In this field of aeronautical science, the flow visualization has been a nebulous observation simply because air is invisible. The use of saw dust, tufts, fluorescent dye in oil with black light, a recently developed laser beam and smoke, are some of th methods and materials used to attempt flow visualization.
The use of saw dust increases the difficulty of maintenance in the wind tunnel and its associated equipment. The tuft and oil methods show flow on the surface of the model. the laser beam has not yet been perfected as a usable tool for this purpose. Whatever the method used, the substance introduced into the tunnel should be non-corrosive, non-toxic, and the equipment must be safe to handle.
The use of smoke appears to combine all the best characteristics of available methods. In the early days, rotten wood was the popular source of smoke, (Ref. 1 and 2). However, smoke produced by the burning wood was an eye irritant and caused an accumulation of tars.
The technique of evaporating oil by various methods was introduced to eliminate some of the problems of the smoke, (Ref. 2). The study of smoke generators has also been reported in England and Australia, (Ref. 3, 4, and 5). In the United States, Princeton University produced smoke using kerosene, (Ref. 6).
(References in the further reading!)