Utilization of turbulent energy transfer rate time-scale in aeroacoustics with application to heated jets

This paper presents a study of the effects of the form of the time-scale used in aerodynamic noise prediction methodologies with application to the prediction of noise from hot jets. It was motivated by the need to improve the spectral shape of predictions obtained using Lighthill Acoustic Analogy based schemes such as the MGBK method in conjunction with a RANS CFD flow simulation. Acoustic Analogy formulations require knowledge of the unsteady characteristics of the turbulence whereas using a RANS calculation as the starting point provides only steady characteristics of the flow and it is then necessary to model the unsteady behaviour in some way. A critical issue is the proper definition of the acoustic time scale which empirical studies have shown to be frequency dependent. The authors discuss how this dependence arises from the underlying physics of the flow by considering a timescale based on the rate of energy transfer through the turbulent cascade. The technique is applied to noise prediction for both isothermal and heated jets using the MGBK method. It is shown that using the new definition of timescale results in good agreement with experimental measurements in both cases. It is suggested that such a definition will prove robust and find application in other areas of aeroacoustic noise prediction