Recent Experience in the RAE (Royal Aircraft Establishment) 5 Metre Wind Tunnel of a China Clay Method for Indicating Boundary Layer Transition

摘要

The China Clay method is based on the differential rate of evaporation, beneath laminar and turbulent boundary layers, of a liquid absorbed on a solid spread on the surface. The liquid is chosen to have a similar refractive index to the solid so that the mixture appears to be transparent until evaporation has taken place, when the colour of the solid becomes visible. The solid layer consists of china clay powder mixture with certain chemicals to form a lacquer, which is sprayed onto the surface. When dry, this forms a semi-permanent white absorbent film. A suitable liquid sprayed on to this film is absorbed by it, causing it to become transparent. Details of a visual method for indicating boundary layer transition are given, with particular reference to tests at high Reynolds number in the RAE 5-meter pressurized low-speed wind tunnel on a slender-body model. The method is compared with other techniques on the basis of ease of use and quality of the indication.