The effect of a baffle on acoustic radiation directivity

Abstract

An investigation is made of the effect of an artificial baffle introduced in Finite Element modeling of acoustic radiation from turbofan inlets. The purpose of the baffle is to limit the extent of the computational domain in order to maximize the efficiency of the scheme. However, the possibility exists that the baffle, if improperly oriented, will generate spurious reflections which interfere with the true radiated acoustic field. A generic example of a turbofan inlet, including typical geometry and non-uniform potential flow in and around the nacelle, is modeled with a baffle as typically introduced limiting the angular extent of the domain around the inlet to approximately 140 degrees from the inlet axis, and with the baffle removed with the domain of computation being the entire half space surrounding the nacelle. Acoustic modes introduced at the source plane are chosen to produce radiated fields with peak directivity at increasing angles from the inlet axis. Acoustic fields are computed using the baffled and unbaffled models and comparisons made of the baffled results with the baseline unbaffled results. It is found that even with radiation patterns with significant lobes at as high as 70 degrees from the inlet axis very little effect of the baffle on radiation directivity is observed except near the baffle. The effect of the baffle on phase accuracy is tested by representing the noise source with multiple radial modes which produce characteristic multiple lobe radiation patterns due to constructive and destructive interference. The baffle is observed to have little effect on radiation directivity except near the baffle, even when significant lobes are present at angles from the axis as high as 70 degrees, suggesting that both amplitude and phase are not significantly affected by the baffle.______ Copyright © 1998 by Walter Eversman. Published by the Confederation of European Aerospace Societies with permission. INTRODUCTION Computation of the acoustic field in and around turbofan nacelles accounting for geometric and flow field details is approached by parallel processing implementations of time accurate finite difference formulations of the non-linear field equations [1] and by finite element method (FEM) formulations of the linearized acoustic field equations for harmonic time dependence [2-6]. Either approach results in a large computational problem because of grid refinement required to resolve acoustic perturbations which have wave lengths which may be considerably smaller than the pertinent length scale of the nacelle. In FEM models the dimensionality of the problem has been controlled to some extent by the introduction of an artificial baffle which limits the computational domain to a region which is judged to be sufficient to capture the essential details of the acoustic field without substantial contamination by spurious reflections introduced by the baffle [2-6]. The artificial baffle can significantly reduce dimensionality, but at the expense of the introduction of uncertainty in the accuracy of the computed acoustic field. Figure 1 shows a computational domain used for inlet radiation. The baffle is swept back at an angle which is typically set by experience to be about 90 degrees behind the angle at which peak radiation directivity is expected to occur. If this condition is met, little adverse effect of the baffle is expected to be observed in the region of peak radiation. However, in many cases the condition on baffle orientation is not met because of the necessity to reconstruct the computational mesh for each case or because of the source characteristics which may generate an acoustic field with multiple peaks in directivity (lobes) and therefore no clear direction of peak directivity. Little is known about how failure to conform to the accepted limit on baffle orientation affects modeling accuracy.