An assessment of a bi-telecentric visualization system for schlieren imaging in high-speed wind tunnels

Abstract

Bi-telecentric visualization systems, recently popularized for machine vision applications, share many similarities with traditional schlieren systems for flow visualization in high-speed wind tunnels. The intent of this work is to assess the feasibility of employing an off-the-shelf bi-telecentric system for schlieren-like imaging. Through benchtop experiments, it is found that a bi-telecentric system behaves equivalently to a schlieren system having a circular cutoff with a diameter larger than the focused beam (thus requiring a finite deflection angle before registering a response). By introducing a slight angular misalignment, however, the response curve of the system can be shifted to provide a finite response about zero deflection, though at the expense of a nonuniform background. This allows the visualization of even weak flow structures in wind tunnel experiments. The sensitivity of the telecentric system is approximately one-third that of a comparable Z-type schlieren; nevertheless, it exhibits a reduced depth of field (minimizing background disturbances), occupies a much reduced physical footprint, and is simpler to align. It is thus proposed that a bi-telecentric system with a modified aperture stop could provide a practical alternative for the visualization of high-speed flows.