## 1 Introduction  
Scramjet engines are widely used in air-breathing hypersonic aircraft, and combustion inside the engine requires sufficient air to serve as an oxidizer [1]. Therefore, enhancing the air capture capability of hypersonic inlets is in great demand. Under normal conditions, the working state of a hypersonic inlet is classified into two forms: start and unstart. Because of the low air-capture efficiency and large flow loss when an inlet unstarts, the thrust performance of the engine degrades substantially, and flameout may occur even in extreme circumstances [2, 3]. Therefore, it is urgently necessary to conduct research about the start problem in hypersonic air inlets. In a hypersonic inlet, throat congestion and shock boundary layer interference are the two main causes of unstart [4, 5, 6]. Most of the time, unstart results from the combined action of both factors [7, 8], and hysteresis occurs in the conversion between start and unstart. The specific performance issue is that the inlet may not restart immediately after returning to the original path when the status converts from start into unstart, and restart does not take place until the situation is resolved [9, 10, 11]. Usually, the process of transitioning from unstart to start is called \"self-start\". Accordingly, the working plane of the inlet is classified into three independent regions: the unstart region, double-solution region, and self-start region [12]. Self-start characteristics are the core of the start problem of hypersonic inlets, and clarifying the flow structure of unstart status is the foundation of the study of self-start characteristics. In a hypersonic inlet, large-scale separated flow is easily induced when intense lip shock compacts into the opposite boundary layer. The status of the inlet then becomes unstart, which is the most conventional unstart mode [13; 14; 15; 16]. The oscillation of the unstart mode is stronger than that of the start mode, which is affected by factors including the boundary layer [17] and back pressure [18]. Self-sustained separation near or in the inlet is considered to be the main reason for the maintenance of the unstart flow mode, and self-start may be achieved by breaking that separation [19]. Based on the relative position between the lip and the separation near or in the inlet (including the resulting separation shock), some scholars have classified the separation into two types: opened separation and closed separation, of which the opened separation is much easier to self-start [20; 21]. Regarding the conventional unstart mode, at most times, inlet self-start can be completed through increasing the Mach number [22; 23; 24]. Because decreasing the angle of attack (alpha) increases the Mach number in front of the inlet, self-start can also be achieved through this process [25; 26]. Furthermore, if unstart is caused by excessive back pressure [27; 28] and other factors allow the inlet to self-start, self-start can be achieved after the back pressure is released [29; 30]. Many advances have been made in the study of the forecast of inlet self-start in recent years. [PERSON] developed the inlet self-start formula for the hybrid compression hypersonic inlet, obtaining results that approximate the Kantrowitz limitation [31]. Based on that research, [PERSON] and [PERSON] further studied the influence of external compression and flow spillage on the ultimate contraction ratio of 2D inlets, which determines the working state (start or unstart) [12; 32]. [PERSON] established a self-start prediction model in the state of \"hard unstart\" flow mode for a 2D hypersonic inlet [33]. A special unstart mode might be induced when the actual inflow Mach number exceeds the designed Mach number [34; 35]. This kind of flow mode is called \"overrated unstart\". In overrated status, the shocks and compression waves produced by the forebody should have gathered in the leading edge of the lip impact the inner surface of the lip.