TIME AND FLOW-DIRECTION RESPONSES OF SHEAR-STRESS-SENSITIVE LIQUID-CRYSTAL COATINGS

Time and flow-direction responses of shear-stress-sensitive liquid cry stal coatings were explored experimentally. For the time-response expe riments, coatings were exposed to transient, compressible flows create d during the startup and off-design operation of an injector-driven su personic wind tunnel. Flow transients were visualized with a focusing schlieren system and recorded with a 1000 frame/s color video camera. Liquid crystal responses to these changing-shear environments were the n recorded with the same video system, documenting flow-visualization response times equal to, or faster than, the time interval between seq uential frames (i.e., 1 ms). For the flow-direction experiments, a pla nar test surface was exposed to equal-magnitude and known-direction su rface shear stresses generated by both normal and tangential subsonic jet-impingement flows. Under shear, the sense of the angular displacem ent of the liquid crystal dispersed (reflected) spectrum was found to be a function of the instantaneous direction of the applied shear. Thi s technique thus renders dynamic flow reversals or flow divergences vi sible over entire test surfaces at image recording rates up to 1 kHz. Extensions of the technique to visualize relatively small changes in s urface shear stress direction appear feasible.