Flight effects on exhaust noise for turbojet and turbofan engines—comparison of experimental data with prediction

Recent experiments on the effects of flight on jet engine exhaust noise have produced apparently conflicting results. Some of these results do not agree with projections based on classical jet noise theories nor with experimental results from model jet simulated flight tests. It has been shown that in some of the cases reported, the proper corrections were not made to account for the distributed nature of the jet noise sources. It is shown herein that the remaining discrepancies can be reconciled by considering the combined effects of jet-mixing noise, internally generated engine exhaust noise, and shock noise. This paper demonstrates that static and in-flight jet engine exhaust noise can be predicted with reasonable accuracy when the multiple-source nature of the problem is taken into account. Jet mixing noise and shock noise are predicted from an improved version of the NASA interim prediction method. A provisional method of estimating internally generated noise is used, based partly on existing prediction methods and partly on recently reported engine data.