Formation and evolution of streamwise vortices in a rectangular-like to circular shape transition inward turning inlet

The complex flow field in a rectangular-like to circular shape transition inlet is investigated to reveal the formation and evolution mechanisms of streamwise vortices. Numerical simulations combined with tomography-like planar laser scattering are conducted at a Mach number of 6. The effects of the sharp/blunt leading edges and with/without suction are also investigated based on Liutex vortex identification method. The results show that in the inlet with sharp leading edge and suction, the cowl shock sweeps the boundary layer on the cowl wall and the sidewall, inducing two corner vortices in turn, which gradually merge into a stronger streamwise vortex in the isolator. In the inlet with blunt leading edge and suction, the rotational flow generated from the swept blunt leading edge converges at the corners of the external compression section to form corner vortices, while also converging near the symmetry plane to form a large-scale counterrotating vortex pair. In the internal compression section, the cowl shock first sweeps the low-momentum flow rolled-up by the corner vortex, and then incident on the upstream counterrotating vortex pair, which induces new streamwise vortices with the opposite direction of rotation to that of the upstream vortices. In isolator, the corner vortices and the counterrotating vortex pair dominates the flow without merging. The structure and strength of the streamwise vortices in the inlet with blunt leading edge and with/without suction are basically the same, indicating that the suction in the internal compression section has few effects on the flow field uniformity in the inlet with blunt leading edge. Due to the influence of the counterrotating vortex pair in the external compression section, the inlet with blunt leading edge exhibits a 26.2% decrease in the maximum Liutex vortex flux of the streamwise vortices in the isolator and a 9.0% decrease in the total pressure recovery coefficient on the exit compared to the inlet with sharp leading edge. The effect of the bluntness of the leading edge should be emphasized when designing and evaluating the performance of an inlet. The corresponding flow control strategy should focus on suppressing the counterrotating vortex pairs derived from the external compression section.