# Experimental Study of Downburst Wind Flow over a Typical Three-Dimensional Hill
## 5 Conclusions  
1. The laboratory tests conducted using a physical downburst simulator revealed remarkable effects of hilly terrain on the downburst wind field. It was found that the speed-up was not only related to the slope, but also to the hill height and ratio of hill  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \(c_{1}\) & \(c_{2}\) & \(c_{3}\) & \(c_{4}\) & \(c_{5}\) & \(c_{6}\) & \(d_{1}\) & \(d_{2}\) & \(d_{3}\) & \(d_{4}\) & \(d_{5}\) & \(d_{6}\) \\ \hline
1.454 & \(-1.182\) & 1.226 & \(-0.6082\) & 0.1118 & 0.1118 & 0.3935 & \(-0.9798\) & 0.9711 & \(-0.4579\) & 0.08016 & 0.08016 \\ \hline \end{tabular}
\end{table}
Table 8: Values of \(c_{i}\) and \(d_{i}\).  
height to jet height. This finding is evidently different from the ABL wind case, and can be largely attributed to the nature of the downburst.
2. Horizontal wind velocity predominated when the hill was placed in the strong horizontal wind region, i.e., when the distance from the stagnation point to the crest was around \(1.0D_{\text{jet}}\). Under this circumstance, the wind tended to flow over the crest when the height of the hill was sufficiently low, and flowed around the hill body in the cases of high hills. As the hill height increased, the speed-up region gradually moved from the crest to the sides of the hill, namely to the vicinity of the windward edges. As a result, the maximum speed-up effect was frequently found on the ridge or at the hill foot, in the cases of high hills. Among all cases, the maximum topographic multiplier was \(1.12\), and appeared at the ridge when the ratio of hill height to jet height was \(5/12\), in terms of horizontal wind. In addition, the corresponding maximum vertical wind velocity value reached about half that of the impinging jet velocity.
3. A strong vertical wind field on the hill was observed when the hill model was placed directly under the impinging jet. It was found that the maximum strong vertical wind appeared at the crest of the hill, and that either a steeper slope or a higher hill led to a greater gradient of vertical wind velocity in the direction of elevation. As the hill height increased, the vertical wind velocity at the relative height of \(0.3D_{\text{jet}}\) would increasingly approach the impinging jet velocity.
4. To inform future structure design, explicit formulas for estimating the envelope values at the typical locations were presented that enable conservative estimations of horizontal downburst wind loads. Moreover, a phenomenological model was proposed to predict the vertical wind velocities on the crest when the hill is directly under the impinging jet, and shows a satisfactory agreement with the test results.
5. The physical downburst simulator employed herein is mainly based on the impinging jet model that has been widely accepted in wind engineering for the experimental study of downburst wind and its effects on structures. However, the simulator is not capable of reproducing the dynamic characteristics of the downburst, in particular the ring vortex that is occasionally observed during downburst events. Accordingly, there is a need for further study to examine the effects of the presence of ring vortices.  
The authors declare no conflict of interest.