P.O.A.L. Davies

Practical flow duct acoustics

Predictions of plane acoustic wave propagation through acoustically reactive flow duct systems are of practical relevance in design studies for industrial, transportation and environmental noise application. Acoustic conditions throughout the flow duct are described by the complex values of the incident and reflected wave amplitudes relative to that of the wave component incident at an open duct termination. Examples are given of the calculation of these relative component amplitudes with a comprehensive range of typical flow duct configurations. The practical consequences of wave attenuation and other losses are quantified, but consideration is restricted to plane wave propagation. In practical application the flux of acoustic energy from a prescribed source through some prescribed system will be governed by the acoustic characteristics of both. To maintain generality of application in this presentation, sources are represented simply by equivalent specified termination conditions. Thus modelling of source characteristics and its coupling with the system is not included here, neither is flow noise, nor is acoustic generation or amplification by flow processes acting as sources. Within these restrictions, effective methods are presented for quantifying and assessing acoustic performance, expressed in terms of realistic predictions for design application to practical situations. The emphasis throughout is based on fluid dynamic and acoustic principles and directed towards the development of physical insight with the identification of the relevant processes controlling wave propagation. Thus the methods described remain general enough for application to any relevant flow duct configuration, as well as those specifically considered here.

PERFORMANCE OF EXHAUST SILENCER COMPONENTS

Abstract This paper describes part of a systematic investigation aimed at producing more accurate theoretical models for internal combustion engine exhaust silencers. The performance of a number of silencer components, e.g. area discontinuities, branched systems, was measured in a very noisy exhaust system. These results were then compared with those predicted by the one-dimensional linearized theory with mean gas flow. It was found that this one-dimensional theory was sufficiently accurate for silencer design purposes.

Flow-acoustic coupling in ducts

Experimental data for two mechanisms of sound generation at area discontinuities in flow ducts are described and discussed. The first step in the process appears to be the development of an ordered train of vortices in the shear layer produced by a separating flow. Though not themselves strong radiators of sound, such vortices can excite resonators strongly. The acoustic field of the resonator provides the sound waves which synchronize the vortex motion, producing a self-sustaining oscillation. Alternatively, synchronization of the vortex motion with an incident acoustic field from a source upstream can enhance the sound by transferring energy from the mean flow.

An experimental investigation of the unsteady pressure forces on a circular cylinder in a turbulent cross flow

Measurements of the mean and fluctuating pressure field on a two-dimensional circular cylinder have been made in the Reynolds number range 0·8×105–6×105. Investigations were carried out for uniform flow and for grid produced turbulence having longitudinal length scales ranging from 0·19D to 0·55D. The experiments for the critical Reynolds number flow cases showed that both the magnitude and the correlations of the pressure fluctuations on the frontal area of the cylinder depend strongly on the parameters of the turbulence in the approaching stream, while the pressure fluctuations on the rear of the cylinder are nearly independent of the turbulence in the approaching flow.

The radiation of sound from an engine exhaust

This paper reports the findings of the first stage of a systematic investigation of the processes occurring in an internal combustion engine exhaust system. It deals initially with finite amplitude pressure wave effects. The boundary conditions imposed on the fluctuating pressure in the exhaust system by the exhaust outlet are then discussed on the basis of measurements made using the tail pipe as an impedance tube. The nett energy flux in the tail pipe including the effect of mean flow is calculated and subsequently related to the level of the sound radiated by the engine exhaust.

The design of silencers for internal combustion engines

Abstract This paper is a review of two theories of silencer design. The traditional approach based on acoustic filter theory is outlined and compared with a new single pulse theory. The latter is based on the well known behaviour of a shock tube. Since the acoustic filter theory describes the harmonic response of a silencer while the single pulse theory describes the transient response it is argued that the two theories are complementary. Comparison with experimental results indicates, however, that the single pulse theory gives a better estimate of the silencer performance unless strong resonances occur.

Some covariance measurements in a subsonic jet

This paper presents covariance and phase velocity measurements in a subsonic cold jet. By using a digital technique, iso-contour maps of the correlation covariance, inside the potential cone and mixing region, yield information concerning the time averaged location of vortices and their velocity of convection.

A recursive approach to prony parameter estimation

Abstract The Prony series method of fitting a series of complex exponentials to a time series can be applied to many acoustic and vibration signals. For example, in the analysis of a structures response to transient excitation the Prony series method can be used to find the natural frequencies, damping ratios, amplitudes and relative phases of the modes of the structure. One of the main problems with this method is the need to calculate and invert matrices. In this paper an algorithm that combines the Prony series method with the recursive least squares algorithm is described. This eliminates the need to invert any matrices and also requires only part of the data to be available at one particular time. The method is applied to analyze a simulated structures response and also to analyze the response of a beam to transient excitation.

Structure of turbulence

Studies of turbulent flow structure at Southampton over the past ten years have been primarily concerned with the quantitative description of those components concerned with aerodynamic noise generation and those concerned with the unsteady loading of structures. The aims of, and experimental approach to, individual experiments has changed over this period with the growth of knowledge and development of facilities. For example, excellent analogue facilities for the measurement of the statistical properties of turbulent flow structure have existed throughout the ten years while fast digital data processing equipment has recently provided what amounts to a new environment for our research. Initially the studies concerned statistical descriptions of the structure of turbulent jets and boundary layers based on time delayed cross-correlation measurements. These provide useful descriptions of practical existing flows but offer no obvious hints how these observed flows may be controlled or modified. More recently the aim has been to describe in more detail the physical structure of the individual eddies that constitute the turbulence and to clearly identify those features of the flow that led to the observed state. Three lines of active development are described, with some promising initial results. The first concerns time domain analysis of the velocity, displacement and pressure records from turbulent flows to identify the details of the characteristic eddy patterns in the flow. The second concerns potential flow modelling of impulsively started flows where arrays of elementary vortices represent vortex sheets and regions of concentrated vorticity. The third concerns modified Orr-Sommerfeld stability analysis of shear flow turbulence. These approaches are clearly complementary and so provide the basis of a comprehensive attack on the problem.

The measurement and prediction of sound waves of arbitrary amplitude in practical flow ducts

A study is presented that explores the influence of the peak to mean pressure ratio on the wave action occurring in highly acoustically reactive ducts with significant flow present. Particular practical applications concern predictions of orifice noise emissions from piston engine or compressor intake and exhaust systems, with the effect of acoustic resonances on both excitation and acoustic power transmission. The results indicate that any practical differences between linear and non-linear predictions remain negligibly small when the pressure ratio remains below 1.1, corresponding to 170 dB spl, provided that the influence of all frequency-dependent physical features is included. Above this level of excitation in any lengths of uniform pipe connecting other system components, some new observations demonstrate the extent to which compression wave steepening may be of practical significance in the spectral distribution of the power propagated.