An iterative method for predicting turbofan inlet acoustics

A new iterative solution technique for predicting the sound field radiated from a turbofan inlet is presented. The sound field is divided into two regions; the sound field within the inlet which is computed using the finite element method and the radiation field outside the inlet which is calculated using an integral solution technique. A "unified" solution is obtained by matching the finite element and integral solutions at the interface between the interior and exterior regions. The applicability of the iterative technique is demonstrated by considering several simple cases for which exact or *'classical" solutions for the sound field are available. These examples show that the proper solution is obtained within five iterations. The overall accuracy of the method is demon- strated by comparison with experimental data. ROWING concern in recent years over the rise in noise pollution, especially that caused by turbofan engines, has created considerable interest in developing methods for predicting the acoustic properties for these engines. The two major sources of turbofan engine noise are the externally generated noise produced by the jet exhaust and the internally generated noise due to the rotating turbomachinery, turbofan blades, and combustion processes. The noise due to jet exhaust has become less intense due to the use of high bypass ratio turbofan engines which have a reduced jet velocity over earlier low bypass ratio engines. This, in turn, has made the noise produced by the turbofan more apparent. In an attempt to reduce the turbofan noise, much effort has been expended in the development of acoustic liners for these engine inlets. Unfortunately, due to the lack of analytical methods for such development, most of the design work for these liners has been performed by costly cut and try methods. For this reason, analytical methods are needed to equip the designer with the tools necessary for the job of designing quieter ef- ficient turbofan inlets. As the first step in a theoretical approach to reducing the amount of noise that will reach an observer, whether it be a passenger in the aircraft or an observer on the ground, an analytical procedure for predicting the acoustic field of the engine must be developed. This analysis should include the effects produced by short inlet ducts with varying geometry and lined inlet walls. To date, no such analytical method for predicting the entire sound field exists. This paper describes a general approach for calculating the acoustic field of a tur- bofan inlet. The approach is not restricted to any particular computational schemes. Thus, many of the analytical ap- proaches presently used in duct acoustics can be extended to provide a realistic description of the entire sound field. Consider the somewhat idealized inlet shown in Fig. 1. Noise produced within the engine propagates down the inlet duct and radiates out to the surrounding environment. The