Practical aspects of designing background-oriented schieren (BOS) experiments for vortex measurements

Abstract

Density-based schlieren methods are a well-known and well-established tool for studying flows. A representative of this category that has become increasingly important in recent years is the background-oriented schlieren method (BOS). Like other schlieren techniques, the BOS method is based on a changing refraction of light rays due to spatial density gradients in the flow field. Schlieren objects are imaged, usually with a digital camera, in front of a structured background pattern, which results in displacements (ud, vd) of the pattern in the image plane. The camera is focused on the background, meaning that the schlieren object itself is recorded with geometric blur. The background shift is evaluated by a moving-window cross-correlation between the measurement image and an undisturbed reference image with no density gradient between camera and background.