# Experimental Study of Downburst Wind Flow over a Typical Three-Dimensional Hill
## 3 Results and Discussion
### Horizontal Wind Velocity  
For illustration, the downburst wind flows over the hill models with \(L=0.5D_{\text{jet}}\) and \(r_{\text{h}}=1.0D_{\text{jet}}\) are first discussed. Figures 7-9 show the profiles of the horizontal velocity at different measuring locations. For the same location, the corresponding profiles in the undisturbed downburst wind field where the wall jet thickness \(\delta\) is about \(0.15D_{\text{jet}}\) are also depicted in the figures. It is evident that the horizontal wind velocity varies with the relative height from the ground/hill surface. For convenience, we herein define \"the global maximum\" as the highest value for the entire wind field over the hill, and \"the maximum\" as the highest value for the same measuring location simultaneously measured at the different relative heights from the hill surface. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline
**Hill Model** & **H1** & **H2** & **H3** & **H4** & **H5** & **H6** & **H7** & **H8** & **H9** & **H10** & **H11** & **H12** \\ \hline Re & 150,934 & 75,467 & 50,311 & 37,733 & 30,187 & 25,156 & 603,735 & 301,867 & 201,245 & 150,934 & 120,747 & 100,622 \\ \hline \end{tabular}
\end{table} 
Table 3: Reynolds numbers for hill models (\(V_{\text{jet}}=12\) m/s). Figure 7: Profiles of horizontal wind velocity on \(x\)-axis ridge (\(L=0.5D_{\text{jet}}\), \(r_{\text{h}}=1.0D_{\text{jet}}\)). Regarding the horizontal wind on the \(x\)-axis ridge, as shown in Figure 7, it was found that the effects of the hill with a low height (\(H/h=1/12\)) were relatively small in terms of the shape of the wind profile, as well as the location where the maximum horizontal wind velocity occurred (around \(1.0D_{\text{jet}}\)). The predominant factor in determining the variation in the wind profile was the distance from the stagnation point. However, provided \(H/h>1/12\), the increase in \(H/h\) was found to reduce the intensity of the horizontal wind in general. It was also observed that the near-ground maximum horizontal wind velocity decreased correspondingly with the increase in the hill height. In the cases of \(H/h>1/3\), the wind velocities measured at the crest were even lower than those measured near the windward hill foot. This may be attributed to the blockage effect caused by the hill becoming increasingly significant as the height of the hill increased, weakening the intensity of the downburst wind flow over the crest. The speed-up phenomenon on the \(x\)-axis ridge was only observed in the case of the hill with low elevation (\(H/h=1/12\)), and occurred near the crest of hill, with a corresponding \(M_{\text{t,max}}\) of \(1.07V_{\text{jet}}\). Furthermore, it was found that the horizontal wind velocities measured on the windward side were generally greater than those measured on the leeward part. The wind profiles on the \(y\)-axis ridge are shown in Figure 8. At the hill foot with a radial distance of roughly \(r=1.12D_{\text{jet}}\), the wind intensity became relatively stronger. The maximum wind velocity at the hill foot was a little greater than the impinging jet velocity. However, regardless of the hill height, the wind profiles measured at the hill foot were generally similar, which implies that the hill height had a negligible effect at this location. For the hills with \(H/h>1/6\), it was found that the horizontal wind velocity was significantly decreased along the ridge, implying that the blockage effect was pronounced. Furthermore, corresponding with the increase in the hill height, the maximum wind velocity decreased. If \(H/h\leq 1/6\), the speed-up phenomenon could be observed along the whole ridge, and the maximum topographic multiplier was found at \(z=H/4\) with a value of \(1.08V_{\rm jet}\). In Figure 9, the profiles of the horizontal wind velocity on the \(45^{\circ}\)-ray ridge (leeward side), as well as that on the \(135^{\circ}\)-axis ridge (windward side), are depicted. Generally, the wind velocities on the \(135^{\circ}\)-axis ridge were greater than those on the \(45^{\circ}\)-ray ridge.