## 3 Self-Start Characteristics of Different Unstart Modes
### Self-Start Characteristics of LSB Flow Mode  
LSB flow mode is the most common unstart form, about which research is the most extensive. The combined action of congestion in the throat of the inlet and shock boundary-layer interaction is usually thought to be the cause of this unstart mode. Normally, self-start can be achieved via increasing the Mach number or decreasing the alpha. A dwindling internal contraction ratio also benefits the self-start for the geometry-variable inlet. Experiments were conducted to reveal the self-start characteristics of the LSB unstart mode for the present models, and the details are as follows. We first introduce the experimental results when M\({}_{0}\) = 4. Figure 11 shows the effects of increasing the alpha for the LSB flow mode. In Figure 11, we can see that for both Model/SB-1 and Model/SB-2, the LSB flow mode withstood values alpha = 0\({}^{\circ}\) to alpha = 10\({}^{\circ}\) and was converted into DSBL flow mode at alpha = 12\({}^{\circ}\), which means that the LSB cannot vanish at M\({}_{0}\) = 4. We then analyzed the experimental results for Model/SB-2 at M\({}_{0}\) = 5. Figure 12 shows the process of decreasing the alpha after inducing LSB at alpha = 8\({}^{\circ}\) by blocking and then releasing the downstream channel. It can be seen that the LSB vanished as the alpha decreased from 2\({}^{\circ}\) to 0\({}^{\circ}\). Because of the small alpha value at this time, a tiny separation was left as a remnant of the LSB (the reason is described in the previous subsection), which formed SSB flow mode. The transient process of the LSB vanishing is displayed in Figure 13, and the green dotted line in (b) indicates the initial time in (a). As seen in these pictures, the time for the LSB to vanish was about 2 ms. Comparing the flow structure of the first picture in Figure 13a with the first and second pictures in Figure 12a, we can see that the  
Figure 11: Maintenance of LSB flow mode and the transformation from LSB to DSBL flow mode—M\({}_{0}\) = 4. (a) Model/SB-1, Schlieren images. (b) Model/SB-2, Schlieren images. Figure 10: Cp-T (pressure coefficient–time). Curves—process of inducing SSB and self-starting. scale of the LSB did not dwindle gradually before vanishing, which is different from the vanishing process of the SSB. As discussed above, the following self-start was completed when the alpha increased to 4\({}^{\circ}\). For Model/SB-1, the LSB vanishing process was not observed when changing the alpha at M\({}_{0}\) = 5. In order to clarify the LSB vanishing characteristics in this situation, we refer to the experiment results from blocking and then gradually releasing the downstream channel. Therefore, the next discussion is the action of flow modes formed by blocking and then gradually releasing the downstream channel in these experiments. Figure 14 displays the results for Model/SB-1 of gradually releasing the downstream channel after blocking it at M\({}_{0}\) = 4/alpha = 0\({}^{\circ}\) and M\({}_{0}\) = 4/alpha = 4\({}^{\circ}\). It can be seen from  
Figure 12: Vanishment of the LSB in the process of decreasing alpha—Model/SB-2—M\({}_{0}\) = 5. (a) Schlieren images. (b) Pressure distributions. Figure 13: Transient process of the LSB vanishing when alpha decreased from 2\({}^{\circ}\) to 0\({}^{\circ}\)—Model/SB-2—M\({}_{0}\) = 5. (a) Transient Schlieren images of the LSB vanishing. (b) High-frequency pressure curves of the LSB vanishing. Figure 14 that the LSB flow mode was formed after releasing the downstream channel when the alpha was small (alpha = 0\({}^{\circ}\)). However, when the alpha increased (alpha = 4\({}^{\circ}\)), the DSBL flow mode appeared. In engineering practice, unstart is often induced by increasing the downstream back pressure (such as blocking the downstream channel). Then, the back pressure is gradually released to observe whether the inlet can self-start. It is generally believed that the critical status of self-start obtained using this method is consistent with the critical status obtained by changing the alpha or M\({}_{0}\).