Use of liquid crystals for qualitative and quantitative 2-D studies of transition and skin friction

The use of properties of liquid crystals in wind tunnels to visualize transition and to measure skin friction is described. The effectiveness of a transition band to trip the laminar boundary layer on a swept wing is demonstrated by the growth of turbulent wedges with Reynolds number. The ability of liquid crystals to reveal intricate surface flow structures is clearly shown by subtle changes of color on an unswept rectangular wing when subjected to the combined effects of transition, separation, reattachment, and a normal shock. The time response of liquid crystals to changes in shear stress is illustrated by the shock pattern on the model surface, which was seen to be oscillating. A method involving the digitization of the video image into its three component colors has the potential for measuring skin friction in great detail. This involves using relationships correlating the component colors with wavelength and shear stress with wavelength