# A Data-Driven Model for Aerodynamic Loads on Road Vehicles Exposed to Gusty Bora-Like Winds
## 3 Experimental Results  
Example records of the experimental velocity time series, simulating periodic features of the bora wind, are presented in Figure 2.  
Steady aerodynamic loads on the van model increase as the flow velocity increases, Table 1; \(\overline{u}\) is average freestream flow velocity. \(C_{FS}\), \(C_{FL}\) and \(C_{MR}\) coefficients are calculated using the same velocity \(\overline{u}\) averaged during low wind periods of the time record (\(\overline{u}=3.8\) m/s). \(C_{FS}\) is considerably larger for the B model (with wheels), as the van in that configuration is higher and less protected by the bridge barrier. \(C_{FL}\) is larger for the A model (without wheels) due to a stronger flow separation and the respective pressure suction on the roof of the van, when there is no underbody flow. \(C_{MR}\) is calculated using the pressure on the sides, and the roof of the van is slightly larger for the B model (with wheels), as in that configuration the contribution to the side force due to higher van model is stronger than the simultaneous reduction in the lift force. Figure 3a shows the power spectral density of longitudinal velocity fluctuations \(f\cdot S_{u}(f)/\sigma_{u}^{2}\) presented against frequency, for the time records presented in Figure 2; \(f\) is the frequency, \(S_{u}(f)\) is the power spectral density and \(\sigma_{u}^{2}\) is the variance of the longitudinal velocity. In Figure 3a, a peak at \(f_{1}\)\(\sim\)0.16 Hz is observed. It is a consequence of intermittent switching between lower and higher flow velocity for 3 s each, which is in accordance with full-scale measurements, a typical feature of bora-like winds. The spectral peak at 0.16 Hz scaled up using the 1:10 time scale would represent wind gusting events lasting 60 s at full-scale. The additional harmonic components are present at \(n\)\(f_{1}\), where \(n=3\), 5, etc., i.e., the first two other harmonics are recorded at 0.48 Hz and 0.8 Hz, a common phenomenon of ideal signals. The power spectra of longitudinal velocity fluctuations are basically the same in the two studied configurations. In the side force fluctuations, there is a peak at 10 Hz for model A, a trend likely a consequence of vortices shed from the bridge deck, as seen in Figure 3b. For model B, the unsteady aerodynamic side force is predominantly affected by wind gusting because a strong peak at 0.16 Hz, and a less exhibited peak at 10 Hz is observed, as seen in Figure 3b. The flow beneath model B weakens the low-frequency peak in the respective side force spectrum. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**(a) Low Velocity Period (Weak Winds)**} & \multicolumn{4}{c}{**(b) High Velocity Period (Wind Gusts)**} \\ \cline{2-10}  & \(\overline{u}\), m/s & \(\sqrt{u^{2}}\), m/s & \(C_{FS}\) & \(C_{FL}\) & \(C_{MR}\) & \(\overline{u}\), m/s & \(\sqrt{u^{2}}\), m/s & \(C_{FS}\) & \(C_{FL}\) & \(C_{MR}\) \\ \hline Model A & 3.8 & 0.26 & 0.16 & 1.21 & 0.37 & 5.0 & 0.38 & 0.38 & 2.08 & 0.68 \\ \hline Model B & 3.8 & 0.26 & 1.19 & 0.72 & 0.46 & 5.0 & 0.37 & 2.14 & 1.39 & 0.87 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Steady aerodynamic side force, lift force and overturning moment coefficients. Figure 2: Example velocity time series in accordance with periodic features of the bora wind: (**a**) model A (without wheels) and (**b**) model B (with wheels). Lift force fluctuations are characterized by a similar trend for both van models, i.e., an exhibited low-frequency peak at 0.16 Hz and a secondary peak at 10 Hz, Figure 3c. The overturning moment for model A is affected by both characteristic flow phenomena, i.e., wind gusting represented by a peak at 0.16 Hz and the vortices shed from the bridge deck, which is observed in the 10 Hz peak. The low-frequency peak at 0.16 Hz is more exhibited for model B, as seen in Figure 3d.