Noise produced by the interaction of acoustic waves and entropy waves with high-speed nozzle flows

Some aspects of the noise generated internally by a turbojet engine are considered analytically and experimentally. The emphasis is placed on the interaction of pressure fluctuations and entropy fluctuations, produced by the combustion process in the engine, with gradients in the mean flow through the turbine blades or the exhaust nozzle. The results are directly applicable to the problem of excess noise in aircraft powerplants and suggest that the phenomenon described is the dominant mechanism. The one-dimensional interaction of pressure fluctuations and entropy fluctuations with a subsonic nozzle is solved analytically. The acoustic waves produced by each of three independent disturbances are investigated. These disturbances, which interact with the nozzle to augment the acoustic radiation, are (i) pressure waves incident from upstream, (ii) pressure waves incident from downstream, and (iii) entropy waves convected with the stream. It is found that results for a large number of physically interesting nozzles may be presented in a concise manner. Some of the second-order effects which result from the area variations in a nozzle are investigated analytically. The interaction of an entropy wave with a small area variation is investigated and the two-dimensional duct modes, which propagate away from the nozzle, are calculated. An experiment is described in which one-dimensional acoustic waves and entropy waves are made to interact with a subsonic nozzle. The response of the nozzle to these disturbances is measured and compared with the response as calculated by the analytical model. The interaction of two-dimensional entropy waves with a subsonic nozzle and with a supersonic nozzle is investigated experimentally. The results are explained in terms of an analysis of the acoustic waves and entropy waves produced by a region of arbitrary heat addition in a duct with flow.