The influence of an adaptive nacelle inlet lip on fan noise propagation

The aeroacoustic performance of an adaptive inlet of a turbo-fan engine is numerically investigated in this paper. The sound intensity and directivity of the fan noise propagation to the far-field, and the sound level at lateral reference points are investigated. The investigation is performed for three Helmholtz numbers, with the influence of the mean flow included, for a single duct mode (8,1). The contour was defined by five movable knots at the leading edge of the inlet. The contour had to fulfil two constraints, namely it had to have a constant length and a convex curvature. The process of contour adaptation was performed in two steps. In the first step, two knots on the inner inlet side were moved in order to attain a certain shape, while other knots were kept fixed. In the second step, the rest of the knots were moved in order to fulfill the constraints. A finite element solver for the Helmholtz equation is used in the inner part of the inlet, with a perfectly matched layer boundary condition close to the inlet entrance. The propagation through the outer part of domain is solved by Kirchhoff integral method. The results show the influence of the inlet shape adaptation on the noise intensity level as well as the directivity of propagation. The maximum peak intensity level of all inlet shapes is increased by increasing the Helmholtz number. This causes the width of intensity distribution to become narrower and shifted towards the symmetry axis of the nacelle. The inlet shape with the most opened nacelle throat has the lowest peak and an intensity distribution shifted towards the symmetry axis, which indicates the influence of the mean flow. Also, the more closed nacelle throat causes a decrease of the effective perceived noise level.