## 2 Materials and Methods
### Experiments in the Wind Tunnel  
Tests in a wind tunnel were carried out at the Wind Engineering Laboratory of the Cracow University of Technology [28]. The tunnel operated in a closed circuit. A fan that sucked the air was located behind the measuring section. The air returned to the initial part of the measuring space through the return duct. The main dimensions of the wind tunnel working section are: 2.20 m wide, 1.40 m high, and 10.00 m long. The first part of the 6 m long working section is used to form the wind speed profile and generate turbulence. Turbulent air flow is created using turbulence generators, such as barriers and spiers with appropriate geometry and shapes, as shown in Figure 8.  
Figure 7: Front views and cross-section of the full-scale blades models with dimensions in mm: (a) the SC blade; (b) the DC blade. The wind profile was determined starting from a height of 300 mm above the upper surface of the measuring table. At this height, the tested blade was attached to a strain gauge aerodynamic balance. The part connecting the blade with the aerodynamic balance was covered by a fairing. The profile of the wind and the intensity of turbulence are presented in Figure 9a,b. According to standard [29], the profiles of the wind were described by the power law formula:  
\[\frac{V(z)}{V_{ref}}=\left(\frac{z}{z_{ref}}\right)^{\alpha} \tag{2}\]  
where \(V_{ref}\)--the reference velocity at a reference height \(z_{ref}\); \(\alpha\)--the power low exponent i.e., the coefficient of roughness, \(z\)--the height, and \(V\)--the average wind speed. Figure 8: Spires and a barrier to generate turbulent flow in the wind tunnel measuring section. Figure 9: (a) Wind speed profile; (b) turbulence intensity. Turbulence intensities \(I_{V}\)(z) were calculated according to the formula:  
\[I_{V}(z)=\frac{\sigma(z)}{V(z)} \tag{3}\]  
where \(\sigma\)(z)--standard deviations. The speed of the wind was calculated based on the pressure measurements, which were carried out using a set of differential pressure sensors with pitot tubes. The pressure sensors were located in a vertical plane, as shown in Figure 10a,b, on the left side of the blade under test. A measurement time of 10 s was assumed. During that time 6000 samples were collected. The average wind speed was calculated based on the average wind profile values for all test cases. The profile of the wind was determined in the height range from 300 mm to 900 mm with a step of 100 mm. The main parameters of the measurements were summarized in Table 2.  
A five-component strain gauge balance was installed under the floor of the working section of the wind tunnel. These five components represent aerodynamic forces \(P_{x}\), \(P_{y}\), aerodynamic torque \(M_{xy}\), and aerodynamic moments \(M_{x}\), \(M_{y}\). A diagram of the measurement system used in the study is shown in Figure 11. The model of the examined blade was installed on the strain gauge balance. The aerodynamic balance was connected to an amplifier, which included data acquisition systems for collecting records of measured aerodynamic forces and moments. The aerodynamic balance was used to determine three components \(P_{x}\), \(P_{y}\), and \(M_{xy}\) referred to the \(x\),\(y\) coordinates presented in Figure 11. The wind angle was changed using a turntable actuated by an electric motor. The turntable is a coaxial part of the aerodynamic balance. A stepper motor, controlled by the stepper motor control system, caused the model to rotate, which simulated changes in the angle of the wind attack. Sensors for measuring pressure, arranged in the vertical plane, were connected with the pressure scanner. Pressure sensors were used to determine the wind