Reference-free digital shadowgraphy using a moving BOS background

Abstract

A method of measuring density second derivative (shadowgraph) as a variant of background oriented schlieren (BOS) is presented. Pairs of images of a moving background with a small time difference are analysed using moving-window crosscorrelation, giving the second derivative of density in the axis of background travel. The advantage over standard BOS is that no reference image is required. In contrast to the existing "reference-free BOS" method, no double-image is produced, making the data interpretation simpler. The sum-of-correlation (ensemble correlation) method can be used on an image series, where the flow is constant over the series time, to significantly reduce the size of the cross-correlation window. The reduction in the window size and sharpening of the resulting image is similar to that achieved by optical flow, but this method is easily automatable, without knowledge of the flow, and can be used with any existing BOS analysis software.

Improvement in systematic error in background-oriented schlieren results by using dynamic backgrounds

“…Unfortunately, the approaches of Schroder and Cozzi increase the complexity of the measurement setup by implementing moving parts. A similar setup can also be used to improve and conduct reference free digital shadowgraphy (RFDS) (Gardner et al 2020).…”

Extraction of a Weak Flow Field for a Multi-Rotor Unmanned Aerial Vehicle (UAV) Using High-Speed Background-Oriented Schlieren (BOS) Technology

“…To extract the flow field structure of the weak wind object accurately, an appropriate background plate design (including background contrast, number of points, and point size) and the layout of the background plate and camera were two important factors. At present, different experiments have their own appropriate experimental background pattern and layout setups [36]. However, these setups should satisfy the following basic requirements: (1) The background pattern should have a clear contrast, such as black dots on a white background or white dots on a black background.…”

Background-oriented schlieren technique for two-dimensional visualization of convective indoor air flows

“…Furthermore, several approaches show that BOS can be combined with other techniques, e.g. BOS velocimetry (Settles 2010), diffractive optical element based BOS (Rajshekhar and Ambrosini 2018;Ramaiah et al 2020), or reference-free digital shadowgraphy using a moving BOS background (Gardner et al 2020). The accuracy of BOS has been analyzed and optimized showing that the dynamic range is approximately 50:1; measurement errors amount to 2-3 % in full range (Raffel 2015).…”