Mechanisms of Jet Noise Generation: Classical Theories and Recent Developments

Abstract

Lighthill's acoustic analogy and several variants of it have been used to interpret, analyze and explain measured data. In this paper, the validity of the main tenets of the classical theories is examined. This assessment is facilitated by a new comprehensive aeroacoustic database. The overall sound power level does not exactly follow the eighth power of velocity (V_j/a), but has a weak dependence on jet temperature. It has been believed that an additional source of noise, with either sixth or fourth power variation with velocity, appears for heated jets; there is also a supposed change in spectral shape at 90°, even though effects associated with convection and flow/acoustic interaction are negligible at that angle. The new data indicate that the velocity exponent for overall power is close to eight even at a very high stagnation temperature ratio of 3.2. Moreover, the spectral shape is invariant with jet Mach number and jet temperature. A careful assessment of Tanna's database, which provided the rationale and justification for many of the theoretical models, reveals that this database is unreliable and exhibits incorrect spectral trends. A fundamental tenet of the classical theories, based on both the formalisms of Lighthill-Ffowcs Williams and Lilley, is the notion of moving sources. This idea immediately leads to a Doppler shift for frequency and a convective amplification factor for the noise radiated to the aft angles. It is demonstrated with the new database that there is no experimental evidence for these conjectured effects of moving sources. The jet temperature ratio (static or stagnation), in addition to jet velocity V_j/a, is an independent controlling parameter. New scaling laws that are valid for all angles have been developed with the current database. There are no multiplicative factors with adjustable constants for the spectrum functions; the spectrum function and the velocity exponent depend on the jet temperature ratio and the radiation angle. This relation represents the main difference between the current and the classical formulations. A single exponent collapses the entire spectrum from jets with different V_j/a, but at fixed jet temperature ratio. Therefore, there is no experimental evidence for the main classical ideas of jet noise, as consisting of multi-poles and moving sources, as they are inconsistent with the new data. The idea of two distinct sources that are related to the fine-scale and large-scale turbulence of the jet plume has been investigated. Experimental evidence for the importance of the noise from large coherent structures even at low velocities is offered. The observation at lower and large aft angles of distinct characteristics in, (1) the far field spectral shapes, (2) the directivities of the peak Strouhal number and the OASPL, (3) the directivity of the velocity exponent, and (4) the polar and azimuthal correlations of the farfield pressure, provides strong evidence to support the above view of two different sources of jet noise.