Experimental investigation on the hypersonic boundary layer transition induced by side-by-side cylinders

The flow-field structure and heat-flux distribution around the isolated cylinder and side-by-side cylinders on the flat-plate at different angles-of-attack (AoA) and free stream conditions are investigated in a Mach 6 low noise wind tunnel, using nano-tracer-based planar laser scattering (NPLS) techniques and the temperature sensitive paint techniques. The results indicate that the interaction of the bow-shaped shock waves between adjacent cylinders generates an “X”-shaped high-heat-flux region, but this high-heat-flux region only appears near the intersection of the shock waves. Downstream of the shock wave intersection, due to the formation of low-density vortices structures between adjacent cylinders, a low-heat-flux region appears. Further downstream, the boundary layer reattaches, resulting in reattachment shock waves and high-heat-flux strips appearing on the wall surface. The NPLS results show that the horseshoe vortices/bow shock wave interaction between parallel cylinders will cause the position of the shock wave formed after the intersection to move forward and increase the thickness of the boundary layer and the number of low-density vortices structures, but the position of the reattached shock wave hardly changes. As the AoA decreases, the length of the low-heat-flux region between adjacent cylinders increases, and the reattachment position of the boundary layer shifts backward. Notably, the mutual influence between parallel cylinders mainly occurs in the outer horseshoe vortices region, while having a relatively small impact on the streamwise vortices directly behind the cylinders.