Toward quantitative wall shear stress measurements: a comparative study on the impact of RGB-to-hue conversion algorithms on liquid crystal diagnostics

Abstract

Liquid crystal diagnostics is a capable tool for determining quantitative wall shear stress distributions with high spatial resolution, which can be applied to almost any surface shape. A standard consumer camera is typically used to record the scattered light of the liquid crystals as red, green, and blue RGB data. This RGB data has to be converted to a hue-based color space in order to perform a state-of-the-art calibration procedure. Algorithms for this purpose are numerous in the literature. However, a considerable number of them show a wide range of resulting hue values due to different trigonometric relations. This renders some conversion algorithms unsuitable for calculating physical wall shear stresses, as their magnitude and distribution depend on the conversion algorithm used. For this reason, the choice of an inappropriate conversion algorithm may compromise the measurement accuracy and subsequent comparability significantly. The main objective of this paper is to give recommendations for the use of appropriate algorithms to determine physical wall shear stresses. In a first step, synthetic liquid crystal data is converted using algorithms described in the literature. The preselected algorithms are then applied to liquid crystal data from a flat plate wind tunnel experiment to illustrate their influence on the determined uncalibrated wall shear stress distribution. The final discussion serves as guidelines for the post-processing of liquid crystal data and their subsequent comparability.