Iain Dupere

The absorption of Sound near abrupt area expansions

Sound incident onto an abrupt area expansion in a channel is investigated both numerically and analytically. In the presence of a mean flow, the incident sound leads to unsteady vortex shedding from the lip of the expansion, thereby converting acoustic into vortical energy. We use an acoustic analogy and Greens functions to determine the sound reflected and transmitted across the area change. We compare predictions obtained from three different Greens functions with source terms derived either using a simple analytical model or from a numerical calculation. The compact Greens function, with zero normal derivative on the duct walls, gives the best results for a low-Mach-number flow. This Greens function contains a singularity at the lip of the expansion (and hence acoustic sources near the lip have the greatest effect). This means that our estimate of the overall vorticity field can be relatively crude, when using the compact Greens function, provided it is accurate near the lip. Therefore, although predictions for the radiated sound field made using all three Greens functions are formally correct, the solution made using the compact Greens function is less susceptible to errors in the source terms and gives more accurate results. In addition, we find that there is a Strouhal number at which sound absorption is maximized and that this absorption can be enhanced by multiple reflections from the duct ends. Our predictions are compared with an experiment.

The absorption of sound in cellular foams

Porous materials are often used as sound absorbers in a variety of situations including architectural and industrial applications. In many cases it is advantageous for the material to be both lightweight and rigid. Metal foams, originally developed for use in catalytic converters in car exhaust systems, offer an attractive mix of properties being both lightweight and rigid. In addition they have good sound absorbing properties and are good heat conductors giving rise to the possibility of enhanced sound absorption through heat transfer. In this paper, we review the use and acoustic modelling of these materials. We compare the predictions made by a number of viscous models developed by the authors for the propagation of sound through open-cell metal foams with an experiment both for the metal foams and for the polymer substrates used to manufacture the foam. All models are valid in the limit of low Reynold’s number which is valid for the typical ligament dimensions found in metal foams provided the amplitude of the waves is below 160dB. The first model considers the drag experienced by acoustic waves as they propagate passed rigid cylinders parallel to their axes, the second considers propagation normal to their axes, and the third considers the propagation passed the spherical joints. All three are combined together to give a general model of the acoustic behaviour of the foams. In particular, the sound absorption is found to be significant and well predicted by the combined model. In addition we describe a post-processing technique for the experiment used to extract the fundamental wave propagation characteristics of the material.Copyright

Computational Modelling of Self-Excited Combustion Instabilities

It is well known that lean premixed combustion systems potentially offer better emissions performance than conventional non-premixed designs. However, premixed combustion systems are more susceptible to combustion instabilities than non-premixed systems. Combustion instabilities (large-scale oscillations in heat release and pressure) have a deleterious effect on equipment, and also tend to decrease combustion efficiency. Designing out combustion instabilities is a difficult process and, particularly if many large-scale experiments are required, also very costly. Computational fluid dynamics (CFD) is now an established design tool in many areas of gas turbine design. However, its accuracy in the prediction of combustion instabilities is not yet proven.Unsteady heat release will generally be coupled to unsteady flow conditions within the combustor. In principle, computational fluid dynamics should be capable of modelling this coupled process. The present work assesses the ability of CFD to model self-excited combustion instabilities occurring within a model combustor. The accuracy of CFD in predicting both the onset and the nature of the instability is reported.© 2000 ASME

Acoustic noise evaluation for overhead line conductors

Research work on audible noise emitted from overhead line conductors is reviewed, and a unique experimental set up employing a semi-anechoic chamber and corona cage is described. Acoustically, this facility isolates undesirable background noise and provides a free-field test space inside the anechoic chamber. Electrically, the corona cage simulates a 3 m section of 400 kV overhead line conductors by achieving the equivalent surface gradient. UV imaging, acoustic measurements and a partial discharge detection system are employed as instrumentation. The acoustic and electrical performance is demonstrated through a series of experiments. One conductor sample is used as an example to illustrate results under various spray conditions.

Numerical Evaluation of the Compact Green’s Function for the Solution of Acoustic Flows

Due to the relatively tiny length scales involved, complex acoustic flows are not always suitable for traditional CFD codes. This paper develops a robust, semi-analytical numerical method for predicting sound fields based on the calculation of the compact Green’s function over a grid of source-observer positions. These calculations often involve singular functions, hence variations of the method are applied to several different 2D problems to investigate the impact of any singularities on the solution. The effect of grid point density and other parameters on execution time and accuracy are explored including the effects of approximating curved geometries using a number of straight lines. Comparison to known analytical solutions for the 2D problems is used to assess the accuracy of the method. For a typical application, we compute the far-field sound generated by a simple source in the vicinity of a compact, ‘2D’ fan blade in a duct. The current method demonstrates calculation of the compact Green’s function both accurately and robustly by avoiding the mapping from unbounded domains and the evaluation of potential models containing singularities. Both are seen as sources of error which have a widespread impact.Copyright

The use of Helmholtz resonators in a practical combustor

The drive to reduce emissions has led to the development of lean premixed combustors. However lean premixed combustion is often associated with combustion oscillations which can be so severe that they can cause structural damage to the engine. Since the associated frequencies are typically of the order of hundreds of Hertz, there is a need for a compact device to absorb the noise which drives the oscillation. Helmholtz resonators are commonly used as absorbers of incident acoustic power. In addition they are relatively compact. However, their use in combustors creates practical issues, such as placement within the chamber, neck length and cooling, which need to be addressed. In this paper we consider these practical problems and describe how to overcome them in a real combustor.Copyright

The Impact of Water Droplet Vibration on Corona Inception on Conductors Under 50 Hz AC Fields

The relationship between the size of water droplets on the surface of ac high-voltage conductors, their vibration, and corona onset is investigated. Measurements have been carried out on individual droplets in high-voltage experiments, including high-speed photography, and discharge inception measurements. These have been correlated with acoustic emission from larger arrays of droplets on overhead transmission line conductors. A numerical model based on the finite element method has also been developed to simulate the vibration of the droplet enabling evaluation of the electric field and the characteristics of the droplet vibration. It is shown that the size of the water droplet is critical in its mechanical response to the field and that corona does not necessarily occur when the droplet is deformed into its most conical state as is the case under dc fields. For larger droplets the phase shift between the field and droplet vibration results in the maximum field occurring when the droplet is in a “flattened” profile rather than when it is “pointed.” The phase relationship between the droplet vibration and the electric field thus controls the onset of corona. It is argued that conductors which facilitate the development of small, uniform, stable droplets on their surface are the cause of abnormally high levels of low-frequency audible noise sometimes observed by transmission utilities. Moreover, the narrow resonant response of the droplet may lead to a difference in acoustic emissions experienced between power utilities with 50 and 60 Hz system frequencies.

Reflection and transmission across partial blockages in fluid‐filled flexible, non‐thin‐walled pipes.

A model is presented for propagation along a flexible pipe whose thickness is not small in comparison with its diameter across a partial blockage with varying sizes and material properties. Comparison is made with Flugge’s well‐known thin‐shell theory for a propagation along a pipe where it is found that the additional computational complexity found in the current model becomes necessary when the thickness of the shell exceeds 10% of the pipe radius. Comparison is also made with experiment both for the propagation characteristics of the pipe and for the reflection from a partial blockage. Two reflection models are presented: a crude area change model with compensation for the mass of the blockage and a more accurate model using high‐order modes and matching to a flexible blockage using co‐location. Reasonable agreement is found for both with the more accurate model, giving better agreement but at the expense of computational efficiency. The work is useful both for blockage detection and for detecting sten...

The Effects of Blockage on the Propagation of Acoustic Waves in the Liquid-Shell Coupled System

The free vibration of a fluid/structure system consisting of a cylindrical blockage submerged in a liquid enclosed by a cylindrical shell is investigated for the purpose of pipe line transportation monitoring. The wavenumbers are obtained and the reflection and transmission characteristics of these waves at the blockage interfaces are investigated theoretically. Reflection and transmission ratios are obtained in the axisymmetric mode, as functions of frequency. High order modes play an important role in the near field of the discontinuity and are taken into account.