# Experimental Study of Downburst Wind Flow over a Typical Three-Dimensional Hill
## 2 Experimental Setup
Assuming a scale of 1:1000 and an average vegetation height of 2.0 m, the vegetation was simulated by gluing 2 mm long plastic fibers onto the surface of the hill models (see Figure 3), according to previous work by [PERSON] et al. [22], and the surface roughness of the hill models was approximately the same for all models. Figure 3 shows the hill models with the simulated vegetation. To examine the strong horizontal wind region, the hill models were first placed at three locations for the experimental study, namely \(r_{\rm h}=0.8D_{\rm jet}\), \(1.0D_{\rm jet}\), and \(1.2D_{\rm jet}\). For the next part of the test, the hill models were placed at \(r_{\rm h}=0.0D_{\rm jet}\), where a strong vertical wind would occur. The Cobra Probe [23] produced by Turbulent Flow Instrumentation Pty Ltd. was employed to measure the wind velocity field of the downburst. The Cobra Probe is a 4-hole pressure probe that provides dynamic, 3-component and local pressure measurements in real-time. The Cobra Probe features a linear frequency-response from 0 Hz (mean flow) to more than 2000 Hz and is available in various ranges for use between 2 m/s and 100 m/s. In addition, 3-component velocity and static pressure measurements within a \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
**Hill Model** & **H1** & **H2** & **H3** & **H4** & **H5** & **H6** & **H7** & **H8** & **H9** & **H10** & **H11** & **H12** \\ \hline \(H\) (m) & 0.1 & 0.2 & 0.3 & 0.4 & 0.5 & 0.6 & 0.1 & 0.2 & 0.3 & 0.4 & 0.5 & 0.6 \\ \(L\) (m) & 0.3 & 0.3 & 0.3 & 0.3 & 0.3 & 0.3 & 0.6 & 0.6 & 0.6 & 0.6 & 0.6 & 0.6 \\ \(H/L\) & 1/3 & 2/3 & 1.0 & 4/3 & 5/3 & 2.0 & 1/6 & 1/3 & 1/2 & 2/3 & 5/6 & 1.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Dimensions of hill models. Figure 3: Photos of hill models: (**a**) \(L=0.5D_{\rm jet}\); (**b**) \(L=1.0D_{\rm jet}\). Figure 2: Experimental setup (**a**) and coordinate systems (**b**). \(\pm\) 45\({}^{\circ}\) acceptance cone can be performed. The measurement accuracies of the velocity and the flow angle are \(\pm\)0.3 m/s and \(\pm\)1\({}^{\circ}\), respectively. Taking advantage of symmetry, the wind velocity profiles at 21 measuring locations were measured. Figure 4a shows the measuring locations at the hill foot (0.0\(H\)), ridge (0.25\(H\), 0.5\(H\), 0.75\(H\)), and crest (1.0\(H\)), respectively. In this study, the ridge is defined as the profile of the hill model, where the crest and the hill foot are excluded, as shown in Figure 4b. The measuring locations were arranged at the ridges of the \(x\)-axis, \(y\)-axis, 45\({}^{\circ}\) ray, and 135\({}^{\circ}\) ray, respectively, namely at the ridges whose projections on the x-y plane coincide with the \(x\)-axis, \(y\)-axis, 45\({}^{\circ}\) ray, and 135\({}^{\circ}\) ray respectively. The 45\({}^{\circ}\) ray and 135\({}^{\circ}\) ray are the closed half-lines in the \(x\)-\(y\) plane, whose initial points are at the origin of the coordinate system for the hill, having the clockwise angles of 45\({}^{\circ}\) and 135\({}^{\circ}\) respectively from the positive direction of the \(x\)-axis. For each measuring location, the arrangement of the measuring points in a vertical line were the same. For illustration, the relative heights of the measuring points for P2 are shown in Figure 4b. To determine the jet velocity for the tests, a preliminary study on the effects of the Reynolds number (Re) was performed in advance. The Reynolds number can be computed according to the formula Re = \(V_{\rm jet}D/\
u\), where \(V_{\rm jet}\) is the jet velocity, \(D\) is the characteristic length equal to the radius of the curvature at the crest, and \(\
u\) = 1.45\(\times\)10\({}^{-5}\) m\({}^{2}\)/s is the kinetic air viscosity at 15 \({}^{\circ}\)C. Four hill models, namely H1, H6, H7, and H12 were selected for the preliminary study. The hill models were placed at the position of \(r_{\rm h}\) = 1.0\(D_{\rm jet}\), and the jet velocity was varied as follows: \(V_{\rm jet}\)= 4, 6, 8, 10, and 12 m/s.