Future challenges in computational aeroacoustics for fan broadband and combustor noise

Abstract

Two critical areas for the design of quiet gas turbine engines, fan broadband and combustor noise, are briefly reviewed. Both of these noise sources are sensitive to local aerodynamic and geometric variations and thus are ideally suited for analysis by computational aeroacoustic methods. To demonstrate this point for combustor noise and show areas where improved understanding is needed, the scattering of an acoustic wave by a localized heat source is studied. The heat source produces gradients in the mean flow and the speed of sound that scatter the incident duct acoustic mode into vortical, entropic, and higher order acoustic modes. Solutions to the Euler equations are computed to (i) understand how variations in the amplitude and axial extent of a steady heat source and (ii) how unsteady heat release can modify the scattered solution. For an acoustic excitation interacting with a steady heat source, significant entropy waves are produced as the net heat addition increases at the expense of the transmitted acoustic energy. When the net heat addition is held constant, increasing the axial extent of the heat source results in a reduction of the entropy waves produced downstream and a corresponding increase in the downstream scattered acoustic energy. When unsteady heat release which is proportional to mass flow fluctuations in the duct is considered, a significant reduction in the entropy occurs and a corresponding increase in the transmitted and reflected acoustic waves occurs. These results suggest that the dominance of the indirect and direct combustor noise mechanisms are highly sensitive to the heat source lengthscale relative to the acoustic lengthscale and the character of the unsteady heat release.