Quantitative estimation of density variation in high-speed flows through inversion of the measured wavefront distortion

• Published in: Optical engineering 2014-12, Vol.53 (12), p.124107-124107
• Main Authors: Medhi, Biswajit, Hegde, Gopalkrishna Mahadeva, Reddy, Kalidevapura Polareddy Jagannath, Roy, Debasish, Vasu, Ram Mohan
• Format: Article
• Language: English
• ISSN: 0091-3286; 1560-2303
• DOI: 10.1117/1.OE.53.12.124107

A simple method employing an optical probe is presented to measure density variations in a hypersonic flow obstructed by a test model in a typical shock tunnel. The probe has a plane light wave trans-illuminating the flow and casting a shadow of a random dot pattern. Local slopes of the distorted wavefront are obtained from shifts of the dots in the pattern. Local shifts in the dots are accurately measured by cross-correlating local shifted shadows with the corresponding unshifted originals. The measured slopes are suitably unwrapped by using a discrete cosine transform based phase unwrapping procedure and also through iterative procedures. The unwrapped phase information is used in an iterative scheme for a full quantitative recovery of density distribution in the shock around the model through refraction tomographic inversion. Hypersonic flow field parameters around a missile shaped body at a free-stream Mach number of 5.8 measured using this technique are compared with the numerically estimated values.