Computation of mode radiation from a generic aeroengine intake

The radiation of acoustic modes from an aeroengine intake duct is reviewed. The method is based upon a computational scheme which allows acoustic waves, propagating inside the intake duct of a generic aircraft engine, to be admitted into a computational domain that includes the duct section, the exit plane of the duct, and the surrounding flow. The method comprises three elements: a matching process to admit acoustic waves into the in-duct propagation region; near-field propagation inside the duct and diffraction at the lip of the duct; and an integral surface for far-field directivity. The wave admission is realised through an absorbing non-reflecting boundary treatment which admits incoming waves and damps spurious waves generated by the numerical solutions. The wave propagation and diffraction are calculated by solving either the linearised Euler equations or the full Euler equations, using high-order compact schemes. Far field directivity is estimated via an integral surface solution of the Ffowcs Williams - Hawkings equation. This paper compares the use of the Euler and linearised Euler equations in determining acoustic radiation. The case of wave propagation from a realistic aeroengine intake with background mean flow is used for the comparison. First, axi-symmetric acoustic radiation is studied using two different mean flows. One simulating the engine at a high power setting, the other a situation where the flow around the lip becomes locally supersonic. Spinning mode radiation is then examined with the solution of the full 3D euler equations and a 2.5D linearised Euler model. The model requirements are discussed along with an assessment of the readiness of the methods for industrial applications.