Measurement of shear-stress distribution over a surface by liquid-crystal coating

Abstract

In this paper, a method for measuring the instantaneous shear-stress distribution over a solid surface in a fluid flow is studied by using a surface-flow visualization technique with a shear-sensitive liquid-crystal coating. The colour response of the liquid-crystal coating is calibrated against the various shear-stress magnitudes and directions with an experimental test section of laminar channel flow. The measurement of instantaneous shear-stress distribution is carried out using two colour CCD cameras, which are situated side by side in stereoscopic arrangement. The colour-to-shear-stress transformation is conducted by analysing the hue distribution of the visualized colour image with the calibrated relationships between the hues from the two colour CCD cameras and the shear-stress magnitude and direction. The usefulness of this technique was demonstrated by the measurement of shear-stress distribution around a circular cylinder on a surface under decelerating flow conditions.

Wind tunnel testing of shear-stress measurement by liquid-crystal coating

“…It was sprayed uniformly onto a polymer film (= 0.1 mm in thickness) after being mixed with the petroleum ether and pasted on the test surface. The calibration of the liquidcrystal coating is conducted by using the calibration equipment, which has been described by Fujisawa et al (2003). This equipment consists of a straight channel having dimensions of 0.75 mm in height, 30 mm in width and 200 mm long.…”

“…The determination of the shear-stress magnitude and direction is made from the calibration curves obtained from the color image analysis of the liquid-crystal coatings. The details of the shear-stress analysis are described by Fujisawa et al (2003). where ρ : density of fluid and τ : shear stress.…”

“…The frequency response of the color change of the liquid crystals is higher than 1 kHz, thus it is applicable to the measurement of instantaneous shear-stress distributions. Very recently, a method of instantaneous shear-stress measurement of flow over a surface has been studied by Fujisawa et al (2003) using a stereoscopic arrangement of the color CCD cameras placed side by side. The magnitude and direction of the shear stresses over a surface are determined from the stereo color images of liquid-crystal coating in combination with the calibrated relationships among the color of the liquid crystal, the shear-stress magnitude and the direction.…”

Detection of separation in S-duct diffusers using shear sensitive liquid crystals

“…Fujisawa et al (2003) in their extensive study on effect of viewing and illumination angles have given some guidelines on the optimum angles. However, the angles given by Fujisawa et al (2003) could not be used here as the present study was on internal flows where the illumination and viewing angles are restricted because of glare from the test section upper surface and proper optical access to the test surface. To eliminate stray ambient light from interfering with the coating pattern, the whole set up was isolated by means of a dark room arrangement.…”

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

“…Previously, Fujisawa et al [7] studied instantaneous surface shear stress distributions from measurements using an SSLC coating. Two synchronized cameras were placed side by side.…”