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. This means that pressure measurements can here be used to study the space-time structure of the two with a view to discerning the causal relationship which links them. In this work, measurements performed in the near pressure field of an isothermal subsonic jet reveal a strong interaction mechanism between the reactive and propagating components of the pressure field associated with the coherent structures of the flow, the essential features of which are captured by a simple model. On account of the success of the model it is possible to draw a number of interesting conclusions concerning the sound production mechanism associated with these structures, the most important of these being that despite the rotational velocity field in which they exist, where their sound production is concerned they can be considered as a quasi-irrotational, or wavy-wall type source mechanism. The same observation in supersonic jets has led to considerable simplifications where noise prediction is concerned – instability wave models being thus appropriate. The results from this work illustrate how similar models may be appropriate for modelling the noise production by this component of the source mechanism, believed to be predominant where peak radiation is concerned.