Design and characterisation of a quiet, low turbulence open jet blow down wind tunnel in ISVR

This technical report presents the design of an open-jet, blow-down wind tunnel that was newly commissioned in the anechoic chamber at the ISVR, University of Southampton, UK. This wind tunnel is intended for the measurement of airfoil trailing edge self-noise but can be extended to other aeroacoustic applications. With the primary objectives of achieving acoustically quiet and low turbulence air jet up to 120m/s through a 150mm x 450mm nozzle, several novel noise and flow control techniques were implemented in the design. Both the acoustical and aerodynamic performances of the open jet wind tunnel were calibrated in detail after its fabrication. It is found that the background noise of the facility is adequately low for a wide range of exit jet velocity. The potential core of the free jet is characterised by a low turbulence level of about 0.1%. A benchmark test by submerging a NACA0012 airfoil with a tripped boundary layer at zero angle of attack into the potential core of the free jet was carried out. It was confirmed that the radiatingaerofoil trailing edge self-noise has levels significantly above the rig noise over a wide range of frequencies. The low noise and low turbulence characteristics of this open jet wind tunnel are comparable to the best facilities in the world, and isbelieved to be the first of its kind in the UK

The measurement of sound power flux in flow ducts

This paper describes the development of robust procedures yielding reliable estimates of the nett sound power flux associated with one-dimensional wave motion under strongly reactive conditions in flow ducts. In such reverberant situations, the measurements must be sufficiently precise to clearly identify the small fraction of the total fluctuating wave energy that is being propagated through the system [1–4]. An expansion chamber, together with its inlet and outlet pipes radiating into a semi-anechoic space, was chosen as a simple but sufficiently representative example of such systems. Various practical problems, such as those arising from low signal-to-noise ratios, or any inadequacies of microphone calibration were investigated in detail, along with various strategies for minimizing their influence on the realism and reliability of the associated measurements. The most effective procedures were identified by performing a sequence of comparisons between the resulting measurements and checking them against data generated with an existing and well-verified prediction code

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

The observed aeroacoustic behaviour of some flow-excited expansion chambers

Expansion chambers are often installed in flow duct systems to reduce pressure pulsations and to act as silencers. However, in certain circumstances they can become flow-excited sound generators rather than attenuators. The physics of the flow acoustic interactions responsible for this are examined in detail. They are then illustrated by a systematic sequence of sound power flux measurements in the downpipe, expansion chamber and tailpipe of some representative examples. These measurements then identify both the position and spectral characteristics of the sources of sustained excitation by the flow in its relation to the local geometry and its associated reverberant acoustic behaviour

I.C. engine intake and exhaust noise assessment

The results of the current studies of intake and exhaust noise from piston engines are described in the context of intake and exhaust system acoustic design. The objectives combine the achievement of sound emission targets with the maintenance of optimum engine performance and fuel efficiency throughout a specified range of operating conditions. Practical experience demonstrates that the relevant technology includes a quantitative evaluation of those factors directly concerned with engine operation and breathing, those factors influencing the excitation and propagation of pressure waves through the relevant system and those factors controlling the emission of sound to the surrounding environment. A summary is provided of previous and current developments in appropriate existing numerical codes for performing the detailed calculations and associated performance assessments with increasing realism