The application of computational aeroacoustics (CAA) to automated liner optimisation of turbofan ducts

Astley, Jeremy, Sugimoto, Rie and Mustaphi, Prateek (2010) The application of computational aeroacoustics (CAA) to automated liner optimisation of turbofan ducts. 17th International Congress on Sound and Vibration (ICSV), Cairo, Cairo, Egypt. 17 - 21 Jul 2010. 8 pp .

Abstract

Acoustic liners placed in the intake and bypass ducts of commercial turbofan aeroengines have played a major role in reducing the environmental impact of turbomachinery noise at take-off and approach. The fan stage is the principal source of turbomachinery noise for a modern High Bypass Ratio (HBR) turbofan engine. Typically a turbofan liner is formed from single or double layers of honeycomb material which are separated from the flow and from each other by porous sheets, and fixed to a rigid backing sheet. The acoustic performance of such panels depends upon the nature and spectrum of the source, on the depth of the honeycomb(s) and the resistance(s) of the internal ‘septum’ (in the case of a double layer) and facing sheet, which in turn depend upon gross parameters such as percentage open area but also by more subtle details such as the diameter and profile of the holes, thickness of the sheet, and whether or not a mesh is overlayed to produce a more ‘linear’ response at high sound pressure levels. Overall liner performance is assessed by the extent to which it contributes to a reduction in whole aircraft noise in the far field, as measured by Effective Perceived Noise Level (EPNL) at three noise certification points (lateral, cutback and approach). The selection of physical liner parameters (depth, hole size, open area etc) to reduce EPNL is a complex task which requires the following components:

1. An impedance model which can translate physical liner parameters, such as honeycomb depth, facing sheet percentage open area, hole diameter etc, into resistance and reactance at the surface of the liner.

2. A source model which defines the modal content of the sound field propagating away from the fan stage at the fan face (intake) and OGV (bypass duct).

3. A propagation model to predict sound attenuation in the intake and bypass duct taking into account geometric non-uniformities, mean flow effects and absorption by acoustically treated segments of the duct wall.

4. A radiation model to propagate the acoustic disturbance from the nacelle to the far field, and to predicts the directivity of the resulting sound field.

5. An optimization procedure which embeds steps 1,2,3 and 4 within an EPNL calculation for the whole aircraft (including other noise sources).

This process represented by steps 2 to 4 is illustrated in figure 1 for the case of exhaust radiation. The current paper deals with potential strategies for implementing step 5 of the above sequence.

The study presented in this paper explores the feasibility of using modern CAA tools to perform steps 3 and 4 in the above sequence, and to run such computations within a modern multi-objective optimization suite in step 5 to automate the liner optimization procedure. The computational resources required are shown to be acceptable for realistic optimization or HBR intake liners and the results are shown to be consistent with those obtained using a less automated approach.

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Contributors

Author: Rie Sugimoto

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