Supersonic backward-facing step flows are characterized by strong separation and unsteady shear layers, whose control remains a challenging problem. The shear-induced breakdown of vortex-generator-induced wakes in supersonic separated flows is investigated with a focus on the effect of spanwise width. Experiments are conducted at Mach 3 using arrays of ramp vortex generators (VGs) with identical height and streamwise length, but different spanwise widths (12, 18, and 24 mm) installed upstream of a backward-facing step (BFS). High-resolution flow visualization is performed using nano-tracer planar laser scattering (NPLS), enabling direct observation of the near-field vortex structures and their coupling with the shear layer. Based on the NPLS images, intermittency analysis and fractal dimension statistics are employed to characterize the differences and connections in the wake evolution quantitatively. The results show that vortex generators with smaller spanwise width produce weaker initial shear, allowing the shear layers to remain largely independent and resulting in a more coherent, stable wake with greater penetration into the freestream. In contrast, increasing spanwise width leads to stronger shear, enhanced coupling between shear layers, and intensified shear-induced breakdown and mixing, which drive the wake toward the wall and hinder its intrusion into the freestream. Despite these differences, both edge frequency mapping and intermittency analysis consistently confirm a new discovery that the upper wake boundary remains nearly unchanged across all cases, suggesting that the penetration limit of the VG wake into the free stream is not governed by spanwise width.
Hu et al. (Wed,) studied this question.