The vapour-screen method of flow visualization

Abstract

The vapour-screen method of flow visualization in supersonic wind tunnels is outlined, and the development of a suitable technique for use in the 3 ft. tunnel at R.A.E. Bedford described, together with the associated optical and photographic equipment.The results of tests to determine the humidity required to produce an optimum density of fog in the working section over the Mach number range 1.3–2.0 are presented and the influence of tunnel pressure and temperature discussed. Numerous vapour-screen photographs of the flow over and behind delta wings are included and some comparisons made with the corresponding surface oil-flow patterns.The process of condensation, the physical and optical properties of the resulting fog, and the formation of the vapour-screen picture are all considered in some detail.The effects of humidity on the Mach number and static pressure in the working section were investigated and the results are compared with theoretical estimates at a nominal Mach number of 2.0. It is shown that the adverse effects of condensation on the flow at high Mach numbers may be alleviated by the use of liquids with a lower latent heat of evaporation than water, and some results obtained at a Mach number of 2.0 using carbon tetrachloride vapour are described.The possibility of extending the vapour-screen technique to transonic and subsonic speeds is also considered, and some results obtained at a Mach number of 0.85 are included.