Contribution to the study of return currents generated by a confined jet

This work is essentially an experimental contribution to the study of the recirculation generated by a confined stream in a cylindrical pipe of circular section. The purpose of the tests was to examine in detail the mixing region for various ambient flows, the jet flow rate and the dimensions of the injection nozzle and chamber remaining fixed. This detailed examination included, on a water-operated device, the global measurement of the flows, a survey of the average velocity field in a diametral plane of the chamber and static pressures at the wall, as well as the visualization of instantaneous trajectories within the liquid. From this experimental program, it has been possible to deduce the evolution, along the pipe and as a function of the ambient flow, of the main characteristics of the jet and in particular the speeds on the axis, the static pressure and the ambient velocity, an effective length showing the expansion of the jet, a shape coefficient of the velocity curves and a coefficient of friction in the vicinity of the axis. The visualization revealed on the one hand strong local fluctuations and an overall movement of the recirculation zone, on the other hand an instantaneous swirling structure, similar to a 'von Karman street' of return currents. In order to specify these observations, the longitudinal fluctuations of speed for an average value of the ambient flow have been measured on an air device. These measurements have revealed very high levels of turbulence downstream of the recirculation core, greater than 50 pc even on the jet axis. Such values, unusual in classical fluid mechanics, pose the difficult and little-known double problem of the validity of mean and fluctuating measurements, and the interpretation of phenomena by means of these measurements and general equations