The Use Of Acoustic Scale Models For Investigating Near Field Noise Of Jet And Rocket Engines

Analytical and experimental studies have been made to examine the feasibility of using acoustic scale models for near field noise investigations. Analyses show that the important characteristics of noise generation, propagation, and measurement can be scaled. A few deviations from this involve small errors which are negligible in the near field. The most straightforward model duplicates the gas flow parameters of the full scale engine. The validity of such models has been demonstrated by a series of tests for a wide variety of nozzle exit conditions, from turbojet through rocket exhausts, and whether in a free field or in the presence of objects which interfere with the flow, such as shaped nozzles and flame deflectors. It is further determined both analytically and experimentally that models may be simplified without impairing the results of a scaled test. Considerations in simplifying a model include: reduction of the nozzle size; absence or presence of reflecting surfaces; use of fewer than the full scale number of engines; and use of a substitute gas which is different from and at a lower temperature than that in the full scale engine.

The report summarizes the results obtained at General Electric during the first phase of the Air Force Supersonic Exhaust Noise - Velocity Model Program. The overall objective of the program is to develop the technology to significantly reduce supersonic aircraft propulsion system noise with minimum associated performance and weight penalties. To fulfill that objective, research is being carried out to develop the experimental techniques and the necessary theory to reveal the basic mechanisms of jet generated noise through the range of velocities and temperatures typical of present and future military and commercial supersonic aircraft propulsion systems. A comprehensive aerodynamic analytical model describing the flow mechanisms in supersonic jets is presented and compared with experimental data. A large number of theoretical models describing supersonic far field jet noise are evaluated. (Author).