Planar laser scattering visualization of streamwise vortex pairs in a Mach 6 flow

A series of cross-sectional flow fields of Counterrotating Vortex Pairs (CVPs) generated by a large-scale ramp vortex generator is observed using an ice-cluster-based Planar Laser Scattering (PLS) method in a shock tunnel with a nominal flow Mach number of 6. Combined with a numerical simulation, two streamwise CVPs with opposite rotating directions are identified in the wake flow of the vortex generator with an absence of a boundary layer, namely, a Primary CVP (PCVP) and a Secondary CVP (SCVP). The wake flow is divided into two stages with different features of the PCVP and SCVP. In Stage Ι, the PCVP and SCVP gradually mature, and the flow is relatively stable. In Stage Ⅱ, the PCVP and SCVP depart from each other, and the flow becomes unstable. The profiles of the transverse velocity in the spanwise symmetry plane induced by the PCVP and SCVP do not obey the scaling law of CVPs immersed in the boundary layer. A new scaling law is proposed, in which the transverse distances between adjacent saddle points in the cross-sectional flow field are used as the characteristic lengths for the PCVP and SCVP. After this new scaling procedure, the profiles of transverse velocity induced by the PCVP and SCVP at different streamwise locations collapse well. Moreover, the PLS images show that the mixing between the CVPs and the outside high-momentum flow becomes evident at approximately 5.5 times the height of the vortex generator, which is earlier than that immersed in the boundary layer. These findings enrich the knowledge of CVPs in the hypersonic regime, especially in the absence of the boundary layer.