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 P = relative circumferential-view angle in plane of test surface measured between observer line of sight and shear vector (positive clockwise) AP = pressure difference used to drive jet flow X = wavelength of light A, D = dominant wavelength p = density T = magnitude of surface shear stress vector <| ) = circumferential angle in plane of test surface, measured positive counterclockwise from origin <| ) T = orientation of surface shear stress vector, directed away from observer with line-of-sight at <| ) = (| ) T Subscripts

Windtunnel studies of surface shear stress vector distribution measurement using shear sensitive liquid crystal coatings

“…In this trichromic system, red (R=1, G=0, B=0) was placed at 0°, green (R=0, G=1, B=0) was placed at 120°, and blue (R=0, G=0, B =1) was placed at 240°, respectively. Read et al [8] found that, under normal illumination and fixed above-plane view angle, for a given shear stress magnitude, the maximum hue of shear sensitive liquid crystal coating is measured when the shear vector is aligned with, and directed away from the observer; changes in circumferential angle to either side of the vector aligned position result in symmetric decrease in hue. The hue- change could be well fit by a Gaussian curve between 90° and +90°, or, by a simple second-order polynomial curve over the limited range of 60° and +60°.…”

“…An example of hue- curves for two different shear stress vectors are given in Figure 2. Based on these findings, a surface shear stress measurement methodology was developed [8]. For each physical point on the test surface, a Gaussian curve could be fitted to the hue- data using the follow expression…”

Color change characteristics of two shear-sensitive liquid crystal mixtures (BCN/192, BN/R50C) and their application in surface shear stress measurements

“…In this trichromic system, red (R=1, G=0, B=0) is placed at 0°, green (R=0, G=1, B=0) is placed at 120°, and blue (R=0, G=0, B =1) is placed at 240°. Read et al [18] found that under normal illumination and fixed above-plane viewing angle, the maximum hue of SSLC coating for a given shear stress magnitude was measured when the shear vector was aligned with, and directed away from, the observer; changes in circumferential angle either side of the vector aligned position resulted in symmetric decrease in hue. The hue-φ change could be well fit by a Gaussian curve between −90° and +90°:…”

Measurement of Wall Shear Stress in High Speed Air Flow Using Shear-Sensitive Liquid Crystal Coating

“…Shear-sensitive liquid crystal coating (SSLCC) technique is a non-intrusive global wall shear stress measurement technique. The technique was first studied by Reda et al [ 6 , 7 , 8 , 9 , 10 ] at National Aeronautics and Space Administration (NASA, Washington, DC, USA) Ames Research Center located in Moffett Field, California in the United States. The principle of this technique is to spray a thin layer of shear-sensitive liquid crystal coating onto the tested surface.…”