# Experimental Assessment of RANS Models for Wind Load Estimation over Solar-Panel Arrays
## 2 Experimental Set-Up  
The experimental study has been carried out in the closed-loop wind tunnel of the Aerospace Engineering Group at the _Universidad Carlos III de Madrid_. The test section of this wind tunnel has a square cross area of 0.4 m \(\times\) 0.4 m with a length of 1.5 m. The ceiling and the lateral walls of the tunnel are made of transparent methacrylate in order to enable optical access. Free-stream turbulence intensity is estimated to be lower than 1% by means of Particle Image Velocimetry (PIV) measurements in the empty tunnel. A flat plate with length of 0.55 m equipped with a sharp leading edge is installed in the test section. The solar-panel array is mounted on top of this plate instead of the tunnel floor in order to reproduce ground effect with a relatively thin boundary layer. The solar-panel array is composed of four panels, each with chord length \(c\) of 0.05 m, thickness \(t\) of 0.005 m and span \(l\) of 0.4 m. The panels are inclined of 45\({}^{\circ}\) with respect to the ground. The pitch between the panels is 2 \(c\). The first panel is located at a distance of 2 \(c\) from the leading edge of the flat plate. The leading edge of the 4 th panel is 3 \(c\) away from the trailing edge of the flat plate in order to reduce the effect of the flat plate end. Figure 1 shows a sketch of the experimental setup. The wind tunnel velocity, \(U_{\infty}\), at the test-section inlet is kept constant and equal to 12.8 m/s for all the experiments in order to achieve a Reynolds number based on the panel chord of about 4.0 \(\times\) 10\({}^{4}\), thus being in the turbulent regime. Figure 1: Sketch of the experimental setup. The streamwise and vertical direction are indicated as \(x\) and \(y\), respectively. Each panel is equipped with 6 pressure taps, 3 on the front side and 3 on the back side, approximately located in the mid-span section. On each side the pressure taps are positioned in the chord-wise direction at 0.25 \(c\), 0.50 \(c\) and 0.75 \(c\). Front and back taps are displaced of 0.20 \(c\) along the span-wise direction with respect to the mid section, respectively on the left and on the right (the total distance between front and back taps is 0.40 \(c\)). Each tap has a 1.2 mm diameter. Figure 2 shows a schematic of the pressure taps location on the solar panels. Pressure measurements are performed with a tube system that connects the pressure taps located on the model surface to a pressure transducer. The pressure transducer used in the experiment has an uncertainty of 0.011 in terms of pressure coefficient \(C_{p}=\frac{p-p_{\infty}}{0.5p_{\infty}\mu\Omega_{\text{s}}^{2}}\), being \(\rho_{\infty}\) and \(p_{\infty}\) the ambient air density and pressure respectively. An ensemble of 10,000 measurements is acquired for each pressure tap, thus reducing the uncertainty on the average pressures to 0.00011 in terms of the pressure coefficient. Velocity field measurements are acquired by means of Particle Image Velocimetry (PIV). The flow is seeded with Di-Ethyl-Hexyl-Sebacate droplets with diameter of approximately 1 \(\upmu\)m. The light source is a Big Sky Laser CFR400 ND:Yag (230 mJ/pulse, pulse duration 3 ns). The acquisition is performed with a Andor Zyla 5.5 Camera (2650 \(\times\) 2160 pixel array, 16.4 mm \(\times\) 14 mm sensor size) with a spatial resolution of about 8.5 pixels/mm and a delay between pulses equal to 90 \(\upmu\)s. The sampling frequency of the PIV measurements is 10 Hz. An ensemble of 2000 image pairs is acquired for each experiment. Image quality is improved with a background removal technique based on proper orthogonal decomposition (POD) [27]. This technique allows to remove laser reflections and illumination background, resulting in unbiased measurements near the wall. The SPIV software, developed at University of Naples Federico II, is used to cross-correlate particle images and to calculate the velocity fields [28; 29].