New Methodology for the Measurement of Surface Shear Stress Vector Distributions

Abstract

When a liquid crystal coating (LCC) is illuminated from the normal direction by white light and observed from an oblique above-plane view angle, its color-change response to shear depends on both shear stress vector magnitude and the direction of the applied shear vector relative to the observer's in-plane line of sight. At any point, the maximum color change is always measured when the local shear vector is aligned with, and directed away from, the observer; the magnitude of the color change at this vector/observer aligned orientation scales directly with shear stress magnitude. Based on this knowledge, a full-surface shear stress vector measurement methodology was formulated. An image-based system, incorporating a three-chip color video camera, linked to a frame grabber and computer, was devised to test the method. Full-surface images of LCC color-change response to a three-dimensional turbulent wall jet flowing over a planar surface were acquired and analyzed to define the surface shear stress vector field.