Accurate computation of multimode propagation in and radiation from a turbofan duct is important for aircraft noise prediction. A formulation of linearised Euler equations (LEE) is proposed for efficient computation. Its solutions are sought in time-domain using high-order computational aeroacoustic (CAA) algorithms. An advantage of the proposed method is that the number of computation is the cut-on mode number at the highest frequency regardless frequency sample interval. Furthermore the

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... ing cost only increases marginally despite a doubling of the governing equations in real space. Tests show that the method could potentially achieve computing cost saving. Examples including a generic bypass engine duct are given to illustrate the reliability and efficiency of the method.