## 4 Wind Tunnel Testing
### Sensitivity of Numerical Outputs with the Spatial and Temporal Discretizations  
One of the key activities, which aims to provide confidence in CFD-based results, is to assess the sensitivity of numerical outputs depending on the levels of spatial and time discretization. To address this task, the g01 sample at a \(0^{\circ}\) angle of attack (AoA) was selected and three different levels of mesh refinement (coarse, medium and fine) were considered, comparing the values obtained for the force coefficients, the standard deviations of the force coefficients and the Strouhal number. For the three meshes, the characteristics of the boundary layer mesh region around the boxes are the same: a first layer mesh height, \(y_{1}/C\), of \(2.42\times 10^{-4}\), an inflation ratio, \(r_{\text{bl}}\), of 1.104, an element size along the wall, \(x_{1}/C\), of \(9.69\times 10^{-4}\) and an overall number of layers of 15. The results are reported in Table 3, which shows a minor impact of the mesh refinement, with only small changes in the absolute value of the lift coefficient and the Strouhal number, with consistent results for the medium and fine meshes. Based on this spatial discretization sensitivity study, the fine mesh characteristics are retained for the CFD simulations that are reported next. Furthermore, the impact of the time-step resolution is further analysed by considering different levels of maximum Courant number in the time-marching simulations, also \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Mesh** & **\# Cells** & \(\mathbf{C_{D}}\) & \(\mathbf{C_{L}}\) & \(\mathbf{C_{M}}\) & \(\widetilde{\mathbf{C_{D}}}\) & \(\widetilde{\mathbf{C_{L}}}\) & \(\widetilde{\mathbf{C_{M}}}\) & \(\mathbf{St}\) \\ \hline Coarse & 166,680 & 0.060 & \(-\)0.071 & 0.015 & 0.011 & 0.089 & 0.022 & 0.079 \\ Medium & 200,460 & 0.062 & \(-\)0.014 & 0.021 & 0.012 & 0.106 & 0.026 & 0.137 \\ Fine & 319,467 & 0.061 & \(-\)0.010 & 0.021 & 0.012 & 0.106 & 0.025 & 0.134 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Force coefficients and standard deviation of the force coefficients for the g01 sample at \(0^{\circ}\) AoA considering different levels of mesh resolution (maximum Courant number: 1). for the g01 sample at 0\({}^{\circ}\) AoA and with fine mesh. The values obtained for the time-averaged, fluctuating force coefficients and Strouhal number are reported in Table 4 for maximum Courant numbers of 4, 2, 1 and 0.5, finding minor differences, remarkably, between maximum Courant numbers of 1 and 0.5; therefore, a maximum Courant number of 1 is retained for the simulations reported in the next sections. In Table 5, for case g01 with fine mesh and a maximum Courant number of 1, the mean and maximum values obtained for the \(y^{+}\) distribution around the boxes are reported (\(y^{+}=(y_{1}u_{*})/v\); \(y_{1}\) is the height of the first prismatic grid layer and \(u_{*}\) is the friction velocity). For the two deck boxes, the maximum values in Table 5 are below the generally accepted limit of eight, meeting the requirements for the low Reynolds wall modelling approach adopted herein. In Figure 6, several images of the fine finite volume mesh for case g01 are depicted, where the refined wake region and the boundary layer mesh around the deck can be perused. In Table 6, the fundamental characteristics of the finite volume meshes for the 15 considered geometries are reported. It should be noted that the difference in the overall number of cells is related to the changes in the geometry of the samples, as the size of the flow domain depends on the deck total width \(B\) and depth \(D\).