Measurement of surface shear stress vector distribution using shear-sensitive liquid crystal coatings

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

“…Within this region, the mean values and the standard deviations were τ = 115.5 Pa ± 0.5% and φ τ = 0.60 ± 0.64°. The standard deviations are much smaller than that obtained in low speed flow [ 11 , 12 , 13 , 14 , 15 , 16 , 17 ]. Note that the mean vector direction was only 0.60°, whereas an orientation of 0° was expected in this region due to the symmetry of the flow.…”

“…The mean value and the standard deviation of the relative fit error for each dataset are −0.13% ± 2.24%, −0.11% ± 1.19%, and −0.16% ± 1.42%, respectively. Although no quantitative curve fit errors were given in literatures [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 ], it is clear to see that the noises in the curve fits in Figure 3 are much smaller than that in literatures by simply comparing Figure 3 with the hue vs. φ plots in the literature [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 ]. The improvement is attributed to the synchronous imaging system consisting of six digital cameras, the significant improvement in the imaging technique over the past two decades, and the higher shear stress levels, which are easier to measure.…”

“…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.…”