Reduction of engine exhaust noise in a jet engine test cell

High amplitude noise is often generated during the running of a turbofan or turbojet aero engine in a test cell. A large proportion of this noise originates from the exhaust noise from the jet engine and is mostly broadband in nature. Low frequency noise can generate structural vibration and certain unpleasant physio-psychological effects in sensitive people whilst the higher more audible frequencies are a nuisance and contribute to the general noise pollution. Efforts to reduce noise emission, especially in the infrasonic range have had varying degrees of success and target either the flow generated noise or the transmission through the cell. This paper seeks to introduce an alternative and novel method of noise control by reducing the exhaust jet noise. This is achieved by reducing the shear rate between the engine exhaust jet and the cell bypass flow through altering the position of the engine in the test cell and the augmenter tube size. A CFD study was carried out to predict the change in cell bypass flow velocity and Lighthill’s formulae were used to predict the change in jet noise. The combination of these two modifications shows a 1–4 dB reduction with the change in augmenter tube size being the more effective method. However a very simple change in the position of the engine within the main engine chamber can still yield a 1.8 dB reduction in the current model.