Compositional inhomogeneities as a source of indirect noise in subsonic and supersonic nozzles

Abstract

Engine-core noise in aeronautical gas-turbines is commonly divided into direct and indirect noise (Strahle 1978; Dowling & Mahmoudi 2015; Ihme 2017). Direct combustion noise is a source of self-noise, and describes the generation of acoustic pressure uctuations by unsteady heat release in the combustion chamber (Figure 1). In contrast, indirect combustion noise represents an induced noise-source mechanism that arises from the interaction between non-acoustic perturbations exiting the combustion chamber and downstream engine components. The indirect noise generation by temperature inhomogeneities arising from hot and cold spots is referred to as entropy noise (Marble & Candel 1977a), and indirect noise from vorticity uctuations is referred to as vorticity noise (Cumpsty 1979). Once sound has been generated, its propagation through the engine core depends on mean ow gradients and the geometry, which distort, di ract and re ect the acoustic propagation. Contributions of indirect noise to the overall core-noise emission have been examined theoretically and experimentally. These studies focused on separating out the contributions to noise from the (i) direct transmission and (ii) entropy noise. Di erent techniques have been employed to determine the transfer functions, including compact nozzle theories (Marble & Candel 1977a) and expansion methods (Stow et al. 2002; Goh & Morgans 2011; Moase et al. 2007; Giauque et al. 2012; Dur an & Moreau 2013). These theoretical investigations were supported by experimental studies (Bake et al. 2009; Kings & Bake 2010). These studies showed that indirect combustion noise requires consideration in the analysis of engine-core noise and can exceed the contribution from direct noise under some circumstances (see, e.g., Dowling & Mahmoudi 2015).