Jet noise and the effects of jet forcing

Abstract

This paper gives a review of the experimental situation regarding the acoustic field of axisymmetric jets subject to controlled excitation. Two very different types of response are found in the literature; in one the response to tonal forcing involves a significant reduction in the broadband noise signal except in frequency bands around the tone and its harmonics and subharmonics, which are strongly amplified, while in the other there is a significant and essentially uniform increase in the entire broadband spectral level at all angles to the jet. In the latter case the forcing tone may or may not protrude above the broadband background in the acoustic far field; if it does, the tone is radiated to the far-field with conservation of acoustic energy at Helmholtz numbers greater than unity, and with a substantial loss of acoustic energy from the incident forcing to vortical instabilities on the jet shear layer at low Helmholtz numbers.It is argued that a turbulent initial shear layer and a Reynolds number ReD = UJD/V greater than about 105 lead to broad band amplification for forcing Strouhal numbers StD = fD/UJ around 0.5 and to broad band attenuation at much higher Strouhal numbers (StD ≈ 2). If the initial shear layer is laminar and ReD < 105 the response to tonal forcing at StD ≈ 0.5 involves an amplification at this Strouhal number and a suppression of broadband levels, most of the unsteady energy being locked into a periodic oscillation at the forcing frequency. These remarks apply to cold subsonic jets. The experimental evidence on forced jets is also reviewed for hot jets, for hot and cold imperfectly expanded supersonic jets, for cold perfectly expanded supersonic jets and for subsonic hot and cold coaxial jets. Practical implications of all these studies are discussed, and it is emphasised that the broadband amplification associated with a turbulent shear layer and high ReDis much morelikely to be relevant in practice than the highly -ordered tonal structure at low ReD A brief mention is made of three current theoretical approaches to these phenomena, involving (i) nonlinear stability theory and turbulence closure modelling, (ii) discrete fully Nonlinear axisymmetric vortex ring analyses, (iii) acoustic source modelling to simulate nonlinear instability waves and vortex pairing in the presence of turbulent fluctuations.