The flow in the mixing region of a jet

Abstract

The object of this work was to investigate experimentally the structure of the early shear layer of high-speed jets and its relation to the mechanism of noise generation. Of special interest was the question of the existence of periodic fluctuations in the velocity field. The experimental investigation is divided in three parts.Optical observation of the jet flow by means of the shadowgraph technique.Measurement of mean Mach number and temperature profiles.Survey by means of hot-wire of the component of the fluctuating velocity field in the mean flow direction.The shadowgraphs show very interesting features of the breakdown process of free shear layers, but fail to show any propagation of strong acoustic disturbances in the near pressure-field.Mean profile measurements show that the flow field in the range 1 [les ]x/d[les ] 4 develops in a conical fashion, i.e. the mean profiles in Mach number and temperature can be expressed in terms of a single conical variable η = (2r–d)/2x. The fluctuating velocity field is described in terms of the intensity of turbulence, its spectral distribution, and two-point space-time correlation functions. Similarity laws are given for the power spectra and the space-time correlation functions. On the cylinderr= ½d, the convection speed of the turbulent field is different for the different eddy sizes, varying from the local mean speed for small eddies to ½Uexitfor the large eddies. Measurements of the angular correlation function are reported which show no correlation of the fluctuations across the jet diameter.