Advances in Flow Visualization Using Liquid-Crystal Coatings

The visualization of boundary-layer transition from laminar to turbulent flow plays an important role in flight and wind tunnel aerodynamic testing. Visualization aids in the understanding of specific causes of transition. In the past, the most popular boundary-layer visualization methods for flight and wind tunnel applications included the sublimating chemical technique and the oil flow technique. However, each method has certain advantages and limitations which constrain the applications. This paper discusses a new method for visualizing boundary layer flows including transition, separation, and shock locations, by the use of liquid-crystal coatings. For flight applications, liquid crystals provide transition visualization capability throughout almost the entire altitude and speed ranges for subsonic aircraft flight envelopes. The method is also applicable to supersonic flow visualizaton and for general use in high- and low-speed wind tunnel and water-tunnel testing.