Experimental Study of Supersonic Boundary Layer Transition Induced by Cylindrical Roughness Elements

Abstract

The boundary layer transition process induced by different heights and quantities of the cylindrical roughness elements at Mach 3.0 is investigated experimentally by using the nano-tracer-based planar laser scattering (NPLS) technique. Fine structures of the boundary layers of both streamwise and spanwise induced by the cylindrical roughness elements in the supersonic flow are obtained, and the development of the boundary layer in the roughness element wakes is observed. The boundary layer after reattachment will keep the laminar state within a certain distance, meanwhile hairpin vortexes can be observed clearly during the development of the boundary layer. The fractal theory is used to quantitatively analyze the NPLS images of the boundary layer. This method can achieve the positions of the boundary layer transition in several cases. The research results show that the interaction between multiple roughness elements can inhibit the boundary layer transition and the evolution of hairpin vortexes. However, when the roughness elements on both sides are higher than those in the middle, the interaction between roughness elements has no obvious inhibitory effects on the development of the boundary layer.

Keywords

• roughness elements
• hairpin vortex
• nano-tracer-based planar laser scattering (npls)
• fractal dimension