An equivalent-source model for simulating noise generation in turbofan engines

Nowadays, computational aeroacoustics (CAA) is used for simulating wave propagation in ducted turbofans, especially as computational fluid dynamics (CFD) is increasingly employed to model the identified noise sources. An efficient way to match the CFD and CAA domains is to make some assumptions on flow and duct geometry, so that disturbance fields can be expanded over incoming/outgoing acoustic modes. Based on this approach, this paper presents an original matching model in which the outgoing modes are generated by means of equivalent monopole distributions defined as source terms in the equations governing the acoustic propagation, instead of a conventional inflow boundary condition (BC). Advantages and limits of the method are discussed. The process to get back to the sources and its numerical implementing are described. Although initially focused on tones, an extension of the method to broadband noise generation is tackled too. The method then is validated on a simplified turbofan exhaust configuration. Numerical solutions obtained by implementing the source terms in a high-order time-domain Euler code are compared to analytical solutions, either in a uniform or in a radially shear mean flow (provided by RANS). The parallel shear flow solution is obtained by solving the Pridmore–Brown equation. The ability to accurately simulate the standing waves due to acoustic reflections at duct ends is also assessed by comparing the numerical solutions computed using both source-based and BC-based options in the Euler solver.