Enhancement of synthetic schlieren image resolution using total variation optical flow: application to thermal experiments in a Hele‑Shaw cell

Abstract

We present an improvement to the standard synthetic schlieren technique to obtain the temperature distribution of a fluid inside of a Hele-Shaw cell. We aim to use the total variation L1-norm optical flow method to treat experimental images and to obtain quantitative results of the development of thermal convection inside a cell, by detecting the gradients of the optical refractive index. We present a simple algorithm to set the optical flow parameters, which is based on the comparison between the optical flow output and the result obtained by digital PIV using the structural index metric. As an example of the application of the proposed method, we analyze laboratory experiments of thermal convection in porous media using a Hele-Shaw cell. We demonstrate that the application of the proposed method produces important improvements versus digital PIV, for the quantification of the gradients of the refractive index including the detection of small-scale convective structures. In comparison with correlation-based digital methods, we demonstrate the advantages of the proposed method, such as denoising and edge capture. These features allow us to obtain the temperature, for this experimental setting, with better image resolution than other techniques reported in the literature.