Measurement of surface shear stress vectors using liquid crystal coatings

Abstract

Under normal white light illumination and oblique observation, liquid crystal coating (LCC) color-change response to shear depends on both shear stress magnitude as well as the direction of the applied shear relative to the observer's line of sight. These color-change responses were quantified by subjecting a LCC to a wall-jet shear flow and measuring scattered-light spectra using a fiber optic probe and spectrophotometer. At any fixed shear stress magnitude, the maximum color change was measured when the shear vector was aligned with and directed away from the observer; changes in the relative in-plane view angle to either side of this vector/observer aligned position resulted in symmetric Gaussian reductions in measured color change. Based on these results, a surface shear stress vector measurement methodology, involving multiple oblique-view observations of the test surface, was formulated. Under present test conditions, the measurement resolution of this technique was found to be ±1 deg for vector orientations and ±5% for vector magnitudes. An approach to extend the present methodology to full-surface applications is proposed. Nomenclature Cf = skin friction coefficient D = jet exit diameter / = spectral scattering intensity N = number of data points used in curve fits or in datareduction technique V = velocity W = white light X, Y = chromaticity coordinates a = above-plane view angle, measured positive upward from zero in plane of test surface