# Experimental Study of the Formation and Evolution of Gas Jets in Supersonic Combustion Chambers
## 3 Results
### Spatiotemporal Evolution of the Flow Field  
Based on the understanding of the instantaneous flow field structure, three typical test conditions are selected in this section to study the spatiotemporal evolution of the flow field, and the influence of the jet angle on the shear vortex. Figure 9 shows the evolution of the shear vortices under different working conditions. It should be emphasized that due to the complex turbulent structure and limited spatial accuracy of observation, the flow characteristics of individual vortices are not clear. Therefore, the area where vortex exist is used as the research object to discuss the spatiotemporal evolution of the flow field. Label 1 or 2 represents the area where the vortex exists, and for ease of description, it is called shear vortex 1 or 2. The images in each column in the Figure 9 correspond to different time instants, separated by 10 \(\upmu\)s, of the same test. Under the conditions of Test 05, the shear vortex labeled 1 and located above the barrel shock wave at time t0, developed into a bow-like shape. At the time t0 + 10 \(\upmu\)s, the shear vortex 1 further developed downstream. As the velocity of the main stream was higher than that of the shear layer, the part of the vortex that was closer to the main stream (see the red dot) stretched, changing the vortex shape from bow-like to straight line. At t0 + 20 \(\upmu\)s, the shear vortex 1 continuously stretched and was inclined downstream. At t0 + 30 \(\upmu\)s, the vortex began to tear apart due to the velocity difference of each part. On the other hand, during Test 11, the bow-like shear vortex labeled 2 kept its general shape when it developed downstream and uplifted, between t0 and t0 + 10 \(\upmu\)s. At t0 + 30 \(\upmu\)s, shear vortex 2 separated from the shear layer and began to tear. However, its morphology did not tilt downstream within the image field of view. The shear vortex evolution found during Test 08 was in between those explained above for Test 05 and Test 11. The main difference occurred during the stretching stage of the vortex. The  
Figure 9: The evolution of the shear vortices under different working conditions. stretching is caused by the velocity difference of the different sections of the shear vortex, which is produced by the variable velocity field of the main stream in that area. In experiments reported by previous works, the intensity of the bow shock typically decreases along the longitudinal direction. This occurs because the velocity of the main stream after the bow shock is inversely proportional to the strength of the latter. Consequently, the velocity of the main stream after the bow shock increases monotonically along the longitudinal direction, and the shear vortex near the main stream is stretched and tilted downstream. This is also the formation and evolution process observed for shear vortex 1 and 2 in Test 05. However, it can be seen that the intensity (angle) of the bow shocks at the area, delimited by the blue square in Figure 9, was greater than that at the area given by the yellow square. This suggests that the strength of bow shock does not change monotonously along the longitudinal direction. As a consequence, the tensile position of the shear vortex is no longer only the portion closer to the main stream. In order to further illustrate the change of the tensile position of the shear vortex, Figure 10 shows the typical spatiotemporal evolution of the flow field registered during Test 15. Meanwhile, using the position of the bow shock acquired in the test image, the mainstream velocity (U) along the longitudinal direction behind the bow shock at each time in Figure 10 was calculated, as shown in Figure 11. The complete evolution of shear vortex 1 in the flow field can be observed. Multiple extreme values of the bow shock angle result in alternating changes of the mainstream velocity along the longitudinal direction, as represented by the orange line segments in the t\({}_{0}\) + 30 \(\upmu\)s image.