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Improved quantitative visualization of hypervelocity flow through wavefront estimation based on shadow casting of sinusoidal gratings

Medhi, Biswajit and Hegde, Gopalakrishna M and Gorthi, Sai Siva and Reddy, Kalidevapura Jagannath and Roy, Debasish and Vasu, Ram Mohan (2016) Improved quantitative visualization of hypervelocity flow through wavefront estimation based on shadow casting of sinusoidal gratings. In: APPLIED OPTICS, 55 (22). pp. 6060-6071.

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Official URL: http://dx.doi.org/10.1364/AO.55.006060


A simple noninterferometric optical probe is developed to estimate wavefront distortion suffered by a plane wave in its passage through 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 continuous-tone sinusoidal grating. Through a geometrical optics, eikonal approximation to the distorted wavefront, a bilinear approximation to it is related to the location-dependent shift (distortion) suffered by the grating, which can be read out space-continuously from the projected grating image. The processing of the grating shadow is done through an efficient Fourier fringe analysis scheme, either with a windowed or global Fourier transform (WFT and FT). For comparison, wavefront slopes are also estimated from shadows of random-dot patterns, processed through cross correlation. The measured slopes are suitably unwrapped by using a discrete cosine transform (DCT)-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 similar to 8 measured using this technique are compared with the numerically estimated values. It is shown that, while processing a wavefront with small space-bandwidth product (SBP) the FT inversion gave accurate results with computational efficiency; computation-intensive WFT was needed for similar results when dealing with larger SBP wavefronts. (C) 2016 Optical Society of America.

Item Type: Journal Article
Additional Information: Copy right for this article belongs to the OPTICAL SOC AMER, 2010 MASSACHUSETTS AVE NW, WASHINGTON, DC 20036 USA
Department/Centre: Division of Mechanical Sciences > Aerospace Engineering(Formerly Aeronautical Engineering)
Division of Mechanical Sciences > Civil Engineering
Division of Interdisciplinary Sciences > Centre for Nano Science and Engineering
Division of Physical & Mathematical Sciences > Instrumentation Appiled Physics
Date Deposited: 08 Oct 2016 07:21
Last Modified: 08 Oct 2016 07:21
URI: http://eprints.iisc.ac.in/id/eprint/54943

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