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A Detached Eddy Simulation (DES) study of a low momentum flux ratio jet, J=(ρu 2) jet/(ρu 2)∞= 0.35, in the HyShot II scramjet system is carried out. The flow structure near the injector, shock pattern in the symmetry plane as well as the instantaneous coherent structures are presented and explained in the paper. Different from most previous studies, over-expanded and under-expanded flow states occur simultaneously at the exit of the current jet porthole. The shock system near the injector is therefore a combination of a detached normal shock and a small three-dimensional barrel shock terminated by a Mach disk. Further insight into the flow physics is conducted by visualizing instantaneous coherent structures. The formation of Ω-shaped vortices, which was observed in experiments previously, but never well-studied numerically, is discussed in detail. A new understanding of the key flow physics and mixing …