Effect of vortex generator spanwise width on vortex size and aero-optical effects in backward-facing step flow field

Flow control is required to suppress aero-optical effects for optical windows with backward-facing steps. This study investigates the impact of vortex generator (VG) spanwise width on vortical breakdown and aero-optical distortions in a Mach 3 flow field over a backward-facing step, using the nano-tracer planar laser scattering technique. Vortex evolution is quantified through fractal dimension analysis and two-point spatial correlation, while aero-optical effects are evaluated via gray-density calibration and ray-tracing methodology. Experimental results indicate that VG with larger spanwise widths facilitate accelerated vortical structure breakdown. However, excessive spanwise widths of VGs lead to a reduction in the global fractal dimension of the flow field and an increase in vortex size. Enhanced interaction between VG wakes and lower-wall turbulent boundary layers (TBLs) is observed for wider VGs. For spanwise widths of 24 and 18 mm, wake-induced TBL vortex size initially decreases before increasing along the streamwise direction. Time-averaged density fields further reveal a closer proximity of wake structures to the lower-wall TBL with increased spanwise widths of VGs. Optical path difference (OPD) in the computational domain initially increases, then decreases, and eventually stabilizes due to vortex breakdown and TBL/wake development. Notably, the 18 mm VG configuration exhibits a higher root mean square deviation of the OPD than other cases. This parametric study offers insights into optimizing optical window flow control strategies to mitigate aero-optical distortions.