Measurement of Surface Shear Stress Vectors Using Liquid Crystal Coatings

摘要

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 observers 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 A, as 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.