Conditional Measurements of Optically-Aberrating Structures in Transonic Attached Boundary Layers

Abstract

This paper is a further extension of an investigation of the optical distortions caused by a transonic, attached, turbulent boundary layer reported in earlier paper. Simultaneous, time resolved x-wire, surface pressure and optical measurements were performed to investigate the physical mechanism of the optical aberrations. A conditional sampling algorithm, triggered on local minima in the optical path difference, was implemented to extract the flow features of optically aberrating flow structures. The conditionally averaged surface pressure showed a pressure well associated with optically aberrating flow structures. A technique to identify coherent vortical structures is applied to the conditionally averaged velocity field, and it shown that a large scale, coherent, vortical structure resides over the optical measurement location. The conditionally averaged vortical structure extends ~δ* in the streamwise direction and ~1.2 δ* in the wall normal direction, where δ* is the boundary layer displacement thickness.