## 4 Results and Discussion
### Aerodynamic Pressure Regrading Train-Intersection  
Discussion in this section will highlight the aerodynamic effect caused by the intersection of HSTs. TPLs acting at tunnel sidewall and train surface are both addressed with research interest inclined to the pressure difference caused by the intersection at different velocity levels. Firstly, Figure 16 displays the wave propagation and pressure history measured at the head and tail of Train A in Test 13. Pressure counterbalance, superposition, as well as its coupling effect performing as TPLs are revealed. Further investigation associated with the pressure waveform considering the intersection effect is illustrated by Figure 17. After the entry of two high-speed trains running through tunnel portals from each side, the initial pressure waves appear at \(t=a\) s exhibited in Figure 16a,c lead to the (1) pressure increase \(\Delta P_{h-1}\) due to the compression wave produced at train head and (2) pressure drop \(\Delta P_{t-1}\) attributed to the expansion wave generated at train tail. Detailed change of pressure waveform can be observed in in Figure 17. The second pressure increase appeared at train  
Figure 14: Histories of micro-pressure wave at \(x_{tu-out}=\)20 m: (**a**) field test (**b**) simulation. Figure 15: Data fitting to determine the dimensionless micro-pressure \(P^{*}_{micro}\) along tunnel centerline: (**a**) peak value of micro-pressure at tunnel entrance; (**b**) \(C_{p}\) coefficient value. head and tail, marked as \(\Delta P_{h-2}\), is clearly explained by the diagram of wave propagation shown in Figure 16b. The reason behind is due to the compression wave caused by the entry of Train B at \(t=b\) s marked in both Figure 16a,c. Continuous pressure decreases in Figure 16a appeared between \(t=c\sim f\) s shown in both Figure 16a,c are respectively resulted by the head-to-head intersection, expansion wave for train head, and head-to-tail intersection for train tail. Increase of the pressure measured after \(t=c\sim f\) s is due to the reflected compression wave, which will not be further discussed. Figure 18 lists the pressure contours recorded at different time in Test \(11\) where train's intersection is conducted at the speed of 400 km/h. As shown in Figure 18a-c, pressure in the overall flow field shows growing trend from the portal side to the tunnel center with all the values being positive before the intersection. As the decease of train's head-to-head distance, area within the high pressure marked within the dotted lines is reduced. After the shortly head-to-head intersection at \(t=4.475\) s, as shown in Figure 18d, a fast pressure decrease in the whole flow field is observed. Afterwards, area of the negative pressure field expands as time goes, resulting the pressure field within the intersected area (marked in Figure 18e within dotted lines) being entirely negative. Figure 16: Pressure wave spread and corresponding evolution curves at train head and tail. Figure 17: Pressures at train nose tips regarding train’s intersection. TPL acting at train body is plotted in Figure 19 considering intersection carried out by both equivalent and unequal train speed (Test 12 and 14 for example). The pressure waveform drawn in Figure 19 show very limited difference in such two conditions. Besides, for a given location at train surface, the pressure load measured at the intersection side agrees well with what obtained from the non-intersection side. Pressure difference exists only when the measuring position moves along the longitudinal direction, which matches well with what observed in the pressure contours exhibited in Figure 18.  
In the similar way referring to Figure 11, Figure 20 reveals the pressure changes measured at different positions regarding train's intersection with both equivalent and unequal train speed (Test 13 and 15 for example). In general, results from both Test 13 and 15 denote values of \(P^{*}_{tu-max}\) first increase and then decrease with \(L^{*}_{tu}\).