The global wall shear stress measurement tech- nique using shear-sensitive liquid crystal （SSLC） is extended to wind tunnel measurements. Simple and common every- day equipment is used in the measurement; in particu...The global wall shear stress measurement tech- nique using shear-sensitive liquid crystal （SSLC） is extended to wind tunnel measurements. Simple and common every- day equipment is used in the measurement; in particular a tungsten-halogen light bulb provides illumination and a saturation of SSLC coating color change with time is found. Spatial wall shear stress distributions of several typical flows are obtained using this technique, including wall-jet flow, vortex flow generated by a delta wing and junction flow behind a thin cylinder, although the magnitudes are not fully calibrated. The results demonstrate that SSLC technique can be extended to wind tunnel measurements with no complicated facilities used.展开更多
The present paper aims at introducing Shear-Sensitive Liquid Crystal Coating(SSLCC)technology into compressor cascade measurement for the first time and serves as a basis for better understanding of the influence from...The present paper aims at introducing Shear-Sensitive Liquid Crystal Coating(SSLCC)technology into compressor cascade measurement for the first time and serves as a basis for better understanding of the influence from the boundary layers. Optical path layout, which is the most significant difficulty in internal flow field measurement, will be solved in this paper by selfdesigned image acquisition device. Massive experiments with different Mach number and incidence are conducted at a continuous subsonic cascade wind tunnel to capture the boundary layer phenomenon. Image processing methods, such as Three-Dimensional(3-D) reconstruction and Hue conversion, are used to improve the accuracy for transition position detection. The analysis of the color-images indicates that complex flow phenomena including transition, flow separation,and reattachment are captured successfully, and the effect of Mach number and incidence on the boundary layer flow is also discussed. The results show that: the Mach number has a significant effect on transition position; the incidence has little effect on transition position, but it has a great impact on the transition distance and leading-edge separation; influenced by the end-walls, the reattachment occurs in advance under positive angle of attack conditions.展开更多
A learning-based approach for solving wall shear stresses from Shear-Sensitive Liquid Crystal Coating(SSLCC) color images is presented in this paper. The approach is able to learn and establish the mapping relationshi...A learning-based approach for solving wall shear stresses from Shear-Sensitive Liquid Crystal Coating(SSLCC) color images is presented in this paper. The approach is able to learn and establish the mapping relationship between the SSLCC color-change responses in different observation directions and the shear stress vectors, and then uses the mapping relationship to solve wall shear stress vectors from SSLCC color images. Experimental results show that the proposed approach can solve wall shear stress vectors using two or more SSLCC images, and even using only one image for symmetrical flow field. The accuracy of the approach using four or more observations is found to be comparable to that of the traditional multi-view Gauss curve fitting approach;the accuracy is slightly reduced when using two or fewer observations. The computational efficiency is significantly improved when compared with the traditional Gauss curve fitting approach, and the wall shear stress vectors can be solved in nearly real time. The learning-based approach has no strict requirements on illumination direction and observation directions and is therefore more flexible to use in practical wind tunnel measurement when compared with traditional liquid crystal-based methods.展开更多
It is known that the adsorption of surfactants at the liquid crystal (LC)/aqueous interface can induce a bipolar-to-radial director configuration of LC droplets dispersed in aqueous solution. In this paper, we study t...It is known that the adsorption of surfactants at the liquid crystal (LC)/aqueous interface can induce a bipolar-to-radial director configuration of LC droplets dispersed in aqueous solution. In this paper, we study the effect of charged polyelectrolyte-coating on the interaction of surfactants and LC droplet cores by observing the director configuration of the LC droplet cores as a function of surfactant concentrations. It is found that surfactants can penetrate into the polyelectrolyte coating and react with the LC droplet cores to induce the bipolar-to-radial transition of the LC inside the droplet cores. However, the concentration of charged surfactants required to induce the configuration transition of the LC droplet cores is affected by the charged polyelectrolyte coating. The effect is significantly enlarged with decreasing the alkyl chain length of charged surfactants. Our results highlight the possibility of engineering polyelectrolyte coatings to tune the interaction of LC droplets with analysts, which is critical towards designing LC droplet based sensors.展开更多
The liquid crystal thermography is a high-resolution and non-intrusive optical technique for full-field temperature measurement.We present detailed calibration data for a wide-bandwidth thermochromic liquid crystal（...The liquid crystal thermography is a high-resolution and non-intrusive optical technique for full-field temperature measurement.We present detailed calibration data for a wide-bandwidth thermochromic liquid crystal（TLC） to investigate the effect of the coating thickness on the hue-temperature characteristics and the measurement uncertainty of the TLC.It is found that the coating thickness has appreciable effect on the TLC hue-temperature curve.For TLC coatings with the thickness over 20μm,a thicker TLC coating shows a relatively smaller measurement uncertainty,but the effect of the coating thickness is non-distinctive on the measurement uncertainty.展开更多
A hafnium strontium oxide(HfSrO)liquid crystal(LC)alignment film was efficiently created through brush coating,and its ability to change the LC alignment direction was confirmed.A brush was applied to HfSrO sol coated...A hafnium strontium oxide(HfSrO)liquid crystal(LC)alignment film was efficiently created through brush coating,and its ability to change the LC alignment direction was confirmed.A brush was applied to HfSrO sol coated on an indium-tin oxide substrate,after which the coating was solidified at var-ious curing temperatures.It was confirmed that a directional micro/nanostructure was formed above 280°C due to the shear stresses caused by the movement of the brush hairs.Surface chemical changes were analyzed by using X-ray photoelectron spectroscopy and contact angle measurements.As the curing temperature increased,the prevalence of oxygen bonds increased and the contact angle decreased,thereby increasing the surface energy.The anisotropic boundary of the micro-grooves and the van der Waals forces due to an increase in surface energy changed the alignment direction of LC mole-cules from vertical to horizontal,as verified through polarized optical microscopy and pretilt angle measurements.Thus,the efficiency of the brush-coating method,which dramatically simplifies the LC alignment film process,was confirmed.The homeotropic/homogeneous LC alignment property of the HfSrO film produced through brush coating depending on the curing temperature provides an innovative approach for LC alignment.展开更多
基金the Doctorate Creation Foundation of Northwestern Polytechnical University (CX200902)
文摘The global wall shear stress measurement tech- nique using shear-sensitive liquid crystal （SSLC） is extended to wind tunnel measurements. Simple and common every- day equipment is used in the measurement; in particular a tungsten-halogen light bulb provides illumination and a saturation of SSLC coating color change with time is found. Spatial wall shear stress distributions of several typical flows are obtained using this technique, including wall-jet flow, vortex flow generated by a delta wing and junction flow behind a thin cylinder, although the magnitudes are not fully calibrated. The results demonstrate that SSLC technique can be extended to wind tunnel measurements with no complicated facilities used.
基金co-supported by the National Natural Science Foundation of China(No.51476132)the Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University(No.CX201713)the 111 Project(No.B17037)
文摘The present paper aims at introducing Shear-Sensitive Liquid Crystal Coating(SSLCC)technology into compressor cascade measurement for the first time and serves as a basis for better understanding of the influence from the boundary layers. Optical path layout, which is the most significant difficulty in internal flow field measurement, will be solved in this paper by selfdesigned image acquisition device. Massive experiments with different Mach number and incidence are conducted at a continuous subsonic cascade wind tunnel to capture the boundary layer phenomenon. Image processing methods, such as Three-Dimensional(3-D) reconstruction and Hue conversion, are used to improve the accuracy for transition position detection. The analysis of the color-images indicates that complex flow phenomena including transition, flow separation,and reattachment are captured successfully, and the effect of Mach number and incidence on the boundary layer flow is also discussed. The results show that: the Mach number has a significant effect on transition position; the incidence has little effect on transition position, but it has a great impact on the transition distance and leading-edge separation; influenced by the end-walls, the reattachment occurs in advance under positive angle of attack conditions.
基金co-supported by the National Natural Science Foundation of China(No.11602107)the Natural Science Foundation of Jiangsu Province of China(No.BK20150733)。
文摘A learning-based approach for solving wall shear stresses from Shear-Sensitive Liquid Crystal Coating(SSLCC) color images is presented in this paper. The approach is able to learn and establish the mapping relationship between the SSLCC color-change responses in different observation directions and the shear stress vectors, and then uses the mapping relationship to solve wall shear stress vectors from SSLCC color images. Experimental results show that the proposed approach can solve wall shear stress vectors using two or more SSLCC images, and even using only one image for symmetrical flow field. The accuracy of the approach using four or more observations is found to be comparable to that of the traditional multi-view Gauss curve fitting approach;the accuracy is slightly reduced when using two or fewer observations. The computational efficiency is significantly improved when compared with the traditional Gauss curve fitting approach, and the wall shear stress vectors can be solved in nearly real time. The learning-based approach has no strict requirements on illumination direction and observation directions and is therefore more flexible to use in practical wind tunnel measurement when compared with traditional liquid crystal-based methods.
文摘It is known that the adsorption of surfactants at the liquid crystal (LC)/aqueous interface can induce a bipolar-to-radial director configuration of LC droplets dispersed in aqueous solution. In this paper, we study the effect of charged polyelectrolyte-coating on the interaction of surfactants and LC droplet cores by observing the director configuration of the LC droplet cores as a function of surfactant concentrations. It is found that surfactants can penetrate into the polyelectrolyte coating and react with the LC droplet cores to induce the bipolar-to-radial transition of the LC inside the droplet cores. However, the concentration of charged surfactants required to induce the configuration transition of the LC droplet cores is affected by the charged polyelectrolyte coating. The effect is significantly enlarged with decreasing the alkyl chain length of charged surfactants. Our results highlight the possibility of engineering polyelectrolyte coatings to tune the interaction of LC droplets with analysts, which is critical towards designing LC droplet based sensors.
基金supported by the National Natural Science Foundation of China under Grant No.50806045
文摘The liquid crystal thermography is a high-resolution and non-intrusive optical technique for full-field temperature measurement.We present detailed calibration data for a wide-bandwidth thermochromic liquid crystal（TLC） to investigate the effect of the coating thickness on the hue-temperature characteristics and the measurement uncertainty of the TLC.It is found that the coating thickness has appreciable effect on the TLC hue-temperature curve.For TLC coatings with the thickness over 20μm,a thicker TLC coating shows a relatively smaller measurement uncertainty,but the effect of the coating thickness is non-distinctive on the measurement uncertainty.
基金This work was supported by the National Research Foundation of Korea[2020R1G1A1013604].
文摘A hafnium strontium oxide(HfSrO)liquid crystal(LC)alignment film was efficiently created through brush coating,and its ability to change the LC alignment direction was confirmed.A brush was applied to HfSrO sol coated on an indium-tin oxide substrate,after which the coating was solidified at var-ious curing temperatures.It was confirmed that a directional micro/nanostructure was formed above 280°C due to the shear stresses caused by the movement of the brush hairs.Surface chemical changes were analyzed by using X-ray photoelectron spectroscopy and contact angle measurements.As the curing temperature increased,the prevalence of oxygen bonds increased and the contact angle decreased,thereby increasing the surface energy.The anisotropic boundary of the micro-grooves and the van der Waals forces due to an increase in surface energy changed the alignment direction of LC mole-cules from vertical to horizontal,as verified through polarized optical microscopy and pretilt angle measurements.Thus,the efficiency of the brush-coating method,which dramatically simplifies the LC alignment film process,was confirmed.The homeotropic/homogeneous LC alignment property of the HfSrO film produced through brush coating depending on the curing temperature provides an innovative approach for LC alignment.
