Cts of Air Humi Ity in §u Ersonic Wind Tunnels

Szcrnmavy.-The available theoretical and experimental information on condensation of water vapour in the supersonic flow of air is reviewed and the influence of condensation on operation of supersonic tunnels is considered. The mechanism of condensation in supersonic flow is of molecular nature and does not depend on the presence of solid condensation nuclei in the air. As estimated by Oswatitsch and confirmed by experimental results, the condensation in supersonic flow of air is primarily a function of the adiabatic supercooling A T,, to ad (defined in Fig. l) , which determines the conditions a t which the condensation shock occurs. For medium-sized supersonic tunnels (say I-ft square working section) the adiabatic supercooling is of the order of 50 deg C. For most test purposes it is essential to eliminate the detrimental effects of condensation, on flow distribution in the tunnel working section. The usual method is to use highly dried air, and the question of the required dryness is considered. It is shown that by increasing stagnation temperature condensation can be avoided usually only at Mach numbers smaller than 1 .5. Alternatively, condensation can be eliminated from the tunnel nozzle by pre-expansion in an auxiliary nozzle, as verified experimentally. Other methods, which do not rely on the dryness of air, are discussed. Iuttroduction.-In the early days of experimental supersonic research the effects of air humidity on the flow in supersonic tunnels were largely neglected. At the 1935 High Speed, Volta Congress, Professor Prandtl" drew attention to the occurrence of a shock system a short distance downstream of the throat of supersonic nozzles, at that time a still unexplained phenomenon. Professor Wieselsberger commented that according to the observations made at Aachen*, the position of a shock system of that type was a unique function of the air humidity. This proved in fact to be the correct explanation, the air humidity being the cause of the formation of disturbances known today as condensation shocks. Fig. 2 shows the appearance of a typical condensation shock while supersonic flow in a two-dimensional 1-86 Mach number nozzle is initiated. With the development in Germany of supersonic research on a large scale during the Second World War, it was.realized that in order to obtain experimental results with the required degree of accuracy the effects of air humidity must be eliminated. The obvious and radical method, which was universally adopted, was to use air dried down to …