## 4 Improving Self-Start with \"Flow Field Reconstruction Method\"  
As previously stated, the self-start characteristics of the DSBL flow mode are very unsatisfactory. Therefore, the theory of first eliminating this inferior unstart form and then reconstructing a new flow structure which is easy to self-start was taken into consideration. This tactic is named the Flow Field Reconstruction Method (FFRM), and the simplest way to destroy a flow mode is applying downstream back pressure (for example, by blocking the downstream channel).  
Figure 22 shows the change in Schlieren images utilizing FFRM with an initial flow mode of DSBL unsatr form--M\({}_{0}\) = 5/alpha = 4\({}^{\circ}\), Model/SB-1. Figure 23 is the variation in high-frequency pressure with time in this process. As shown in these figures, by continuing to block the downstream channel, the DSBL flow mode was eliminated and converted into LSB at the moment indicated by the blue arrow in Figure 23. At this time, the pressure dropped sharply. Then, the scale of the body-side separation was enlarged by continuously increasing the extent of the block of the downstream channel. The point indicated by the black arrow is the max extent of the block of the downstream channel, and after that, the process of releasing the downstream channel began. Self-start was achieved in this process, indicated by the green arrow.  
Figure 22: Self-start process of FFRM—Schlieren images. (Model/SB-1 at the status of M\({}_{0}\) = 5/alpha = 4\({}^{\circ}\)).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Unstart Modes** & **Separation Vanishing/Self-Starting Method** & **Point of Separation Vanishing or Inlet Self-Start** & **Feature** \\ \hline Small separation on & \multirow{2}{*}{Increasing alpha} & Model/SB-1: alpha = 4\({}^{\circ}\)(M\({}_{0}\) = 5) & Easy to self-start \\ body & & Model/SB-2: alpha = 4\({}^{\circ}\)(M\({}_{0}\) = 5) & Easy to self-start \\ \hline Large separation on & \multirow{2}{*}{Acceleration/decreasing alpha} & Model/SB-1: M\({}_{0}\) = 5/alpha = a, a \(\geq\) 5\({}^{\circ}\) & \multirow{2}{*}{Easy to self-start} \\ body & & Model/SB-2: M\({}_{0}\) = 5/alpha = 0\({}^{\circ}\) & \\ \hline Dual separation on & \multirow{2}{*}{Through the intermediate flow mode} & \multirow{2}{*}{Cannot vanish directly at M\({}_{0}\) = 5 and} & Very difficult to \\ both body and lip & & of LSB by decreasing alpha & M\({}_{0}\) = 6 & self-start directly \\ \hline \hline \end{tabular}
\end{table}
Table 3: Self-start characteristics of different unstart modes.  
Figure 24 shows the high-speed Schlieren images of releasing the downstream channel. It can be seen from these pictures that the whole process could be divided into countless quasi-stable states, and the body-side separation oscillated around a certain range in each quasi-state. However, for a large time span, the body-side separation dwindled with the oscillating process. At a certain moment, the inlet self-started and DSBL did not emerge in the whole process of releasing the downstream channel.  
Figure 23: Self-start process of FFRM—pressure variations. (Model/SB-1 at the status of M\({}_{0}\) = 5/alpha = 4\({}^{\circ}\)).  
Figure 24: Transient process of releasing downstream channel—Schlieren images. (Model/SB-1 at the status of M\({}_{0}\) = 5/alpha = 4\({}^{\circ}\)).