Ann Dowling

Combustion noise is becoming increasingly important as a major noise source in aeroengines and ground based gas turbines. This is partially because advances in design have reduced the other noise sources, and partially because next generation combustion modes burn more unsteadily, resulting in increased external noise from the combustion. This review reports recent progress made in understanding combustion noise by theoretical, numerical and experimental investigations. We first discuss the fundamentals of the sound emission from a combustion region. Then the noise of open turbulent flames is summarized. We subsequently address the effects of confinement on combustion noise. In this case not only is the sound generated by the combustion influenced by its transmission through the boundaries of the combustion chamber, there is also the possibility of a significant additional source, the so-called ‘indirect’ combustion noise. This involves hot spots (entropy fluctuations) or vorticity ...

Combustion noise is becoming increasingly important as a major noise source in aeroengines and ground based gas turbines. This is partially because advances in design have reduced the other noise sources, and partially because next generation combustion modes burn more unsteadily, resulting in increased external noise from the combustion. This review reports recent progress made in understanding combustion noise by theoretical, numerical and experimental investigations. We first discuss the fundamentals of the sound emission from a combustion region. Then the noise of open turbulent flames is summarized. We subsequently address the effects of confinement on combustion noise. In this case not only is the sound generated by the combustion influenced by its transmission through the boundaries of the combustion chamber, there is also the possibility of a significant additional source, the so-called ‘indirect’ combustion noise. This involves hot spots (entropy fluctuations) or vorticity ...

The reduction of pollution and noise emissions of modern aero engines represents a key concept to meet the requirements of the future air traffic. This requires an improvement in the understanding of combustion noise and its sources, as well as the development of accurate predictive tools. This is the major goal of the current study where the low-order thermo-acoustic network (LOTAN) solver and a hybrid computational fluid dynamics/computational aeroacoustics approach are applied on a generic premixed and pressurized combustor to evaluate their capabilities for combustion noise predictions. LOTAN solves the linearized Euler equations (LEE) whereas the hybrid approach consists of Reynolds-averaged Navier–Stokes (RANS) mean flow and frequency-domain simulations based on linearized Navier–Stokes equations (LNSE). Both solvers are fed in turn by three different combustion noise source terms which are obtained from the application of a statistical noise model on the RANS simulations and ...

A semianalytical model for installed jet noise is proposed in this paper. We argue and conclude that there exist two distinct sound source mechanisms for installed jet noise, and the model is therefore composed of two parts to account for these different sound source mechanisms. Lighthill’s acoustic analogy and a fourth-order space–time correlation model for the Lighthill stress tensor are used to model the sound induced by the equivalent turbulent quadrupole sources, while the trailing-edge scattering of near-field evanescent instability waves is modelled using Amiet’s approach. A non-zero ambient mean flow is taken into account. It is found that, when the rigid surface is not so close to the jet as to affect the turbulent flow field, the trailing-edge scattering of near-field evanescent waves dominates the low-frequency amplification of installed jet noise in the far-field. The high-frequency noise enhancement on the reflected side is due to the surface reflection effect. The mode...

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. The potentials of using large eddy simulation (LES) with low Mach number (incompressible) formulation to predict combustion noise from premixed flames in open environment are investigated. The spatio-temporal variation of heat release rate obtained from LES of these flames are used to compute the far field sound pressure level and its power spectral density for various equivalence ratios and turbulence levels. Computational results are compared to measurements to assess the efficacy of the LES approach and the agreement is found to be very good. The advantages and limitations of this LES approach are identified and discussed.

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.

Certain combustion processes are inherently unstable due to the coupling of the flame and the acoustic modes of the combustion chamber. Such instabilities lead to oscillating heat release and pressure, which are undesirable. A means to eliminate these oscillations is to design a controller to monitor the fluctuating pressure upstream and modulate the fuel injection accordingly. Making use of a

ABSTRACT The paper is devoted to the first order delayed linear system with relay output controlled by the proportional-integral (PI) regulator. The deterministic system exhibits stable oscillations, and, since the system itself is stable, it can be suitable to switch off the controller if there are no disturbances during a long time interval. In the present work, the random disturbances are modelled by a Poisson stream of impulses, and the goal is to determine the instants of switching on (off) of the PI controller. After several assumptions and quantization of the time axis, we construct the new optimal control problem which is successfully solved with the help of the dynamic programming approach.

... [1] WM Dobrzynski, LC Chow, P. Guion, and D. Shiells, "A European Study on Landing Gear Airframe Noise Sources," in 6th AIAA/CEAS Aeroacoustics Conference: AIAA ... and R. Sen, "An Experimental Investigation of Airframe Noise Using a Model Scale Boeing 777 ...

Lean premixed prevaporized (LPP) combustion can reduce NOx emissions from gas turbines but often leads to combustion instability. Acoustic waves produce fluctuations in heat release, for instance, by perturbing the fuel-air ratio. These heat fluctuations will in turn generate more acoustic waves and in some situations linear oscillations grow into large-amplitude self-sustained oscillations. The resulting limit cycles can cause structural damage. Thermoacoustic oscillations will have a low amplitude initially. Thus linear models can describe the initial growth and hence give stability predictions. An unstable linear mode will grow in amplitude until nonlinear effects become sufficiently important to achieve a limit cycle. While the frequency of the linear mode can often provide a good approximation to that of the resulting limit cycle, linear theories give no prediction of its resulting amplitude. In previous work, we developed a low-order frequency-domain method to model thermoacoustic limit cycles in LPP combustors. This was based on a “describing-function” approach and is only applicable when there is a dominant mode and the main nonlinearity is in the combustion response to flow perturbations. In this paper that method is extended into the time domain. The main advantage of the time-domain approach is that limit-cycle stability, the influence of harmonics, and the interaction between different modes can be simulated. In LPP combustion, fluctuations in the inlet fuel-air ratio have been shown to be the dominant cause of unsteady combustion: These occur because velocity perturbations in the premix ducts cause a time-varying fuel-air ratio, which then convects downstream. If the velocity perturbation becomes comparable to the mean flow, there will be an amplitude-dependent effect on the equivalence ratio fluctuations entering the combustor and hence on the rate of heat release. Since the Mach number is low, the velocity perturbation can be comparable to the mean flow, with even reverse flow occurring, while the disturbances are still acoustically linear in that the pressure perturbation is still much smaller than the mean. Hence while the combustion response to flow velocity and equivalence ratio fluctuations must be modeled nonlinearly, the flow perturbations generated as a result of the unsteady combustion can be treated as linear. In developing a time-domain network model for nonlinear thermoacoustic oscillations an initial frequency-domain calculation is performed. The linear network model, LOTAN, is used to categorize the combustor geometry by finding the transfer function for the response of flow perturbations (at the fuel injectors, say) to heat-release oscillations. This transfer function is then converted into the time domain through an inverse Fourier transform to obtain Green’s function, which thus relates unsteady flow to heat release at previous times. By combining this with a nonlinear flame model (relating heat release to unsteady flow at previous times) a complete time-domain solution can be found by stepping forward in time. If an unstable mode is present, its amplitude will initially grow exponentially (in accordance with linear theory) until saturation effects in the flame model become significant, and eventually a stable limit cycle will be attained. The time-domain approach enables determination of the limit cycle. In addition, the influence of harmonics and the interaction and exchange of energy between different modes can be simulated. These effects are investigated for longitudinal and circumferential instabilities in an example combustor system and the results are compared with frequency-domain limit-cycle predictions.

The Silent Aircraft airframe has a flying-wing design with a large wing planform and a propulsion system embedded in the rear of the airframe with intake on the upper surface of the wing. In the present paper, boundary-element calculations are presented to evaluate acoustic ...

Certain combustion processes are inherently unstable due to the coupling of the flame and the acoustic modes of the combustion chamber. Such instabilities lead to oscillating heat release and pressure, which are undesirable. A means to eliminate these oscillations is to design a controller to monitor the fluctuating pressure upstream and modulate the fuel injection accordingly. Making use of a

The generation of sound by turbulent boundary-layer flow over a rough wall has been investigated theoretically and numerically. The roughness elements were speculated to behave like point dipoles with the dipole strength due to scattering of the near turbulent pressure fluctuations by a roughness element. The roughness-generated noise was shown to be comparable to the trailing-edge noise on an aircraft. Experiments were also performed and the results confirmed that roughness noise is not a negligible sound source. In this study, phased arrays were applied to localize the possible dipole sound sources due to roughness elements on a flat plate. The radiated sound from two rough and one smooth plates in an open jet was measured by 48-channel phased arrays at three locations. The rough regions were manufactured by a square distribution of rigid, hemispherical bosses on a rigid plate. The beamforming source maps obtained at different locations have displayed some features of dipole sourc...

ABSTRACT 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.