Investigation of large-scale structures in supersonic planar non-reactive shear layer with a base flow region

Abstract

Large-scale structures in supersonic planar non-reactive shear layer with a base flow region are experimentally investigated. By utilizing the recently developed nanoparticle-based planar laser scattering (NPLS) method, the fine transient streamwise and spanwise flow fields are acquired, and the large-scale structures in the reattachment region, initial turbulent shear region, and developing turbulent shear region are analyzed. The predominant instability is convective for the two separated shear layer, whereas the instability in the base flow zone is still absolute according to the linear instability analysis. From the initial to developing turbulent shear layer region, the predominant instability transited gradually from absolute to convective instability, and the corresponding scales of the large-scale structures turn from about half to whole of the shear layer thickness. The entire streamwise mean flow topology and the streamwise and transverse velocity fluctuations are compared with different pressure ratios and with incompressible blunt trailing-edge shear layer. Linear instability analysis shows further evidence to the instability evolution of the supersonic shear layer with a base flow along the streamwise. It implied that instability induced by unmatched pressure in the wake zone can increase the absolute instability and improve the receptivity to inflow perturbations. The reattachment points are approximated with the intersection points of the reattachment shock pairs, which can be obtained from the transient NPLS images. The distribution of the reattachment points suggests that: the principal axis of the reattachment point distribution is dominated by the high-speed flow, and unmatched pressure increases the perturbation in the wake zone, result in a high absolute instability.