In high speed wind tunnels, the china clay method has been replaced by methods based on the sublimation of a solid. The sublimating solid is diluted in a spraying solution in a highly volatile liquid that evaporates before the test. In continuous supersonic tunnels, a solid is used with a slow sublimation rate such as azobenzene; in intermittent supersonic wind tunnels, a solid with a fast sublimating rate like exachloroethane is used. In subsonic and transonic wind tunnels, solids with an average rate of sublimation like acenaphthene are widely used. Suitable solvents are benzene and xylene. The method has the disadvantage of providing images with a poor contrast (Figure 6.13).
Boundary layer transition visualized with a sublimating material
Laminar Flow Turbulent Flow