## 3 Numerical Simulation
### Application of Simulation Technique  
In order to improve the accuracy of numerical simulation regarding blast pressure in a close-range blast, fine mesh size and ALE Fluid-Structure Interaction are commonly used. However, with the increasing number of meshes, various issues, such as restrictions on the analysis model and mesh size, have been derived therefrom. In order to address this issue, in this study, the ANSYS/LSDYNA Mapping Technique was used for numerical simulation in testing to overcome the limit and increase the accuracy of numerical simulation computation.  
In this study, due to a large-sized ammunition depot, a large-scale numerical model was required, which led to long computation time; therefore, the ANSYS/LSDYNA Mapping Technique was utilized. A small model was first utilized for the blast simulation; then, the simulation was gradually mapped to a large model. Finally, blast pressure was applied to the vent opening and walls of the ammunition depot to save computation time. Element erosion conditions were incorporated to remove elements subject to severe distortion due to the termination of computation, which was a result of a negative volume of wall meshes due to severe distortion.  
Regarding the application of the Mapping Technique in a blast in free air with an 80\(\times\)80 cm computation model; for example, the side length of the element was set at 0.03125 cm and the model was divided into more than 6 million elements, in order to increase the accuracy of simulation. Such a  
\begin{table}
\begin{tabular}{c c c c} \hline
**Strain Rate (1/\(\mu\)s)** & **Dynamic Amplification Coefficient** & **Strain Rate (1/\(\mu\)s)** & **Dynamic Amplification Coefficient** \\ \hline \(-3\times 10^{-2}\) & 9.70 & \(3\times 10^{-11}\) & 1.00 \\ \hline \(-3\times 10^{-4}\) & 9.70 & \(1\times 10^{-10}\) & 1.03 \\ \hline \(-1\times 10^{-4}\) & 6.72 &  
huge number of elements will lead to time-consuming computation. If the side length of the element was set at 0.0625 cm, and the number of elements was more than 1 million, while computation efficiency was improved, the computation accuracy was ignored. In order to maintain both the accuracy and efficiency of computation, the Mapping Technique was used to address this issue, and its computation steps are shown in Figure 14. Step 1, meaning the side length of the element, was set at 0.03125 cm, the boundary size of the model was set as a 40\(\times\)40 cm model of a blast in free air, and simulation computation was ceased upon the arrival of blast pressure at the boundary. Step 2 implemented numerical imaging, where the size of the elements (0.0625 cm) and boundary (80 \(\times\) 80 cm) of the model were scaled up two times; the blast pressure computation result from the 40 \(\times\) 40 cm model was taken as the initial value for the 80 \(\times\) 80 cm model, and then, computation was continued.  
The overall ALE Fluid-Structure Interaction numerical model of the internal blast in the ammunition depot was assumed as a cube (structure length: 60 cm, width: 60 cm and height: 60 cm, the ground length: 240 cm, width: 60 cm, and thickness: 1 cm), and it was assumed as a rigid body, as shown in Figure 15. Mapping is ceased when the blast pressure is mapped close to walls, and ALE Fluid-Structure Interaction computation is implemented in order to assess the external blast pressure value, as a result of the blast wave leaking through the vent opening in the wall. In the case of a fragile wall, element erosion is generated by the wall in order to simulate blasting through the fragile wall (Figure 16).  
Figure 14: Schematic diagrams of computation steps of numerical mapping [12].  
Figure 15: Schematic diagram of ALE Fluid–Structure Interaction numerical model of internal blast in the ammunition depot.