1,720,982 research outputs found
Jet noise prediction using the Lighthill acoustic analogy
No full textA jet noise source model based on the Lighthill acoustic analogy is presented. Although much of the theory used is well known, a new feature of the model is the inclusion of frequency dependence for the time and length scales used in the turbulence two-point correlation function. It is found that allowing for this experimentally observed dependence markedly improves the agreement of the model's prediction with experimental far-field data. To illustrate this agreement the case of a single turbulent jet is considered. Using well-respected scaling laws for the mean and turbulent properties of such jets a prediction for a single jet noise spectrum is obtained which shows very good agreement with the prediction using the empirically based ESDU database. The effect of altering the frequency dependence of the moving axis timescale is briefly discussed and it is indicated how the source model can be generalized to use RANS and other CFD data to predict jet noise, for single and coaxial jets and also for more novel nozzle geometries
Aeroacoustics research in Europe: The CEAS-ASC report on 2001 highlights
No full textThis paper summarizes some highlights of aeroacoustics research in Europe in 2001, compiled from information provided to the Confederation of European Aerospace Societies (CEAS) Aeroacoustics Specialists Committee (ASC). The CEAS comprises the national Aerospace Societies of France (AAAF), Germany (DGLR), Italy (AIDAA), The Netherlands (NVvL), Spain (AIAE), Sweden (FTF), Switzerland (SVFW) and the United Kingdom (RAeS)
Theoretical and analytical comparison of stochastic noise generation and radiation (SNGR) method and acoustic analogy using a new timescale
No full textCones of silence, complex rays and catastrophes: high-frequency flow-acoustic interaction effects
No full textIn this paper we develop a novel ray solver for the time-harmonic linearized Euler equations used to predict high-frequency flow-acoustic interaction effects from point sources in subsonic mean jet flows. The solver incorporates solutions to three generic ray problems found in free-space flows: the multiplicity of rays at a receiver point, propagation of complex rays and unphysical divergences at caustics. We show that these respective problems can be overcome by an appropriate boundary value reformulation of the nonlinear ray equations, a bifurcation-theory-inspired complex continuation, and an appeal to the uniform functions of catastrophe theory. The effectiveness of the solver is demonstrated for sources embedded in isothermal parallel and spreading jets, with the fields generated containing a wide variety of caustic structures. Solutions are presented across a large range of receiver angles in the far field, both downstream, where evanescent complex rays generate the cone of silence, and upstream, where multiple real rays are organized about a newly observed cusp caustic. The stability of the caustics is verified for both jets by their persistence under parametric changes of the flow and source. We show the continuation of these caustics as surfaces into the near field is complicated due to a dense caustic network, featuring a chain of locally hyperbolic umbilic caustics, generated by the tangency of rays as they are channelled upstream within the jet.</p
Jet noise prediction using different turbulent scales
No full textThe turbulent energy dissipation rate time-scale and length-scale has been routinely used for the prediction of noise from turbulent flows, particularly jet streams. However, this is not the only possible choice. In general, scales evolving in a turbulent medium are threefold. First, those associated with the mean flow; second, those attributed to the turbulence and the mean flow interactions; and third, scales related to the turbulence-turbulence interactions. In this paper, special attention will be paid to further study of the underlying physics of aerodynamic noise by examining various time-scales. To do so, three time scales, namely, dissipation, production, and strain rate time scales, are defined and used in the source modelling to emphasis the effect of the turbulence structures at different jet regions on the jet noise production mechanism. The required mean value and turbulence parameters are obtained using a modified k ? ? turbulence model, and Lighthill’s Acoustic Analogy is used for the prediction of the emanated noise. <br/
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