Application of Atmospheric Attenuation Correction in Scaling Jet Spectra

Abstract

S CALE model tests of exhaust nozzles are usually carried out to establish the characteristics of jet noise because of significantly lower cost relative to full-scale engine tests. Model tests also allow the flexibility to independently control and vary the jetMach number and jet total temperature, which is not possible with jet engines. Further, acoustic data may be acquired in a controlled anechoic environment with systematic parametric variation. Much of the knowledge on jet noise gained in the last six decades has come mainly from scale model tests conducted at various laboratories. Suitable theories that explain the observed characteristics, and scaling laws to collapse spectra, have also been attempted. To facilitate direct comparison of jet noise spectra obtained fromnozzles of different diameters and varying test-day weather conditions, the measured data must be normalized in some fashion. Recently, Viswanathan [1] investigated several issues that are relevant in the scaling ofmodel scale spectra so as to permit comparisonwith engine noise. Some of these issues addressed include the requirements of the instrumentation system for model and engine tests, a suitable methodology for the calculation of the atmospheric attenuation coefficients, the propagation effects, the repeatability of data, the effects of the disparate Reynolds numbers, etc. It was clearly demonstrated that 1) spectra obtained with nozzles of different diameters can be collapsed perfectly at all frequencies; and 2) a model scale nozzle emits the same jet noise as a jet engine. Viswanathan [2] developed new scaling laws based on the explicit recognition that 1) the variation of the overall sound power level with jet velocity has a weak dependence on jet stagnation temperature ratio; and 2) the variation of the overall sound pressure level with velocity at every radiation angle is a function of jet stagnation temperature ratio. Excellent collapse of the spectra over the entire measured frequency range, at various jet stagnation temperature ratios and at several radiation angles, has been shown in [1–4]. However, some confusion still prevails on the application of the atmospheric absorption corrections in the scaling of spectra. It is well established that the amplitude of acoustic waves is attenuated by the atmosphere during sound propagation. It is also well known that the effect of atmospheric absorption is a strong function of frequency, with the coefficients of absorption increasing with increasing frequency. Several methods have been developed for calculating the absorption coefficients. It was shown in [1] that the method due to Shields and Bass [5] is best suited for the high frequencies of interest in model tests. Some clarification on the proper application of the atmospheric corrections, with concrete examples, is provided in this paper.