## 3 Self-Start Characteristics of Different Unstart Modes
### Self-Start Characteristics of SSB Flow Mode  
The start flow mode was gained at M\({}_{0}\) = 5/alpha = 0\({}^{\circ}\) (M\({}_{0}\): inflow Mach number, alpha: angle of attack) when the wind tunnel started, both for Model/SB-1 and Model/SB-2. This process is called pulse start. Then, the SSB flow mode could be obtained by blocking and then releasing the downstream channel gradually through the throttle system, as shown in Figure 7. The forebody shock, bump shock, and separation shock caused by the small-scale separation can be seen in these figures. Observing the Schlieren images carefully, we can see the beginning section of the lip shock, which was a detached shock caused by the rounded corner of the leading edge. The rest lip shock hit behind the sideboards, so it could not be observed. Increasing the alpha from the initial SSB flow mode, we found that the inlets could self-start. For both Model/SB-1 and Model/SB-2, self-start took place whilst alpha increased from 2\({}^{\circ}\) to 4\({}^{\circ}\). Figures 8 and 9 separately exhibit the Schlieren images and pressure distributions along the flow direction in this process. As shown, even though the SSB is difficult to distinguish in the Schlieren images (oscillation could be observed more clearly in the process of continuous animation), the pressure increased at the points indicated by black arrows at the beginning of the SSB. Congestion in the throat of the inlet and shock boundary-layer interaction are the two main causes which give rise to inlet unstart. Additionally, the compression of the forebody abdominal surface strengthens as the alpha rises, which results in the Mach number decreasing in front of the inlet and the boundary-layer thinning in the forebody abdominal surface [52]. If the main cause of the SSB is congestion in the throat of the inlet, self-start should occur as the alpha decreases, which is incongruent with our experimental results. If the main cause of the SSB is shock boundary-layer interaction, self-start should take place as the alpha increases because the resistance to the flow separation of the thinner boundary layer is stronger [34], which is consistent with our experimental results. Therefore, the shock boundary-layer interaction accounts for the SSB unstart mode. For Model/SB-1, Figure 10 displays the variations in high-frequency pressure in the process of blocking and then releasing the downstream channel gradually at M\({}_{0}\) = 5/alpha = 0\({}^{\circ}\) and then changing the alpha. The locations of the sensors are shown in Figure 1. As can be seen in Figure 10, the pressures of all the sensors increased after the flow mode converted from start to SSB unstart by blocking and then gradually releasing the downstream channel. However, the pressures of sensor 1 (which was affected by the separation shock of the SSB) and sensor 2 decreased when the alpha reached 2\({}^{\circ}\), and the low limitation was closed to the pressures of the start mode, which was at the same status. Unilateral oscillation was ongoing based on this low limitation, and the oscillation intensit \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**M\({}_{0}\)** & **p\({}_{0}\) (MPa)** & **p (Pa)** & **T\({}_{0}\) (K)** & **T (K)** & **Unit Re/10\({}^{7}\)** \\ \hline
4 & 0.43 & 2832.0 & 298 & 70.95 & 1.95 \\
5 & 1.1 & 2079.03 & 405 & 67.50 & 1.94 \\
6 & 2.12 & 1342.7 & 405 & 49.39 & 2.53 \\ \hline \hline \end{tabular} p\({}_{0}\)—total pressure; p—static pressure; T\({}_{0}\)—total temperature; T—static temperature; Re—Reynolds number.  
\end{table} 
Table 2: Parameters of wind tunnel inflow in the experiments. Figure 7: Schlieren images of forming SSB flow mode by blocking and then releasing downstream channel gradually—M\({}_{0}\) = 5/alpha = 0\({}^{\circ}\); Mode/SB-1. sensor 1 but weakened in sensor 2. As for sensor 3, the pressure increased continuously as alpha increased from 0\({}^{\circ}\) to 2\({}^{\circ}\), while the oscillation was constant. This means that the SSB was dwindling as alpha increased, and so was the scope of influence.