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

Abstract

A previously determined surface shear stress measurement technique using shear-sensitive liquid crystal (SSLC) coating is extended for use in wind tunnel-like conditions. Simple and common everyday equipment is used in the measurement; in particular a tungsten halogen light bulb provides illumination. The color change characteristics of two SSLC mixtures, BCN192 and BN/ R50C, are investigated. BCN/192 is found sensitive to both magnitude and direction of the shear stress at different shear stress levels with little noise, and is suitable for surface shear stress distribution measurements. The spatial shear stress vector distribution beneath a tangential jet is obtained using BCN/192, although the magnitude is not fully calibrated. BN/R50C is found to be sensitive to shear stress magnitude, but only slightly sensitive to shear stress direction at both low and high levels. Moreover, jumps in the reflected hue are found when the viewing orientation is perpendicular to the shear stress direction. These characteristics render the use of BN/R50C for surface shear stress measurement difficult.shear-sensitive liquid crystals, surface shear stress measurement, tangential jet flow Citation:Zhao J S, Scholz P, Gu L X. Color change characteristics of two shear-sensitive liquid crystal mixtures (BCN/192, BN/R50C) and their application in surface shear stress measurements.

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

“…The light bulb was placed normally above the anodized aluminum plug, with a distance of 120 cm to reduce the light angle difference across the test surface. The characteristics of this illumination source were discussed in Zhao et al [ 16 ]. The maximum difference of the light angles across test plug (10 cm × 10 cm) was less than 8°, which had little effect on the SSLCC color change, according to Wilder et al’s findings [ 36 ].…”

“…As a result, the camera required a relative long exposure time, and it therefore did not enable instantaneous flow measurements, as discussed latter. The illumination source could not be simply replaced with a high intensity one because most illumination sources of high intensity usually have a large size, and thus provide complex light directions, which are not suitable for the SSLCC technique [ 16 ].…”

“…In the literature, the SSLCC technique has been used in used in several measurements including shear stress over planar surface in the flow beneath a tangential wall jet [ 9 ] and an oblique impinging jet [ 10 ], shear stress over planar surface around a cylinder in steady flow [ 11 ], and instantaneous flow [ 12 ], and even shear stress over the curved surface [ 13 ]. Extension of the SSLCC technique to the general wind tunnel measurement is available in Zhao et al [ 14 , 15 , 16 , 17 ]. There are also other global wall shear stress measurement techniques, such as surface stress-sensitive film (S3F) technique [ 18 , 19 ], the oil-film interferometry (OFI) technique [ 20 ], the luminescent oil-film technique [ 21 , 22 ], the global skin friction diagnostic technique [ 23 , 24 , 25 ], and the global technique based on soft substrates [ 26 ].…”

“…Although some encouraging and impressive results have been obtained by the SSLCC technique under special arrangements, most wind tunnel studies of the SSLCC technique were performed in very low speed flow [ 11 , 12 , 13 , 14 , 15 , 16 , 17 ], exactly speaking, no more than 30 m/s. Some exceptions can be found in Disimile et al [ 34 ] and Zharkova et al [ 35 ], where only the shear magnitude field was measured.…”

“…Some exceptions can be found in Disimile et al [ 34 ] and Zharkova et al [ 35 ], where only the shear magnitude field was measured. Note that the wall shear stress in low speed flow is very small and it is not easy to measure, and so there are considerable noises in the measured wall shear stresses [ 11 , 12 , 13 , 14 , 15 , 16 , 17 ]. It is believed that the shear stress that was measured using the SSLCC technique should be more accurate at higher wind tunnel velocities, because in that case the shear stresses are larger and thereby easier to measure.…”