Systematic Application of Background-Oriented Schlieren for Isolator Shock Train Visualization

Abstract

The background-oriented schlieren technique is applied to visualize Mach 2.5 isolator shock train structures in 3.0- and 6.0-aspect-ratio rectangular cross-section ducts. To optimize the background pattern, a parametric study is performed by using a 10 deg compression ramp calibration flowfield producing a reference density gradient. In conjunction with a multigrid cross-correlation algorithm, the optimized background pattern adheres to three classic particle image velocimetry design rules: 1) interrogation window width four times the expected particle displacement, 2) magnified particle diameter between 2 and 5 pixels, and 3) image density greater than 10. A modified -factor analysis and qualitative observations of the density gradient range resolved suggest an optimal image density of 75 and a magnified particle diameter range of 2–5 pixels. Background-oriented schlieren visualization of the primary shock train element in both aspect ratio ducts shows the strongest density gradients to be located in the refracted shock region between the oblique shock intersection and the primary normal shock. A previous study confirms that this region is characterized by an increasing shock angle and Mach number as it approaches the duct centerline.