Application of the background-oriented schlieren technique to visualize indoor airflows

Abstract

This study focuses on further developments and the application of the background-oriented schlieren (BOS) technique to visualize indoor airflows. It is divided into two parts. First, the BOS setup and its specifications are introduced. Since indoor airflows are usually large-scaled and defined by very small density gradients and small velocities, the setup needs to have a high sensitivity, high resolution, and a large field of view. The setup consists of a high resolution (8688  5792 pixels) DSLR camera Canon EOS 5DS R, a background structure of random black pixels (edge length 0.22 mm, density 25 %) on white ground, and two flashlights ELINCHROM BRX 500 (each 500 Ws max.) illuminating the background. For image analysis, the cross-correlation-based program PIVview2C is used. Additionally, the sensitivity and possibilities of the developed setup are presented. Using the settings described, it is possible to visualize temperature gradients of ΔΘ<0.3 K and virtual image shifting of Δy<0.1 pixels. In the second part, the developed BOS setup was used to visualize the spread of breathing air two-dimensionally when playing wind instruments and singing. Here, various brass and woodwind instruments, as well as the human voice, were studied to assess the spreading range of the breathing air when playing different notes, scales, or musical pieces at different pitches, with different dynamics and articulations. Additionally, velocities of the escaping airflows were measured to assess their extent and to validate the BOS evaluations.