Applications of liquid crystals in aerodynamic testing
This thesis investigates the applications of liquid crystals in three areas of aerodynamic testing, namely temperature, shear stress and pressure measurement over surfaces. The use of the selective reflection colours from encapsulated chiral nematic materials to map surface temperatures is assessed. This method is very successful in a wide range of applications for determining heat transfer rates, but has limitations where high heat transfer rates are present, due to the thermal response time of the material structure, and the effects of temperature gradients. The thermal time constant is determined as a function of material viscosity. It is typically 10ms for a chiral nematic material at room temperature. The effect of a temperature gradient on the selective reflection is studied in terms of the pitch gradient produced in the material structure. Two improved methods are then proposed. The first makes use of the cholesteric to isotropic transition to indicate an isotherm on a heated surface, while the second uses changes in the birefringence colours produced by an aligned nematic layer applied to a surface and viewed between crossed poiarisers. Changes in the selective reflection colours from cholesteric liquid crystals can be used to assess surface shear stress levels. This method is assessed and determined to be too problematical for accurate measurements. A new method is proposed, based on the shear induced texture change from the focal-conic to the Grandjean texture in the cholesteric phase. This method may be used to quantify surface shear stress levels as well as to provide excellent flow visualisation once flow alignment has occurred. Tests have been conducted on the pressure sensitivity of chiral nematic liquid crystal materials. The results indicate that the change in transition temperature with pressure is of the order of 40°/kbar. which may not be sensitive enough for wind tunnel purposes.