# Experimental Study of the Formation and Evolution of Gas Jets in Supersonic Combustion Chambers
## 3 Results
### Discussion on Instantaneous Flow Field Structure  
Figure 7 shows the instantaneous flow field structure captured during Test 06, evidencing a clear typical flow field structure that includes the following features: (a) Bow shock (labeled 1), which is formed in the upstream of the nozzle by the obstruction effect of the transverse jet on the mainstream. (b) Separation shock (labeled 2) is produced by the adverse pressure gradient along the wall, which causes the separation of the boundary layer. (c) Barrel-like shock wave (labeled BS) and Mach disk (labeled MD) at the outlet position, formed by the jet expansion and acceleration in the supersonic inlet flow. (d) The latter also produced a reflected shock wave (labeled RS) at the end of the barrel-like shock wave. (e) Shear vortex (labeled SV) on the windward shear layer of the jet, produced by the Kelvin-Helmholtz instability resulting from the velocity shear between the jet and the surrounding gas. In addition, the angle (\(\beta\)) of the bow shock and the deflection angle (\(\theta\)) of the mainstream are also marked. The influence of jet angle and jet momentum flux ratio on the flow field structure was qualitatively demonstrated by comparing the instantaneous flow structure diagram under  
Figure 6: Schematic diagram of the image processing technique used to obtain the mean temporal characteristics of the flow field measured during the experiment. Figure 7: Transient flow field structure of Test 06.  
various working conditions, see Figure 8. The images in each row of Figure 8 have the same jet angle, while each column presents the same jet momentum flux ratio. The legend gives information on the test ID, jet angle and jet momentum flux ratio. In addition, the scale grid in the images of Figure 8, and all the images in the rest of the paper, have a step size (given by the purple or grey rectangles) of d.  
Compared with the momentum flux ratio, the jet angle had a greater influence on the windward shear vortexes in the flow field, such as scale, quantity, and spatial distribution. The shear vortices in the jet flow field with an acute angle were smaller, more numerous, and had a wider spatial distribution range of the flow direction compared with an obtuse angle. In addition, a shear vortex coherent structure was formed in the shear layer for the acute angle jet field. This originates from the intermittent collision between the high-speed mainstream and the relatively low speed of the jet boundary. It is concluded that the velocity difference between the jet flow and the local mainstream is the main factor affecting the size of the shear vortex. Moreover, the evolution of the windward shear vortex is mainly influenced by the compressibility effect of the shear layer. It can be inferred that, when an acute angle injection is adopted, the jet has little influence on the main flow obstruction, and the mainstream after the bow shock still maintains supersonic flow, which leads to longer spatial distance for shear vortex dissipation in the flow field. By comparing the instantaneous flow field diagram of the images in each row of Figure 8, it can be seen that an increase of the jet momentum flux ratio improves the jet penetration depth, and also raises the lower edge of the jet wake. This effect favors fuel mixing and combustion. On one hand, it can increase the contact area between jet and main stream, improving mixing efficiency. On the other hand, the high temperature area can be located in the center of the flow passage, relaxing the requirements on the chamber wall thermal protection. By comparing the instantaneous flow field diagram of each column in Figure 8, it is evidenced that a change of the jet angle produces significant differences in the jet wakes. As the jet angle increased, the shear layer between the jet and the main stream decreased, as did the spatial distribution range of the shear vortex. In addition,  
Figure 8: Instantaneous flow field diagrams for different working conditions (see the legends).