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Journal of Flow Visualization and Image Processing

ISSN Print: 1065-3090
ISSN Online: 1940-4336

Journal of Flow Visualization and Image Processing


27 pages

A NEW ALGORITHM FOR ANALYZING SHADOWGRAPH IMAGES

John C. Patterson
School of Civil Engineering The University of Sydney Sydney, NSW 2006, Australia
G. B. Brassington
College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Admin. Bldg., Corvallis, OREGON 97331, USA
M. Lee
School of Engineering, James Cook University

ABSTRACT

A new algorithm for constructing shadowgraph images from approximate density fields is presented with the primary motivation of performing accurate laboratory shadowgraph analysis. Available image construction algorithms are noisy and produce discontinuous errors even for small gradient density fields. Discontinuity errors are serious, being indistinguishable from real optical focusing which occurs frequently. The new algorithm completely eliminates these errors. Image improvements are demonstrated for realistic synthetic refractive index fields. Favorable comparisons of the new algorithm are also demonstrated with laboratory shadowgraph of natural convection flows in a cavity which feature large density gradients. A second motivation of the paper is to accurately analyze approximate shadowgraph images derived from a linearized analytical model for refraction. The linearized shadowgraph images are correlated with the artificial shadowgraph images of the new algorithm. Preliminary results indicate that quantitative information from shadowgraph images of larger gradient density fields could be obtained by iterating about the linear solution.