Areal measurements of surface shear stress vector distributions using liquid crystal coatings

For normal white-light illumination and oblique above-plane observation, liquid crystal coating (LCC) 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 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 rotatable three-chip color video camera, linked to a frame grabber and computer, was devised to test the method. Full-surface color images of LCC response to a three-dimensional turbulent wall jet flowing over a planar surface were acquired and converted to surface shear stress vector fields. Comparisons of these first-ever such data sets to conventional point measurements, obtained via the oil-drop interferometry technique, yielded very good agreement, thereby validating the new method. Full-field vector resolution to ±5 percent in magnitude and ±3 degrees in orientation is achievable. Extension of the technique to curved surfaces is under investigation. a Senior Research Scientist, Fluid Mechanics Laboratory Branch; Associate Fellow, AIAA. 4 Research Scientist, MCAT Inc. c NASA-Ames Graduate Student Researcher, Stanford University. d Research Scientist, Fluid Mechanics Laboratory Branch. e Systems Engineer, Data Analysis Branch. •^ Computer Engineer, Data Analysis Branch. 3 Software Specialist, Sterling Software Co. Copyright 1996 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for government purposes. All other rights are reserved by the copyright owner. Nomenclature D jet exit diameter H hue HSI hue, saturation, intensity LCC liquid crystal coating n index of refraction of oil drop PO wall jet stagnation pressure on the exit plane centerline RGB red, green, blue s fringe spacing t time oil drop exposed to flow V velocity x axial coordinate on test surface measured from jet exit plane y transverse coordinate on test surface measured from jet centerline a above-plane view angle, measured positive upward from zero in plane of test surface A angles A wavelength of light H viscosity of oil circumferential angle in plane of test surface, measured positive counterclockwise from origin at jet centerline (j>T orientation of surface shear stress vector directed away from observer with an in-plane line of sight at = T r average shear stress vector orientation