any other quantity. Each load cell has two strain gages that are connected to the same Wheatstone bridge circuit to provide an amplified output. The voltage from the bridges is measured with a data acquisition card, which allows a measuring frequency up to 20 kHz. Note that this is especially relevant when unsteady phenomena are to be studied with this kind of device. The rotation of the balance was performed with a mechanical orientation system which granted the variation in the pitching angle using a worm gear pair (Figure 1b). The system was manufactured by fused deposition modelling (FDM) and allowed a minimum angular step of 0.5◦. Finally, measurement data analysis and calculations were performed with custom MATLAB codes in a computer.

Three different prototypes were tested in the aerodynamic balance for this work: two flat plates (of different dimensions) and the DU06-W-200 airfoil (Figure 1c). One of the flat plates was designed to perform as a theoretical 3D plate, featuring an aspect ratio (*L*/*c*, where *L* is the span and *c* is the chord or width) of 3.2, which was proved to be sufficient for the purpose. On the other hand, the 2D plate had a span as wide as the wind tunnel test section (a clearance of tenths of a millimeter was left so that there is no contact with the walls) and the same width of the 3D flat plate. Note that this allowed testing both at the same wind velocity with an equal Reynolds number. Hence, the 2D flat plate had an aspect ratio of 7.2. The airfoil prototype also had a span as wide as the wind tunnel, but the chord was chosen so that, at the objective Reynolds, the obtained forces were coherent with the range of the aerodynamic balance used. Thus, the resultant aspect ratio was 3.8, which, given the results obtained, proved to be sufficient to obtain 2D airfoil coefficients over this wall-to-wall prototype. The dimensions of the tested prototypes are included in Table 1.


**Table 1.** Tested prototypes and dimensions.

The three models were made of PLA and manufactured with FDM, requiring subsequent sanding and polishing to achieve an adequate final roughness. The prototypes have in their core a steel rod to increase their stiffness. This rod has a fixed support connection to the balance and cylindrical joint in the wall of the other end, avoiding movements in the measurement plane and prototype bending. The balance calibration procedure already accounts for the effect of the second support.
