The noise of ejectors

Abstract

Summary. An extensive model-test programme has been carried out on the noise properties of a series of fifteen axi-symmetric ejectors, using a two-inch diameter cold jet as the primary discharge and inducing the secondary flow from atmosphere. These ejectors had (D/d)² ratios of 2, 3 and 4, and (L/d) ratios which lay between 3 and 32, ('D' and 'L' are the diameter and length of a particular ejector and 'd' the diameter of the cylindrical primary nozzle). Parallel experiments at the Royal Aircraft Establishment, Farnborough, have investigated the static aerodynamic properties of these ejectors. This report concentrates on the features of the broad-band discharge, after making a full survey of the types of noise present. No noise came from the ejector casing, and radiation of noise from the ejector intake was prevented by the use of an absorber box. Radiation from the exhaust for certain choked conditions was found to contain shock noise similar in nature to the unshrouded case and some unstable high-frequency broad-band noise was detected. Further discrete noise could emanate when the flow was entirely subsonic, and though the walls of the ejector were excited by the internal airflow this was not the source of this noise which had the properties of a non-linear oscillation within the shroud. The acoustic efficiency of this type of noise depended on the modes of excitation and was greatest for the shorter shrouds operating at pressure ratios of less than about 1.4. In many cases this noise was of sufficient strength to dominate the broad-band efflux noise. In certain cases the contribution due to the discrete noise could be removed by analytic means to leave an estimate of the broad-band discharge. It is shown that this latter noise appeared to be more related to the mixing conditions of the primary jet than to the conditions of the mixed stream at discharge. Defining the attenuation of an ejector as the difference between the peak (broad-band) noise levels measured along a line parallel to the jet axis with and without the ejector in position, such attenuation increased with increase in ejector length to a maximum of about 7 db but was independent of diameter. A survey and assessment of other published noise work on ejectors is included.