Broadband wave propagation from an Aero-engine duct

Accurate and efficient computation of multimode/broadband noise propagating into and radiating out of a duct has both fundamental and practical interests, particularly for broadband turbofan noise problems. In this work, a new complex formulation of the linearized Euler equations is proposed in the time domain for accurate and efficient multimode noise computations via a single-mode summation approach. The approach achieves incoherent results by summation of individual predictions from excitation modes at all discrete frequencies. The solutions are obtained using high-order computational aeroacoustics algorithms. An advantage of the proposed formulation is that the number of computations is equal to the cuton mode number at the highest frequency regardless of the frequency sampling rate. Furthermore, the computation cost only increases marginally despite a doubling of the governing equations in real space. Tests show that the method could potentially achieve significant computational cost savings. This approach is compared with a time-domain random phase approach with which incoherent results are obtained by averaging the results from repeated simulations with random phases added to the wave inputs in each computation. Examples including a generic bypass engine duct are included to illustrate the reliability and efficiency of the method