NEW METHODOLOGY FOR THE MEASUREMENT OF SURFACE SHEAR-STRESS VECTOR DISTRIBUTIONS

When a liquid crystal coating (LCC) is illuminated from the normal dir ection by white light and observed from an oblique above-plane view an gle, its color-change response to shear depends on both sheer stress v ector magnitude and the direction of the applied shear vector relative to the observer's in-plane line of sight. At any point, the maximum c olor 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 wi th shear stress magnitude, Based on this knowledge, a full-surface she ar stress vector measurement methodology was formulated. An image-base d system, incorporating a three-chip color video camera, linked to a f rame grabber and computer, was devised to test the method, Full-surfac e 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, Comparisons of these fir st-ever such results to conventional point measurements, obtained via the oil-drop interferometry technique, yielded very good agreement, co rroborating the new method, Based on present results, it has been show n that the liquid crystal coating technique can be utilized to obtain areal measurements of surface shear stress vector distributions on pla nar surfaces with accuracies equivalent to conventional point-measurem ent techniques.