# Influence of Incident Shock on Fuel Mixing in Scramjet
## 4 Results and Discussion
### JET Flow Structure  
The high-speed airflow encountered the jet stream from the injection hole, creating a phenomenon akin to a \"wall of resistance\", resulting in a reduction in flow speed and an increase in pressure. This high-pressure environment, together with the low-pressure area upstream of the injection hole, formed a significant adverse pressure gradient, facilitating the formation of a recirculation zone upstream of the injection. As shown in Figure 18, the label \"S\" indicated the starting point of separation, and we could see that in case 6, the separation point was located further upstream due to the lower pressure before injection. Conversely, in case 5, since the injection was situated in a localized high-pressure area, the separation region prior to the injection was relatively smaller. This indicated that the pressure environment where the injection was located had a significant impact on the flow field characteristics nearby. During the injection process, fuel was discharged from the injection hole, leading to a swift reduction in both velocity and pressure on the leeward side of the jet [40]. Owing to the closeness of the injection hole to the leading edge of the cavity, the entire flow structure within the cavity was markedly affected by the injection. The shear layer that formed from the injection intermixed with the cavity's recirculation zone, moving upwards towards the core flow and consequently enlarging the area of recirculation. When compared with the cold flow cases, the recirculation zone shifted closer to the core flow and further toward the cavity's leading edge. Such alteration enhanced the conveyance of fuel to the upper wall region, ensuring earlier fuel entry into the cavity [15]. In case 5, it is notable that the boundary layer at the leading edge of the cavity experienced separation. This detached region subsequently merged with the cavity's internal recirculation zone, engendering a  
Figure 16: Mass-weighted average pressure distribution. Figure 17: Pressure distribution of the lower wall surface. pronounced region of reverse flow anterior to the cavity. This, in turn, led to the elevation of the jet shear layer proximal to the core flow. To elucidate this phenomenon, the study scrutinized it through the lens of shock wave structure. Figure 19 depicted a three-dimensional representation of the jet shock structure, delineated by iso-surfaces colored according to density, utilizing a density gradient of 5 to demarcate the shock waves. The z = 0 m plane denoted the central longitudinal section of the jet, while the z = 0.042 m signified the interaction plane where the bow shock waves from two jets intersected. Figure 20 provided an illustration of the shock waves and velocity contours at the z = 0 m and z = 0.042 m planes, with black solid lines signifying the u = 1000 m/s contour lines. In case 4, the pronounced separation region ahead of the injection orifice induced a separation shock with a relatively gentle slope. This separation shock engaged in an interactive process with the bow shock and was concurrently influenced by the incident shock, resulting in a diminished intensity and reduced angle of inclination for the bow shock. As illustrated in Figure 15, the non-uniformity of the flow cross-section resulted in an elevated pressure distribution near the upper wall surface before the bow shock, which consequently had a significant impact on the shape of the bow shock. It is notable that at both the z = 0 and z = 0.042 planes, the initiation angle and starting position of the bow shock were highly comparable, closely resembling the characteristics of a two-dimensional profile, as shown in Figure 19. In case 5, the reduced size of the separation region caused by the jet led to a more pronounced inclination of the separation shock; due to the similar angles between the separation shock and the bow shock, they rapidly merged.