## 3 Numerical Simulation
### Internal Explosion Flow Field at the End Cover of the Vessel
#### 3.2.3 Influence of Aspect Ratio on Internal Explosion Flow Field  
In addition to the scaled distance of charge, factors such as the aspect ratio of the container also affect the internal flow field distribution [25]. Therefore, explosion containment vessel models with different aspect ratios were designed for numerical simulation and the charge was uniformly set to 65 g.  
The evolution of the internal explosion flow field with a container aspect ratio of 1.00 was taken as an example for analysis. As shown in Figures 8 and 9a, at about 215 us after the charge explosion, the primary explosion wave and its reflection occurred simultaneously at the pole and at the central section torus of the containment vessel. Then, the reflection point moved from the torus and the pole to the corner of the container, and finally the reflection points met near the corner, at 335 us. At the same time, all the reflection shock waves converged toward the center of the container and compressed to the extreme at about 535 us, then outward to form a series of secondary shock waves. The ellipsoidal end cover had radians, which helped the reflection shock waves to converge toward the focal point. Therefore, the reflection shock waves did not converge at the center of the container, but formed two upper and lower convergence points. Due to the influence of the location of the convergence point, at about 760 us the end cover pole first received the secondary shock wave and the reflection was formed at the central section torus. It is noteworthy that, along the inner wall of the end cover converging towards the pole, there was still a shock wave propagating. As a result of the pressure of this secondary shock wave, its reflected wave at the pole and the convergence wave were in the same order of magnitude, and they formed a complex interaction at the pole, resulting in the formation of three peaks between 760 us and 860 us, as shown in Figure 9a. After that, the reflection shock waves from the vessel wall converged again at about 1505 us, and the formed tertiary shock wave reached the end cover pole at 1595 us. Moreover, through the complex interaction of the tertiary wave itself,  
Figure 7: **(a)** The pressure curve at the pole under the 8 g charge. The detail is in the top right corner. **(b)** Time interval between the convergence wave peak (red dot solid line) and the secondary shock wave peak (black dot solid line) under the different charges.  
its reflected wave and the convergence wave, a maximum pressure peak was finally formed at around 1645 \(\upmu\)s.  
Through setting up containment vessels with different aspect ratios in the range of 1.00 to 2.50 for numerical simulation, the change of the interval time between the convergence wave and the secondary shock wave was investigated as shown in Figure 9b. When the aspect ratio of the container was greater than 1.75, the interval between the two peaks was practically the same, about 150 \(\upmu\)s. When the aspect ratio was lower than 1.60, the interval between the two peaks decreased gradually. Until the aspect ratio came to 1.40, due to the same intensity of the secondary shock wave and its reflected wave at the pole as the convergence wave, it was impossible to distinguish the convergence wave peak from the secondary wave peak after the complex interaction. When the aspect ratio was less than 1.40, the convergence wave and the secondary shock were fused to form the complex fusion wave.