The principle and characteristics of an image fibre Background Oriented Schlieren (Fibre BOS) technique for time-resolved three-dimensional unsteady density measurements

An image fibre Background Oriented Schlieren (Fibre BOS) technique has an advantage in cost-effectiveness as well as portability and flexibility for a time-resolved quantitative three-dimensional unsteady density measurement. The reason why the Fibre BOS technique has these advantages is that a transparent medium can be visualized at several projection angles, which are required for a tomographic reconstruction, using the flexible optical image fibres instead of expensive high-speed cameras, simultaneously. The basic and demonstration experiments were conducted to investigate the performances of the Fibre BOS technique in this study. The estimation accuracy of a deflection angle is increased when a background-dot size is more than twice as large as a core which is the component of the image fibre if the core size is larger than a pixel size on an image sensor. Multiple images going through the image fibres from several projection angles are simultaneously captured using a single camera, and temporal variation of an unsteady complex flow is measurable. The Fibre BOS technique has a potential to conduct the high time-resolved three-dimensional unsteady density measurement using a single high-speed camera.