Development and Design of 2D and 3D Setups for the Improvement of Tomographic Background Oriented Schlieren Reconstruction Method

Abstract

The Background Oriented Schlieren Method is a measurement system suitable for every kind of flow with variations in the refractive index of the medium. The last ones can be caused by compressible effects, temperature gradients, density changes or mixing of different phases. Since quantitative information can be obtained, the method is able to detect large fluid structures as well as small turbulent ones; anyway there is the necessity to study the inaccuracies in the procedure. Moreover, there has to be the possibility, for a given experimental setup, to give at least a first estimate of the error “before doing the measurement”. That is, if someone wants to use the Background Oriented Schlieren Method as a “validating” measurement system (“au pair” of other more complex measurement chains), he should be able not only to give some information about the error bar connected with the extracted values, but to predict (given a known setup) which will be the amount of the final error. That is way it has been created a simple 2D theoretical model and extended it to more complex 3D cases, trying to give a reasonable estimate of the inaccuracies in the measurements. The attempt to prove the theoretical simple model is successful in the 2D images error evaluation, while should be taken with care if used with 3D Backprojection algorithms, since they create strong artifacts. That is why it can be suggested to pass from Anti-Radon Transform to ART algorithms, or to improve the first with different formulations.