Rodney H. Self

A RANS based jet noise prediction scheme

A jet noise prediction scheme for single isothermal turbulent jets is presented. The acoustic source model is based on the Lighthill acoustic analogy and, although much of the theory used is well known, the model differs significantly from others by taking account of the frequency dependence of time and length scales used in modelling the correlation function. The input required for the source model can be obtained from a fast running CFD code such as a Reynolds Averaged Navier Stokes (RANS) scheme using a simple kturbulence model. Flow-acoustic interaction effects arise because of the propagation of the noise through the mean flow of the jet and this is accounted for by using a corrective flow factor which is angularly dependent. The model is used to compute the far-field acoustic intensity spectra of a single jet and the results obtained are shown agree well with those predicted for the same jet using an empirically derived database. It is briefly indicated how the source model can be generalized to coaxial jets as well as more novel nozzle geometries.

Improved jet noise modeling using a new acoustic time scale

To calculate the noise emanating from a turbulent flow (such as a jet flow) using Lighthills analogy, knowledge concerning the unsteady characteristics of the turbulence is required. Specifically, the form of the turbulent correlation tensor together with various time and length-scales and convection velocities are needed. However, if we are using a RANS calculation then we obtain only steady characteristics of the flow and it is then necessary to model the unsteady behaviour in some way. While there has been considerable attention given to the correct way to model the form of the correlation tensor (or equivalently the spectral density), less attention has been given to underlying physics that dictate the proper choice of timescale. In early studies various authors tended to assume that the acoustic timescale was proportional to the turbulent dissipation rate but later studies have revealed that a frequency dependent relationship gives better results. In this paper we recognise that there are several time dependent processes occurring within a turbulent flow and propose a new way of defining an acoustic timescale. An isothermal single-flow M0.75 jet has been chosen for the present study and essential fluid dynamic information and turbulent parameters have been obtained using a modified k-e method. The jet noise prediction at 90 deg is found using Lighthills analogy and directivity is estimated using an asymptotic solution of Lilleys formulation. Predictions reveal good agreement between the noise predictions and observations. Furthermore, the new time-scale has an inherent frequency dependency that arises naturally from the underlying physics thus avoiding supplementary mathematical enhancements to the model.

Aeroacoustic catastrophes: upstream cusp beaming in Lilley's equation

The downstream propagation of high-frequency acoustic waves from a point source in a subsonic jet obeying Lilleys equation is well known to be organized around the so-called ‘cone of silence’, a fold catastrophe across which the amplitude may be modelled uniformly using Airy functions. Here we show that acoustic waves not only unexpectedly propagate upstream, but also are organized at constant distance from the point source around a cusp catastrophe with amplitude modelled locally by the Pearcey function. Furthermore, the cone of silence is revealed to be a cross-section of a swallowtail catastrophe. One consequence of these discoveries is that the peak acoustic field upstream is not only structurally stable but also at a similar level to the known downstream field. The fine structure of the upstream cusp is blurred out by distributions of symmetric acoustic sources, but peak upstream acoustic beaming persists when asymmetries are introduced, from either arrays of discrete point sources or perturbed continuum ring source distributions. These results may pose interesting questions for future novel jet-aircraft engine designs where asymmetric source distributions arise.

Microphone Position and Atmospheric Eects in Open-air Engine Noise Tests

Typically, outdoor engine measurement test sites (such as the Rolls-Royce test bed at Hucknall, UK) utilise near-ground based microphone arrays that are up to 3m below the engine exhaust exit. The rationale for choosing a near-ground microphone array over pylon-mounted arrays (typical in some anechoic installations such as the QinetiQ facility at Pyestock, UK) is that it overcomes any need to account for ground reections. However, there are a number of possible disadvantages associated with ground-based microphones, which have not been systematically investigated before. These include refraction eects due to variations in atmospheric conditions with height, and loss of coherence due to turbulence in the surface boundary layer. If these can be properly accounted for it may be the case that the use of pylon mounted microphone arrays would yield more information than the near-ground counterpart. Equally, a model of these atmospheric eects will facilitate a better understanding of the relationship between open-air test site measurements and those made in controlled anechoic environments. This paper presents a rst step towards such an investigation.

Parametric Study of Jet Nozzles Using a RANS-Based Jet Noise Prediction Tool

The development of computationally fast and robust RANS-based jet noise prediction tools are essential to improve the design of quieter engine nozzles. Various jet noise methodologies have been proposed since the 1990’s, but most of these methods have remained as research tools instead of being commercial viable. All RANS-based jet noise methods include model coefficients associated with eddy length-scales and time-scales. Computation of these model coefficients are dependent on measured acoustic spectra at 90 far field polar angle, thus making it difficult to predict noise from new conceptual nozzle configurations. In the current work, improvements to a RANS-based jet noise prediction method are proposed. The source model is based on Lighthill’s acoustic analogy and ray theory is used to compute the propagation effects. The current work focuses on arriving at a formalism to determine the RANS-associated length-scale and time-scale coefficients with reduced dependency on experimental data. The formulation is based on a parametric study of a range of jet nozzles and varying flow conditions. As an outcome, the effect of various jet noise parameters on the far field noise is investigated.