A MODELLING OF THE NOISE FROM SIMPLE COAXIAL JETS, PART II: WITH HEATED PRIMARY FLOW

Abstract

This paper reports the second part of a continuing study of the noise of coaxial jets, and describes modifications to the model developed previously to allow for the effects of a heated flow from the primary nozzle. The essential feature of the model described previously for the prediction of the noise from isothermal coaxial jets was the identification of three flow regions, within the coaxial jet flow, the noise production of which could be estimated from single-jet prediction methods. In particular, it was shown that noise from the principal interaction zone could be calculated by using single-jet prediction methods as long as account was taken of the fact that measured turbulence levels in this region were lower than those observed in a single isolated jet at the same centerline velocity. For isothermal flows, for which only quadrupole sources exist, allowance for this reduced turbulence level was entirely straightforward. However, for heated flow both dipole and quadrupole sources exist, and these have different dependencies on the turbulence level. Hence to predict the noise one needs to know the relative contributions of the dipole and quadrupole sources. In the present work, use has been made of previously published results for these relative contributions, as a function of jet velocity and temperature, for single jets. This then permits prediction of the noise from the interaction zone, which is subsequently combined with that from the secondary jet shear layer and fully mixed flow region, as before. Comparison between data and prediction over a range of jet velocity, temperature and angle of observation again show very acceptable agreement.