Shear-sensitive Liquid Crystal Research Articles

液晶による瞬時せん断応力分布の可視化と画像計測

A method for measuring the shear stress distribution over a solid surface in a flow was developed with a use of shear-sensitive liquid crystal coatings and the stereoscopic observation by two color CCD cameras located side by side. This new technique was successfully applied to the shear-stress measurement over the side wall of the cylinder in a uniform flow.

Shear-Sensitive Liquid Crystal Coating Method Applied Through Transparent Test Surfaces

Research conducted at NASA Ames Research Center has shown that the color-change response of a shear-sensitive liquid crystal coating (SSLCC) to aerodynamic shear depends on both the magnitude of the local shear vector and its direction relative to the observer's in-plane line of sight. In conventional applications, the surface of the SSLCC exposed to aerodynamic shear is illuminated with white light from the normal direction and observed from an oblique above-plane view angle of order 30 deg. In this top-light/top-view mode, shear vectors with components directed away from the observer cause the SSLCC to exhibit color-change responses. At any surface point, the maximum color change (measured from the no-shear red or orange color) always occurs when the local vector is aligned with, and directed away from, the observer. The magnitude of the color change at this vector-observer-aligned orientation scales directly with shear stress magnitude. Conversely, any surface point exposed to a shear vector with a component directed toward the observer exhibits a non-color-change response, always characterized by a rusty-red or brown color, independent of both shear magnitude and direction. These unique, highly directional color-change responses of SSLCCs to aerodynamic shear allow for the full-surface visualization and measurement of continuous shear stress vector distributions. The objective of the present research was to investigate application of the SSLCC method through a transparent test surface. In this new back-light/back-view mode, the exposed surface of the SSLCC would be subjected to aerodynamic shear stress while the contact surface between the SSLCC and the solid, transparent wall would be illuminated and viewed in the same geometrical arrangement as applied in conventional applications. It was unknown at the outset whether or not color-change responses would be observable from the contact surface of the SSLCC, and, if seen, how these color-change responses might relate to those observed in standard practice.

Shear-sensitive liquid crystal coating method applied through transparent test surfaces

Shear-sensitive liquid crystal coating method applied through transparent test surfaces

811 液晶を用いた壁面せん断応力のフィールド計測に関する研究

811 液晶を用いた壁面せん断応力のフィールド計測に関する研究

Measurement of Continuous Pressure and Shear Distributions Using Coating and Imaging Techniques

The pressure-sensitive paint (PSP) method and the shear-sensitive liquid crystal coating (SSLCC) method were sequentially applied to measure the areal pressure and shear stress vector distributions on a planar test surface beneath an inclined, axisymmetric, turbulent impinging jet. The combined results provide the first-ever consolidated measurements of the continuous normal and tangential force distributions beneath a fundamental flowfield. Results indicate that the PSP method can be extended to the measurement of small pressure differences (< ∼0.1 psig) encountered in low-speed atmospheric flows. Further, these results provide the first demonstration of the capability of the SSLCC method to measure continuous shear stress vector distributions on planar surfaces beneath flowfields where shear vectors of all possible orientations are present

Measurement of continuous pressure and shear distributions using coating and imaging techniques

The pressure-sensitive paint (PSP) method and the shear-sensitive liquid crystal coating (SSLCC) method were sequentially applied to measure the areal pressure and shear stress vector distributions on a planar test surface beneath an inclined, axisymmetric, turbulent impinging jet. The combined results provide the e rst-ever consolidated measurements of the continuous normal and tangential force distributions beneath a fundamental e owe eld. Results indicate that the PSP method can be extended to the measurement of small pressure differences (<» 0:1 psig)encountered in low-speed atmospheric e ows. Further, these results provide the e rst demonstration of the capability of the SSLCC method to measure continuous shear stress vector distributions on planar surfaces beneath e owe elds where shear vectors of all possible orientations are present.

Performance variation of scramjet nozzle at various nozzle pressure ratios

An experimental study of scramjet nozzle was conducted to investigate how the nozzle flow is affected by ambient pressure. In order to elucidate the aerodynamic properties of nozzle flow, detailed measurements of thrust and wall pressure were carried out using cold nitrogen. Nozzle flow was also visualized using a shear sensitive liquid crystal. Wall pressure and shear stress distributions in an overexpanded nozzle showed that nozzle flow includes a crossing shock wave made at the side-fences. This flowfield can be approximated as a supersonic inlet flow compressed by side walls. The high-pressure region on the nozzle ramp generated by the shock waves results in a higher performance in scramjet nozzle than that estimated for a two-dimensiona l separation from the ramp. bient or pressure in flight. The change in back pressure can significantly alter the thrust and pitching moment gen- erated in the nozzle, especially when the nozzle's internal flow is affected by external conditions. Thus, investigation of noz- zle performance in off-design conditions is important for es- timation of the feasibility of SERN for hypersonic vehicles. Both experimental and numerical studies on SERNs have been carried out to investigate the interaction between the nozzle internal flow and the external conditions. Simulated flight conditions in these studies have recently been extended to the hypersonic regime. Ruff in et al.4 calculated the flow- field in SERN in an underexpanded condition with an implicit Navier-Stokes solver and compared the results with experi- mental data. Watanabe5 investigated the flowfield in SERN in both underexpanded and overexpanded conditions exper- imentally. He also examined the effect of side-fence shapes on the flowfield. Zeutzius and Beylich6 conducted an exper- imental investigation on the effects of parameters such as aspect ratio and scarf angle on the flowfield in a scarfed SERN. These studies, however, concentrated on the investigation of the flowfield in SERNs, while the effect of the interaction, namely the separation, on SERN performance remains un- investigated. The objective of the present study is to investigate the variation of nozzle performance with the change in external conditions. A scarfed nozzle6 was selected to minimize the three dimensionality of the flow on the nozzle ramp. The effect of ambient pressure was measured in terms of nozzle pressure ratio (NPR), defined as the ratio of the nozzle total pressure to the ambient pressure. Total pressure of the nozzle flow was varied to change the NPR. Thrust delivered by a scramjet nozzle was measured in both underexpanded and overexpanded conditions. Obtained thrust efficiency was compared with predicted values. A simple prediction method based on the assumption that flowfield is two dimensional was adopted to predict the variation of nozzle performance. In the overexpanded condition, the treatment of interference of the internal flow with the external flow, viz., the separation of flow, becomes very important in the prediction of the noz- zle performance. In the present study, two-dimensional sep- aration was assumed. The flowfield in the nozzle in the over- expanded condition was observed to investigate the nature of the interference between the nozzle internal flow and the external conditions.

Use of shear-sensitive liquid crystals for surface flow visualization

Use of shear-sensitive liquid crystals for surface flow visualization

A novel technique for response function determination of shear sensitive cholesteric liquid crystals for boundary layer investigations

A description of the design and setup of an experimental technique for measurement of the response function in shear sensitive liquid crystals has been reported. Utilizing the selective reflection characteristics of cholesteric liquid crystals, the method is capable of measuring the delay, rise, and relaxation times in response to a given dynamic shear stress as a function of the wavelength of the incident light. Application of a step input shear stress results in a liquid crystal time response that can be described as consisting of an initial delay, a shear induced helix deformation, and a relaxation to the initial state through diffusion processes. The method has been used for quantitative calibration of a shear sensitive liquid crystal by observing the peak in reflected light intensity, at a given wavelength, as a function of the shear stress. The selective reflection wavelength (λs) decreases linearly with an increase in shear stress (σ) and has a slope dλs/dσ≊10 nm/psi. The results suggest that in order to obtain quantitative information on the flow parameters over a test surface, three reflection spectra are required: one from a white surface and the other two from the liquid crystal surface one before and one in the presence of shear stress respectively. Free energy calculations for the elastic deformation of the liquid crystal suggest a time response of the liquid crystal for a step input in shear stress. Transport of the broken helix swarms has been suggested as a means for the helix relaxation to zero shear state even in presence of shear.

Apparatus for the Calibration of Shear Sensitive Liquid Crystals

An experiment was designed in which shearing forces acting on a film of shear sensitive liquid crystals could be measured directly while additional instrumentation was used to measure the peak spectral intensity of the light scattered by the substance. A liquid crystals sample of low temperature sensitivity was tested and a calibration curve was obtained with a linear slope of 0.208 (g/cm2)/nm from 0 to 3 g/cm2.