## 4 Simulation Results
### Impact of Vent Opening Size  
Simulation analysis was implemented based on one pound of TNT at a height of 10 cm from the ground. The ground was assumed as a rigid body. In the finite element model, the air element is 0.5 cm\({}^{3}\) and mapping is 0.25 cm\({}^{3}\). The simulation result of the extreme value of blast pressure at 10 cm above the ground is shown in Figure 18. Beyond 50 cm from the blast point, the result of the blast at a blast height of 10 cm is close to that of the ground blast in UFC3-340-01, which indicates that the finite element model is available. Within 50 cm from the blast point, the blast pressure is less than that of the ground blast, as that blast point is not a semicircle attached to the ground.  
Figure 16: Schematic diagram of blasting through a fragile wall and numerical simulation [18].  
Figure 17: Finite element model.  
The results show that the confinement effect of the ammunition depot is weak and leakage pressure remains roughly unchanged when the vent opening size is above \(40\times 40\) cm (as shown in Figure 20), i.e., when the vent opening is small, the confinement effect is significant, and thus, leakage pressure is low; however, the impulse on the interior wall surface reaches 36.717 bar/ms and blast pressure on the interior wall surface reaches 128 bar, which is not subject to the effect of vent opening size. The two values fall between the incident blast pressure and reflected blast pressure (or impulse), as compared with the blast pressure and impulse in free air shown in Figure 1, which is mainly because the finite element is still not small enough. While blast pressure shows a decreasing tendency with the increasing height of the observation point, it is not certain, as impulses are subject to the cumulated effect of earth-reflected waves.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{6}{c}{**1-Pound TNT–Full Vent Model (Blast Height 10 cm)**} \\ \cline{2-7}
**Distance** & \multicolumn{6}{c}{**Observation Point Height**} \\ \cline{2-7}
**from Blast** & \multicolumn{2}{c}{**10 cm**} & \multicolumn{2}{c}{**20 cm**} & \multicolumn{2}{c}{**30 cm**} \\ \cline{2-7}
**Point** & \multicolumn{2}{c}{**Blast Pressure Bar**} & \multicolumn{2}{c}{**Impulse Bar**} & \multicolumn{2}{c}{**Blast Pressure Bar**} & \multicolumn{2}{c}{**Impulse Bar**} & \multicolumn{2}{c}{**Impulse Bar**} \\ \hline
30 cm & 37.4 & 0.903 & 38.7 & 0.927 & 35.3 & 0.995 \\
40 cm & 33.1 & 1.920 & 23.9 & 1.060 & 22.7 & 1.100 \\
50 cm & 28.6 & 1.720 & 16.2 &  
The comparison between the computer simulation value of a \(20\times 20\) cm vent opening and the leakage pressure value in UFC3-340-02 (Figure 10) are shown in Figure 21. The results show that the error between the computer simulation result and the specified value can be within 10%. Regarding the \(40\times 40\) cm vent opening, while the error between the two reaches \(-30\%\) (Figure 22), the tendency is consistent.  
The impact of the charge weight on the computer simulation result is shown in Figure 23, which shows that the results of different charge weights (1 lb and 1/8 lb) are consistent, i.e., the scaled distance is applicable and the result of a small ammunition depot can be applied to a large ammunition depot.  
Figure 19: Schematic diagram of duration of blast pressure of the \(20\times 20\) cm vent opening.  
Figure 20: Impact of vent opening size on leakage pressure.  
Figure 21: Comparison between leakage pressure of a \(20\times 20\) cm vent opening and the specified value in UFC3-340-02.  
Figure 22: Comparison between leakage pressure o a \(40\times 40\) cm vent opening and the specified value in UFC3-340-02.