## 3 Self-Start Characteristics of Different Unstart Modes
### Self-Start Characteristics of LSB Flow Mode
However, this method is only applicable in the cases where only the conventional unstart mode (LSB flow mode) exists. Therefore, in order to make the above method available when multiple unstart flow modes exist, we revised it as follows (considering the analysis results shown in Figure 14): if the alpha is not big enough to form DSBL flow mode, blocking and then gradually releasing the downstream channel could result in the same critical status for LSB vanishing, just like that which was obtained by changing the alpha or M\({}_{0}\). To verify the correctness of this revised method, an experiment was conducted at M\({}_{0}\) = 5 for Model/SB-2: alpha = 0\({}^{\circ}\) (pulse start) \(\rightarrow\) alpha = 2\({}^{\circ}\) (blocking and then gradually releasing the downstream channel) \(\rightarrow\) alpha = 0\({}^{\circ}\) (blocking and then gradually releasing the downstream channel). The Schlieren images and pressure distributions along the flow direction are separately shown in Figures 15 and 16. From these figures, it can be seen that the inlet flow mode was converted into LSB unsstart from the start, after releasing the downstream channel at alpha = 2\({}^{\circ}\), and then was converted to the SSB flow mode when the alpha dropped back to 0\({}^{\circ}\). At this time, the LSB did not appear again after we blocked and then gradually released the downstream channel. Thus, whether the downstream channel was blocked and then gradually released or the alpha decreased, the same critical status for the LSB vanishing was obtained (alpha = 0\({}^{\circ}\)), which proves the correctness of the revised method. Figure 14: Schlieren images of blocking and then releasing downstream channel—Model/SB-1—M\({}_{0}\) = 4. (a) Blocking/releasing at small degree of alpha. (b) Blocking/releasing at medium degree of alpha. Figure 15: Schlieren images of seeking the critical status for LSB vanishing—Model/SB-2—M\({}_{0}\) = 5. Based on the above analysis, we introduced the self-start characteristics of the LSB flow mode at M\({}_{0}\) = 5 for Model/SB-1. Figures 17 and 18 show the pressure distributions and Schlieren images before and after blocking and then gradually releasing the downstream channel when the degrees of alpha were 5\({}^{\circ}\) and 6\({}^{\circ}\). Self-start was achieved in the experiment where alpha = 5\({}^{\circ}\). As for the experiment where alpha = 6\({}^{\circ}\), the DSBL flow mode was induced, and this flow mode was maintained after the throttle system fully opened. From these experiments, we can conclude that the critical alpha of LSB self-start is not smaller than 5\({}^{\circ}\). Figure 16: Pressure distributions of seeking the critical status for LSB vanishing—Model/SB-2—M\({}_{0}\) = 5. Figure 17: Pressure distribution before and after blocking/releasing downstream channel—Model/SB-1—M\({}_{0}\) = 5. In summary, both acceleration and a decreasing alpha contribute to the vanishment of LSB. For Model/SB-1, LSB cannot vanish at M\({}_{0}\) = 4, and the critical alpha of LSB self-start is not smaller than 5\({}^{\circ}\) at M\({}_{0}\) = 5. The LSB of Model/SB-2 is more stable and it also cannot vanish at M\({}_{0}\) = 4; its vanishing degree of alpha is 0\({}^{\circ}\) at M\({}_{0}\) = 5.