Experiments on the measurements of transition position by chemical methods

Abstract

The smoke filament technique for detecting transition points is limited to speeds below 180 ft./sec, for orthodox atmospheric tunnels with good lighting and viewing conditions. Also many tunnels are of the enclosed pressure type in which observation is impossible, whilst many atmospheric wind tunnels have poor facilities for lighting and viewing. Hence the need for methods which will supplement the smoke filament technique at high gpeeds, and which can be applied to pressure tunnels and to flight. The work was almost wholly confined to a particular technique, which consisted in allowing a gas in high concentration to ooze out of an orifice near the nose and flow over the wing surface, which was coated with a suitable sensitive paint, thus producing a stain. Various combinations of gases and paints were tried as suggested by well-known 'indicator' tests in chemical analysis. The flow of smoke from the orifice was also studied. With suitable choice of gas and paint the transition position can be determined satisfactorily. The controlling factor which decides the definition of the stain in the laminar region is the 'threshold' of the paint to the action of the gas. This should be such that only a very faint trace is produced in the turbulent region, whilst an intense and well defined stain occurs in tile laminar region. Care must be taken at moderate Reynolds numbers, when laminar separation is likely to be present. With gas oozing from a hole in the nose only, its presence is likely to pass undetected, but it can be found by introducing the gas near the end of the laminar flow region. For this reason, if facilities for observation exist, it is recommended that the smoke filament technique be used at low wind speeds. If a permanent or semi-permanent record is desired, then ammoniff in conjunction with mercurous chloride is very good, as also is hydrogen sulphide on white lead, and ammonia on congo-red (an organic pigment). A 'fugitive' stain which disappears in about 10 minutes can be produced by passing ammonia over Brom-cresol-purple which has been given a suitable 'threshold'. Very small amounts of gas are used - from 3 to 6 cubic inches, supplied at a rate of about 1 cubic inch per minute. A rough metering arrangement is desirable. 'Evaporation' methods are being investigated at the Royal Aircraft Establishment and the National Physical Laboratory. It is desirable that the technique described in this report should be applied to flow at high speeds and high Reynolds numbers; if successful, then it can be extended to pressure tunnels and to flight.