Hypersonic flight within the atmosphere is of current national interest, both military and civilian. Numerous challenges are associated with hypersonic flight. Two important technical challenges that involve the use of jets into a high-speed flow are propulsion and reaction control. The flow structure of perpendicular injection of an under-expanded, which is the prevalent operating condition for both scramjet and reaction control applications, sonic or supersonic jet into a supersonic freestream has been thoroughly documented. In summary, the first flow feature, progressing in the streamwise flow direction, is the interaction or bow shock produced as the freestream impacts the injection stream tube. For injector configurations, where is on the order of one or more, a separation region and a lambda shock form upstream of the injector port. A horseshoe vortex forms between the jet and the interaction shock. After entering the freestream, the underexpanded jet undergoes a rapid Prandtl-Meyer expansion surrounded by a barrel shock. A shock wave normal to the jet path, known as the Mach disk, terminates the barrel shock, and compresses the flow to the effective back-pressure. Downstream of the Mach disk, a counter rotating vortex pair forms within the jet plume. Wake vortices are also formed downstream of the injector, between the plume and the floor. The plume vorticity and turbulent mechanisms induce the required large scale mixing between the jet fluid and the freestream.
Numerical simulations were performed to study the flow fields generated by transverse sonic injection through diamond and circular injectors into a Mach 5.0 freestream flow. The simulations were performed using the SST RANS and DES turbulence models in Cobalt. A sample grid used for these simulations are shown in Fig. 1. Apart from providing a high resolution result of the flow field, the DES model provided a basis for the evaluation of the performance of the RANS model in predicting this flow field. All of the numerical results were compared against available experimental data and the performance of the models was found to be adequate.
A detailed characterization of the shock and vortex structures in the flow was performed for both a diamond and circular shaped injector. For the diamond part, two new vortex structures were identified. The leading edge mixing mechanism was prominent in the diamond injector case and has the benefit of increasing fuel/air mixing. The circular injector also generated a leading edge mixing mechanism with a different configuration. The lateral counter-rotating vortex pair was observed only in the diamond injector configuration and has the potential to act as a gasdynamic flame holder. These new structures were observed in both RANS and DES simulations (time-averaged and instantaneous). The near field flow structure around the diamond injector is shown in Figs. 2a and b. The different colors of the streamlines highlight the various flow features.
Large scale structures were observed in the plume/wake region of the flow field for both injector geometries (Figs. 3 and 4). Numerical shadowgraph plots of the lateral view and the top view of the diamond injector case show structures that are more organized as compared to the circular injector case. Multiple horseshoe vortex pairs can be observed in both injector simulations (see Fig. 5). An animation of the plume structure along a lateral plane was obtained by using data from 500 consecutive time steps. This is shown in Video 1. For these flow fields, it was found that the RANS model performance was adequate when compared to the averaged DES results.
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Figure 1: Near field structures of diamond injector.
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Figure 2: Z=0 plane showing grid.
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Figure 3: Z=0 plane showing shadowgraph contours for diamond injector.
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Figure 4: Z=0 plane showing shadowgraph contours for circular injector.
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Figure 5: Horseshoe vortex structure of diamond injector.