A Note on the China-Film Technique for Boundary Layer Indication

Extract

The “ China-Film ” technique was developed by Murphy and Phinney from the “ China Clay” technique for indicating transition originated by Richards and Burstall. It consists in applying a suspension of china clay in a volatile liquid to a surface subsequently subjected to air flow. Not only is transition indicated, but when all the liquid has evaporated, the china clay remains deposited on the surface in patterns which may be interpreted to yield information on the direction of air flow near the surface and the incidence of shocks thereon.

Experiments were carried out, at supersonic and subsonic air speeds, to determine the process by which the patterns form and to establish the aerodynamic meaning of the “ flow lines ” of the pattern. The influence of the viscosity of the carrier liquid was also studied. Indications of flow direction were found to be unreliable near a trailing edge, on a vertical surface at low speeds and in regions where the surface shear stress is only of the same order of magnitude as the product of pressure gradient and film thickness.