Influence of non-symmetric configurations on fan intake aeroacoustics

Realistic turbofan inlet configurations are usually non-symmetric and this can considerably influence the radiated soundfield. In this paper a three-dimensional finite element scheme is used to model such inlets in the frequency domain. By employing an irrotational mean flow assumption, the aeroacoustics problem is posed in terms of the acoustic velocity potential. Thus, the formulation represents aeroacoustic propagation through a three-dimensional potential flowfield. The duct and exterior, field regions are fully coupled and simultaneously solved by employing a single continuous inlet and farfield mesh. Conventional elements are used in the nearfield and a wave envelope mesh in the farfield. A preconditioned iterative solver is used in conjunction with a sparse storage scheme which enables relatively large problems to be solved using modest computing facilities. For three separate modes, the method is used to predict the acoustic field of three radius-lipped inlet configurations. These comprise an axisymmetric, a 15° scarfed and a combined Sduct / scarfed inlet combination. The FE scheme is used to compute both the three-dimensional potential flowfield and the acoustic fields where the source is prescribed as an incident mode at the fan face. Although single modes are used here, in principle, an entire discrete modal spectrum can be handled in a single run. For certain selected modes, the scarfing to have a significant effect on directivity.