V. P. Maslov

Noise Predictions for Chevron Nozzle Flows

In spite of recent advances in LES methods, RANS based jet noise prediction models appear to be the only realistic, near term, approach for the development of a jet noise prediction procedure that is applicable to chevron nozzle flows, or other advanced nozzle designs. The present paper describes the application of such a model to a series of chevron nozzle flows and the model predictions are compared with experimental data. It is shown that the chevr ons affect t he flow in two important ways. The f irst effect is that the axial vortices cause a very rapid increase in the width of the mixing layer. This process, in effect, leads to a local im balance between the turb ulence and the mean flow, a reduction i n the turbulence production, and an overall reduction in the peak turbulence level, thus contributing to a noise reduction at the lower frequencies. The second is more complex and appears to involve an interaction between the axial vortices and the jet tur bulence that causes more of the high frequency noise from this region to be radiated to the far field. The strength of this new noise source appears to scale with the strength of the axial vortices.

On the prediction of turbulent jet noise using traditional aeroacoustic methods

Several experimental, computational and theoretical results devoted to turbulence jet noise are described. All these results were obtained at CIAM in collaboration with the Boeing Company. The selected topics are combined and described from a single point of view. In spite of the many publications in the classical literature devoted to jet noise, many jet noise problems are not completely solved, and most current jet noise prediction methods are still highly empirical and rely heavily on an existing experimental data base. At the same time, many of the available experimental results are not fully reliable and some are even contradictory. The goal of this paper is, therefore, to explore and discuss some of the possibilities and limitations of classical aeroacoustics methods.

An Experimental Study of Flow Asymmetry in Co -axial Jets

An exper imental study of the flowfields from nominally axisymmetric, co -axial, nozzle configurations showed that these flows are very sensitive to small nozzle distortions. In spite of considerable effort, a fully axisymmet ric flowfield was never achieved in the p resent study. Adding a pylon to the nozzle oriented the flow asymmetry and made the flowfield less sensitive to small nozzle distortions. The resulting flow and noise still tended to change when the nozzle was taken apart and reassembled again, although le ss so than for the axisymmetric configurations. It was demonstrated, however, that if the nozzle was designed to minimize geometry distortions when heated and carefully assembled using special tooling, these changes could be kept within acceptable limits. I. Introduction In an effort to meet increasingly stringent airplane noise standards, attempts are being made to better understand the detailed physics of airplane noise sources. The hope is that with this better understand will come an ability to control an d reduce these noise sources, with a minimum impact on airplane performance. Nowhere is this truer, or arguably more necessary, than for the problem of jet noise. Although it has now been more than fifty years since Lighthills famous landmark paper 1

A RANS based Jet Noise Prediction Procedure

In spite of recent progress in the Large Eddy Simulation (LES) of jet flows, it is argued that a Reynolds Averaged Navier-Stokes (RANS) based jet noise prediction procedure is the only near term approach that can deliver a significant improvement in jet noise predictions for realistic nozzle geometries. Such a model is described in this paper. The flow model used is a modified version of a standard k-e turbulence model and the outputs of this model are input to an empirical jet noise prediction model. Flow and noise predictions from this model are compared with experimental data for flows that range from single stream jets to more complex coaxial nozzle configurations. The overall results are very encouraging.

Investigation of unsteady processes, flow properties, and tonal acoustic radiation of a swirling jet

The results of an investigation of the unsteady flow structure in a turbulent swirling jet obtained using the PIV technology are presented. The greater part of the measurements is carried out at the swirl intensity W0 ≈ 1.7. A part of the data is obtained under other conditions of the swirl jet outflow. To establish the relation between disturbances of different types the phase averaging technique is employed with the pressure fluctuation in the acoustic field of the jet taken as a reference signal. The flow structure is numerically calculated. The results of the investigation show that a quasisteady inhomogeneity observable in the jet flow executes rotational motion relative to the mean flowfield in the jet cross-section, or “precession”. It causes disturbances in the flow ejected by the jet, which transform into acoustic disturbances far away from the jet. The frequencies of the dynamic disturbances near the jet and the acoustic disturbances far away from it coincide with the precession frequency.

Experimental and numerical investigation of jet flap interaction effects

The influence of the wing and flaps geometry on the flow field and acoustic radiation of the jet are experimentally and numerically investigated. The azimuth non-uniformity of jet acoustic field was experimentally defined with a help of far-field microphones, flow field within the jet was measured using PIV technique and numerically calculated with a help high resolution combined RANS/ILES method. The jet flaps interaction (JFI) effects was experimentally and numerically investigated at facility under modeling of real by-pass nozzle configuration and outflow parameters. The comparison between measured flow-field and noise of a jet shows that JFI excess noise is mainly related to jet deformation and increasing of velocity gradients and turbulence within jet which are induced by tip flap vortexes.

Experimental study of propagation of instability waves in a submerged jet under transverse acoustic excitation

An experimental study was conducted on the specific features of instability wave propagation in the mixing layer of a turbulent jet when the jet is excited by an external acoustic wave. We used the technique of conditional phase averaging of data obtained by particle image velocimetry using the reference signal of a microphone placed near the jet. The influence of the excitation frequency on the characteristics of large-scale structures in the mixing layer was investigated. It is shown that the propagation patterns of the instability waves agree well with previously obtained data on the localization of acoustic sources in turbulent jets.

Deflected Fan Flows: Nozzle Installation Effects

In this paper, the sensitivity of jets from coaxial nozzle configurations to small asymmetries in the nozzle geometry is discussed and noise results are presented from an experimental study of four coaxial nozzle/pylon configurations. These include BPR5 and BPR10 nozzle configurations, with and without chevrons on their primary nozzles. All four nozzle configurations are designed so that the fan nozzles can be rotated, in the plane of the pylon, through a small angle. Because of the sensitivity of these jets to small asymmetries in the nozzle geometry, a small deflection of the fan stream can lead to strongly asymmetric flows. This flow asymmetry can, in turn, lead to asymmetry in the noise radiated in different azimuthal directions, with a noise reduction from the side of the jet where the fan stream is widest. This potential jet noise reduction concept was first discovered in the mid 1970’s and has been studied now, on and off, for about 35 years. While most of these studies claimed an overall jet noise reduction, the concept has never been used on any production airplane. The reason for this, it is argued here, is that, despite numerous claims to the contrary, deflecting the fan stream down will not give any useful noise benefit for a real engine installation. The reasons for this is that, unlike real nozzle configurations, most previous studies have been conducted using isolated nozzles without a pylon and with no external stream. While adding an external flow will reduce the noise benefit of deflecting the fan stream, the main problem with using isolated nozzles is that the important effect of the pylon is ignored. The presence of a pylon deflects the primary stream up relative to the fan stream, thickening the fan flow under the primary jet and thinning it on the top. This asymmetry, it is shown, will generally be enough, or more than enough, to provide any noise benefit that may be available from deflecting the fan flow down and that a further thickening of the fan stream is unlikely to give any additional benefit.