Modus Operandi of the Air-Jet Pulsator

Abstract

The so-called air-jet-generator for acoustic waves has been several times described^1–5. We may therefore in this introduction confine ourselves to a brief statement of the fundamental principle. The generator depends on an air-jet with super sound-velocity. Such a jet is produced if air streams from a container, in which the excess-pressure exceeds 0.9 atm, into the free atmosphere. It is known from investigations carried out by E. Mach and P. Salcher⁶, R. Emden⁷ and others, that the jet exhibits a peculiar structure of which a picture may he obtained by means of the method of striae (Fig. 1). As is generally known the explanation of the picture has been given by L. Prandtl⁸. To the periodic aspect of the structure corresponds a periodic variation of the static pressure and of the velocity in the jet, as proved for instance by Stodola⁹. The static pressure has minima in the centres of the jet-sections and maxima in the borderlines between the said sections while for the velocity the opposite distribution prevails. If the sound of a simple Pitot-apparatus, (Fig. 2), is moved along the axis of the jet, the aperture being directed against the flow, the reading of the apparatus will also vary in a periodic way. Fig. 3 shows a Pitot-curve found with a jet-orifice of 0.535 mm and an excess-pressure of 141 cm Hg. The maxima are nearly in the centres of the jet-sections.