## 3 Results and Discussion
### Effect of Compression Direction on Start Performance of Inlet  
Additionally, the self-starting performance of a hypersonic inlet is crucial for ensuring the stable operation of hypersonic vehicles. The self-start Mach number reflects the inlet's ability to transition from an unstart state to a started state. This section aims to reveal the differences in starting characteristics between normal and inverted layouts. To achieve this, we compared the acceleration self-starting Mach numbers of these two inlet layouts at a \(0^{\circ}\) angle of attack. Initially, a low Mach number of three was set to establish an unstart  
Figure 8: Comparison of Mach contours and streamlines on symmetry. Figure 9: Comparison of flow regimes at the throat of the inlet for different inlet layouts. state for the inlet. Subsequently, the inflow Mach number was incrementally increased by \(\Delta\)Ma = 0.02 until the inlet successfully transitioned to a started state. Figures 10 and 11 detailly show the specific self-starting processes of the two layouts, indicating that the normal and inverted configurations achieve self-starting at Mach numbers of 5.52 and 3.90, respectively. Notably, the self-starting Mach number of the inverted inlet layout is lower, indicating its superior self-starting capability. Figure 12 also describes the changes in the flow coefficient and total pressure recovery at the throat of the inlet during acceleration. Although the flow rate did not change significantly for the inverted configuration from Mach 3.88 to 3.90, there was a sudden change in the total pressure recovery coefficient. Similarly, for the normal inlet layout, while the flow rate did not change significantly from Mach 5.50 to 5.52, there was a sudden change in the total pressure recovery coefficient. This also confirms that the normal and inverted configurations achieve self-starting at Mach numbers of 5.52 and 3.90, respectively. Figure 11: Self-starting process of the inverted inlet layout. Figure 12: Comparison of the variation curve of aerodynamic parameters during the starting process. Figure 10: Self-starting process of the normal inlet layout. Figure 13 depicts the non-starting flow field states of the normal and inverted inlet layouts at Ma = 3.88. Diverging blue and red color maps illustrate pressure distribution on the fuselage surface, while a smaller rainbow-colored map indicates Mach number distribution on the symmetry plane. Streamlines near the wall surface and lip are also depicted in Figure 13a,b. In the normal inlet layout, the spanwise wall pressure distribution on the fuselage side is relatively uniform, with no pressure gradient, hindering the effective sweep of low-energy flow. The lip shock wave forms a separation zone in the internal contraction section, obstructing the overflow of low-energy flow and making self-starting challenging. Conversely, the inverted inlet layout exhibits unique advantages. Here, the incident internal cone shock wave is projected onto the fuselage, forming a local high-pressure region. Together with low-pressure regions on both sides unaffected by the shock wave, a significant transverse pressure gradient is formed, effectively discharging low-energy flow near the wall through overflow windows, reducing the scale of the separation region and enhancing self-starting performance. We have conducted an analysis to assess the effect of spillage on Mach number (Ma) and pressure recovery. The results of the calculations, including the mass-weighted average total pressure recovery \(\sigma_{inlet}\), mass flow rate \(\overset{\bullet}{\mathbf{m}}_{air}\), and mass-weighted average Mach number Ma for both normal and inverted inlet layouts, are presented in Table 3 below. From the table, we can observe that the inverted inlet layout achieves a significantly higher mass-weighted average total pressure (0.802) compared to the normal inlet layout (0.296). This suggests that the inverted configuration is more effective in maintaining higher total pressure, which is beneficial for pressure recovery.