## 3 Self-Start Characteristics of Different Unstart Modes
### Self-Start Characteristics of DSBL Flow Mode  
First, experimental results of the DSBL flow mode at M\({}_{0}\) = 5 are introduced. The DSBL flow mode was obtained at an alpha of 6\({}^{\circ}\) and 12\({}^{\circ}\), respectively, for Model/SB-1 and Model/SB-2 by blocking and then gradually releasing the downstream channel. Then, the alpha was decreased. The variation in Schlieren images and pressure distributions are shown in Figure 19. As these figures depict, the DSBL vanished when the alpha decreased to 0\({}^{\circ}\) for Model/SB-1 and the flow mode was converted into SSB unstart. The following self-start could be completed by increasing the alpha again, as discussed above. With regard to Model/SB-2, the flow mode was converted into LSB as the alpha decreased from 4\({}^{\circ}\) to 2\({}^{\circ}\). As previously stated, allowing the alpha to keep decreasing would let the LSB vanish; the remnant of the LSB--the SSB--would vanish as the alpha increased, and then self-start could be achieved. Figure 19: _Cont._  
Figure 18: Schlieren images of blocking/releasing downstream channel—Model/SB-1—M\({}_{0}\) = 5. (a) M\({}_{0}\) = 5\({}^{\circ}\)—self-start. (b) M\({}_{0}\) = 5/alpha = 6\({}^{\circ}\)—form DSBL. High-speed Schlieren images and high-frequency pressures regarding the vanishment of DLSB for Model/SB-1 (Figure 19a,c) are shown in Figure 20, and the green dotted line in (b) indicates the initial time in (a). The transient process is clearly shown in these figures. Firstly, the separation on the lip side was swallowed downstream, while the separation on the body side moved slightly upstream; then, the body-side separation moved downstream again so that the LSB flow mode was formed. This process corresponds to 0.5\(\sim\)2.5 ms in Figure 20a and region I in Figure 20b. Then, the LSB flow mode was maintained for about 3 ms, which corresponds to 2.5\(\sim\)5.5 ms in Figure 20a and region II in Figure 20b. After that, the LSB was also swallowed downstream in 2 ms--5.5-7 ms in Figure 20a and region III in Figure 20b. From this, we can see that the vanishment of the DSBL could not be achieved in one stroke. The DSBL flow mode was converted to the intermediate form of the LSB flow mode at a small alpha first, and then the LSB continued to vanish if it could not be maintained. For Model/SB-2 (Figure 19b,d), the flow mode was converted from DSBL to LSB with the condition that the LSB could be maintained; so, the flow mode did not continue changing. According to the analysis above, the critical alpha of LSB self-start is bigger than 5\({}^{\circ}\), but the DSBL is maintained even if the alpha decreases to 2\({}^{\circ}\) at M\({}_{0}\) = 5 for Model/SB-1, which indicates, to some extent, that the self-start performance of the LSB flow mode is better. Next, we examine the self-start performance of the DSBL flow mode at M\({}_{0}\) = 6. Figure 21 displays the Schlieren images as alpha decreased after DSBL was induced at M\({}_{0}\) = 6/alpha = 12\({}^{\circ}\) for Model/SB-1. As shown in these pictures, DSBL was maintained even when the alpha dropped to \(-\)3.5\({}^{\circ}\). This reveals that DSBL is difficult to vanish via only increasing the Mach number. In other words, once DSBL is induced, the inlet cannot easily self-start unless the flow mode is converted into LSB. The essence of the self-start of hypersonic inlets is the vanishment of separations near or in the inlet. As described above, three types of unsstart modes of hypersonic inlets were analyzed in detail, from which the characteristics of the separation vanishing and inlet self-start were obtained. A summary of this is shown in Table 3.  
Figure 19: Maintenance of DSBL flow mode and the flow mode transformation—M\({}_{0}\) = 5. (**a**) Model/SB-1—Schlieren images. (**b**) Model/SB-2—Schlieren images. (**c**) Model/SB-1—pressure distributions. (**d**) Model/SB-2—pressure distributions. Figure 21: Maintaining of DSBL flow mode—Model/SB-1—M\({}_{0}\) = 6.