## 5 Flow Mechanism for the Poor Self-Start Ability of DSBL Flow Mode  
Compared with the conventional unstart (LSB), the DSBL flow mode is a special unstart form whose self-start ability is far inferior. We have compared the flow characteristics of these two unstart modes to reveal the intrinsic mechanism of the poor self-start ability of the DSBL flow mode. We chose to study Model/SB-1. First, we analyzed the LSB unstart mode. Figure 25 shows the comparison of flow structures and pressure distributions along the flow direction between the experiment and CFD calculation at M\({}_{0}\) = 4/alpha = 0\({}^{\circ}\). As shown in Figure 25, both CFD and experimental results clearly demonstrate the flow field structure, and their results are consistent. Among them, the CFD results display more detailed flow information. Figure 26 depicts the flow structures (the Mach number contours and pressure contours) of different longitudinal sections for the LSB unstart mode (Z-c is the dimensionless width; Z-c = 0 and Z-c = 1 represent the plane of symmetry and the inner wall plane of the sideboard, respectively) based on the CFD results. In these contours, we can observe the overall detailed flow field structure of the LSB unstart mode. As shown, two vortexes of an equivalent scale were located in the separation from the symmetry plane. The flow structure in the longitudinal section of Z-c = 0.375 was similar to that of the symmetry plane. For the longitudinal sections of bigger Z-c, the body-side separation shrank as the Z-c increased. This regularity indicates that there was significant broadwise spillage deriving from the body-side large separation and flowing through the triangle window, which was formed by the sweepback sideboards and forebody. Figure 27 displays a spatial streamline from the body-side separation and through the longitudinal section of Z-c = 0.062, which exhibits flow characteristics of the body-side separation finely. Dense longitudinal vortexes were spread within a certain transversal span close to the symmetry plane. However, with the approach to sideboard where the flow direction points outward, longitudinal vortexes became increasingly sparse. In conclusion, the flow characteristics expressed in Figures 26 and 27 are consistent. Figure 25: Comparison of results between experiment and CFD calculation—LSB unstart mode. (**a**) Comparison of flow structures (Schlieren image and pressure contour of symmetry plane). (**b**) Comparison of pressure distributions along flow direction. We then analyzed the DSBL unsstart mode. The comparison of flow structures and pressure distributions between the experiment and CFD calculation is shown in Figure 28 (at M\({}_{0}\) = 4/alpha = 8\({}^{\circ}\)), and the results of both are also consistent. Figure 26: Flow structures of different longitudinal sections—Model/SB-1 at the status of M\({}_{0}\) = 4/alpha = 0\({}^{\circ}\), LSB flow mode. (**a**) Mach number contours. (**b**) Pressure contours (d1 \(\leq\) d2 \(<\) d3 \(<\) d4 \(<\) d5). Figure 27: Spatial streamline from the LSB—Model/SB-1 at the status of M\({}_{0}\) = 4/alpha = 0\({}^{\circ}\). The flow structures of different longitudinal sections for the DSBL flow mode based on the CFD results are shown in Figure 29. As seen from the symmetry plane in Figure 29a, two vortexes of an equivalent scale existed in the lip-side separation, while a single vortex was maintained around the position of the lip separation shock impact. Comparing the flow structures of different longitudinal sections, we found that the lip-side separation enlarged gradually as it approached the sideboard, while the body-side separation was diminished (although the enlargement trend in the lip-side separation in Figure 29b is not very distinct, it can clearly be seen in Figure 29a). This indicates that the broadwise spillage of the lip-side separation was very weak for the resistance of sideboards. On the other hand, the junction flow near the corner between the sideboards and lip made the separation flow more complex.