Faculty of Mechanical and Energy Engineering, Shahid Beheshti University (SBU), Bahar Blvd., Hakimieh, Tehran, Postal Code 16589-53571, Iran
Background-Oriented Schlieren (BOS) is a non-intrusive and non-destructive technique for measuring density gradient in transparent media. Given the dependency of the refractive index and density of a medium, its static density ﬁeld can be determined as a secondary measurand from BOS measurements. For an ideal gas, if the pressure variation is negligible, the static temperature ﬁeld can be determined as a tertiary measurand. While the BOS technique for far-ﬁeld measurements has been extensively researched, the implementation of this technique for near-ﬁeld measurements still needs to be investigated. Near-ﬁeld BOS is especially important for the measurement and visualization of internal ﬂows, such as turbomachine ﬂows, in which access to ﬂow passage is limited and might be feasible using endoscopes (borescopes). This paper presents the features and challenges of the endoscopic BOS technique and presents methods and solutions for the application of this method in near-ﬁeld measurements. The features of endoscopic BOS records as well as the comparison of endoscopic BOS and BOS measurements with camera are presented.