Coherent Structures in Subsonic Jets: A Quasi-Irrotational Source Mechanism?

Abstract

The true global source of the farfield sound radiated from a subsonic jet is the entire dynamic found within the confines of the hydrodynamic field, a dynamic which comprises an unsteady compressive excitation of the medium resulting from turbulent mixing and unsteady temperature fluctuations, these being driven by a wide range of turbulence scales. Improved understanding, and subsequent modelling of the space-time character of the global source term has been acheived in the past through study of the different physical mechanisms implicated in its dynamic. The phenomena which have to date been accepted as important in terms of the radiated sound are (i) turbulent mixing and shear, (ii) fluctuating entropy, (iii) convective amplification and (iv) refraction and scattering of sound by the mean and turbulent components of the velocity field. Once identified as important, specific modelling strategies can and have been developed in order to deal with these phenomena. The existence of coherent structures in turbulent jets was identified as important in the 1970's and their role in the production of sound has received considerable attention in more recent years. However, direct identification of the causal link between this component of the turbulence and its sound field is a delicate matter due to the virtual impossibility of directly measuring the source dynamic of the flow, this being due to an overwhelming dominance of hydrodynamic energy in the source region, and a total absence of any hydrodynamic signature in the linear, acoustic region. As a result, modelling strategies where this component of the source term is concerned have remained, at best, highly empirical.An interesting region of the flow where our understanding of the relationship between the hydrodynamic cause and its acoustic effect is concerned is the near pressure field, found just outside the rotational region of the flow. In this region the signature of the sound production mechanism and its resultant sound field are both present, coincident in space and in time.

On least-order flow representations for aerodynamics and aeroacoustics

“…The loud flow structures of both LR-OID mode pairs are in qualitative agreement with experiments (see e.g. Juvé et al 1980;Guj, Carley & Camussi 2003;Hileman et al 2004;Coiffet et al 2006). In figure 10(g-l), the first six LE-OID modes are shown.…”

Direct Computation of the Noise Generated by a Hot Coaxial Jet

“…The influence of the choice of the control surface on noise prediction may therefore be weak, which will be investigated quantitatively later. There is also a fair agreement with the near field determined by the LES, even if contributions of aerodynamic pressure 42,43 can be significant in the vicinity of the jet flow in the LES. In addition, the LES pressure field looks to be somewhat dissipated as the radiation distance increases.…”

Three-dimensional evolution of flow structures in transitional circular and chevron jets

“…Nevertheless, as the acoustic production is strongly related to the large-scale flow motion, 32,33,40 the observation of the temporal evolution of large-scales structures is of major importance to understand the physical mechanism behind the production of acoustic noise.…”