Applied Scientific Research 58: 207–216, 1998. A. Biesheuvel and GJ.F. van Heijst (eds), In Fascination of Fluid Dynamics. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 207 Unsteady Liquid Jets G.E.A. MEIER, S. LOOSE and B. STASICKI DLR, Institute of Fluid Mechanics, Bunsenstrasse 10, 37073 Göttingen, Germany Abstract. The behaviour of unsteady liquid jets in a gas atmosphere is mainly governed by the conservation of momentum and the interaction with the environment. In this article it will be shown that many of the particular effects in the propagation and desintegration of unsteady jets are simply explained by the conservation of initial momentum. Many of the distortions and peculiar shapes of the liquid elements of the jet can be explained by a time and space development of weak initial distortions of momentum in travelling waves during propagation. The first part of the paper is devoted to the droplet formation in liquid jets of moderate Weber numbers in the order of ten. In this regime and higher the surface tension is not the governing influence of instability of jet decomposition and hence of the droplet formation. If there is a high frequency distortion of the velocity which is artificially implemented in our experiments by a special device, but under usual conditions also existing by turbulence or source distortions of the nozzle flow, the concentration of mass in packets in a kind of shock formation is the main reason for droplet formation. The frequency of droplets in space and time is given by the frequency of the distortion. The amplitude and phase is also given by the initial values of the driver for these quantities. For large amplitudes and low frequencies the jets are decomposed in very peculiar shapes, which can be very simply explained by the collision of packets of concentrated mass on the axis of the jet motion. The whole range of phenomena can be understood in a theoretically elegant manner by a La- grangian transform of the nonlinear equations of motion together with some plausible considerations about the lateral motion. Solutions for the droplet formation and the formation of liquid films in different shapes can be achieved this way. As an illustration of the whole range of beautiful and regularly reproducible details of jet decom- position some photographs will be shown which summarize the phenomena and give an idea of the importance of systematic investigation of the sometimes very confusing appearance of fluid jets. Key words: jet, liquid/fluid jet, unsteady liquid jet, gas atmosphere, distortion, high frequency distortion, Weber number, Lagrangian transform, non-linear equations of motion, liquid films. 1. Introduction In many practical applications jets of particles or fluid have only weak interaction with the environment. This is mainly due to the high density difference which results in small forces on liquid or solid particle jets in an gas environment. Also the influence of surface-tension on jet instability can be neglected in case of liquid jets if the jet diameter and the velocity fluctuations are comparatively large. The Weber number is defined as