Studies of the influence of density on jet noise

In recent years, studies of exhaust noise from turbofan or turbojet engines at low thrust have emphasized the lack of consistency of jet noise prediction methods, particularly at low exhaust velocities. This situation has arisen mainly from difficulties with engine noise analysis and from the presence of rig noise in many model test facilities. These problems have masked the true effect of density on jet noise. Two independent research programmes on model convergent nozzles have been proceeding concurrently at the SNECMA and the NGTE using advanced research facilities to study the effect of density by varying the jet temperature at velocities from 150 to 800 m/s. Data for the sound power and peak noise levels are presented together with a range of spectra and directivity patterns. A principal result of these studies is that, whilst at high jet velocities the jet noise decreases with decreasing density, at low velocities the trend is reversed. It is shown that the noise data can be empirically correlated by using a velocity-dependent function of the fully-expanded jet density.