# Experimental Study of the Formation and Evolution of Gas Jets in Supersonic Combustion Chambers
## 3 Results
### Statistical Properties of the Flow Field
The total number of pixels in the strong fluctuation region was used to quantify it. Figure 16 shows the normalized strong fluctuation region area for different jet angles and jet momentum flux ratios. The blue line represents the mean area for each group of three momentum flux ratios that share the same jet angle. By comparing each line (same jet angle) and each column (same jet momentum flux ratio) in Figure 15, and combining with Figure 16, the following conclusions can be drawn. (a) When the jet angle was obtuse, the flow field fluctuated more violently and the fluctuation area was larger. (b) The fluctuation area was more sensitive to the jet angle. (c) For the same jet angle, the fluctuation area was positively correlated to the change of the jet momentum flux ratio. (d) For the same jet momentum flux ratio, the fluctuation area first increased and then decreased with the monotonic increase of the jet angle. Figure 14: Velocity and deflection angle of the mainstream behind the bow shock. In particular, for the jet angle of \(120^{\circ}\), the fluctuation area was the largest, and the bow shock showed violent fluctuations. According to this analysis, when the jet angle was \(120^{\circ}\), the mainstream velocity after the bow shock was lower compared with \(90^{\circ}\) (see Figure 14), and the jet had a countercurrent velocity. Therefore, the jet stayed in the flow field for a longer time and the shear vortex had more time to move upward. At the same time, the longitudinal momentum of the jet was only reduced by about \(13\%\), and the jet could be lifted upward with a larger longitudinal momentum. As a result, the shear vortex had a wider distribution range in longitudinal space, which also led to the largest fluctuation area when the jet angle was \(120^{\circ}\). The influence of the momentum flux ratio and jet angle on the main flow was quantitatively analyzed in this work. Considering that bow shock is the most direct manifestation of the jet's influence on the mainstream, an average bow shock angle was defined. This  
Figure 16: Area of the strong fluctuation regions for different jet angles and momentum flux ratios. Figure 15: Diagram of flow field standard deviation (fluctuation). 1. The jet angle had a greater influence on the flow field shear vortex than the momentum flux ratio. When an acute jet angle was used, the number of shear vortexes increased, the flow direction spatial range widened, and the vortexes shrunk, mainly due to the gas compressibility. 2. For all the tests run, the shear vortexes were periodic and their evolution had three distinctive stages, namely, formation, stretching, and tearing. However, the tensile positions were different. When the jet angle was obtuse, the structure of the flow field (bow shock, barrel shock wave, and Mach disk) showed fluctuations due to the periodic generation of shear vortexes. 3. When the jet angle was 120\({}^{\circ}\), the fluctuation area of the flow field strength was the largest, and the bow shock also showed violent fluctuations. In between 30\({}^{\circ}\) and 60\({}^{\circ}\), there existed a certain jet angle that made the flow field structure more sensitive to it. The instantaneous image of the flow field obtained in this paper can clarify the formation and evolution of the transverse gas jet, and provide verification data for numerical simulations. Among the tested working conditions, and based on the area of the strong fluctuation region and the average bow shock angle of the flow field, a jet angle of 120\({}^{\circ}\) is recommended. However, this conclusion is only considered from the perspective of flow field mixing. There are other factors, such as the working performance of the scramjet engine, flow loss, combustion heat release, etc., that should be further considered. Based on the research presented here, an experimental study of transverse jet mixing combustion in supercomputational velocity flow field is planned.