Rafael Piscoya

Equivalent Source Method and Boundary Element Method for Calculating Combustion Noise

In this paper, the application of the Equivalent Source Method (ESM) and the acoustical Boundary Element Method (BEM) for the prediction of the noise generated by an open diffusion flame is investigated. These acoustical methods have been coupled with a Large Eddy Simulation (LES) of the turbulent flow, which simulates the flow and combustion processes in a source region in the vicinity of the flame. Among the hybrid methods which are being used to predict the sound produced by turbulent flow, the ESM and BEM have the advantage that only one acoustic variable must be known at a surface surrounding the source zone (fewer data has to be processed) and that the far field can be directly computed. The sound power generated from two open diffusion flames have been calculated with both the ESM and the BEM, using the velocity distribution over cylindrical control surfaces, which enclose the source region. The results of the calculations are presented and compared with the measured sound power of the same flames. For one configuration good agreement between measurement and simulation at low and middle frequencies is obtained. Possible reasons for the differences for the other configuration will be discussed.

Results of an implementation of the dual surface method to treat the non-uniqueness in solving acoustic exterior problems using the boundary element method

The problem of non-uniqueness (NU) of the solution of exterior acoustic problems when using the boundary element method (BEM) is well known. Methods like the Burton-Miller technique or the CHIEF method are used to solve this challenge at the expense of more complex procedures for handling hypersingular integrals and/or higher computing times due to higher complexity of the algorithm or additional equations. The dual surface method, commonly used for electromagnetic problems, was adapted for acoustic radiation and scattering problems. The basic principles of methods to solve the NU problem are outlined and results for different models and solution procedures are presented, taking into account quality, solution time, and the numerical advantages when using iterative solvers.

Determination of the Far Field Sound Radiation from Flames Using the Dual Reciprocity Boundary Element Method

The present work addresses the calculation of the radiated sound power spectrum in the far field of open or enclosed flames based on the application of the dual reciprocity boundary element method (DRBEM). The focus of this work is the development of a technique that allows a good representation of the sound field, assuming that the sound sources originated by the combustion are known. Following the acoustic analogy approach, the sound radiation of open flames can be evaluated by a volume integral over the equivalent acoustic source terms in the combustion zone. The DRBEM allows replacing the volume integral by a series of surface integrals. The technique is applied to the theoretical model of a spherical flame. The accuracy of the DRBEM model can be studied by comparing it with available analytical solutions for the sound radiation of this flame type. For enclosed flames, the DRBEM is used to calculate the sound propagation in the region of temperature gradients that appears at the exhaust of the combustion chamber. The sensitivity of the radiated sound field to this adjacent inhomogeneous temperature field is investigated. The non-uniqueness problem of the boundary integral equation method is tackled by the Burton and Miller approach. The DRBEM allows an effective determination of the radiated acoustic field because it can take into account inhomogeneities and satisfies automatically the radiation condition at infinity.

Acoustical Green’s Function and Boundary Element Techniques for 3D Half-Space Problems

This paper presents a review of basic concepts of the boundary element method (BEM) for solving 3D half-space problems in a homogeneous medium and in frequency domain. The usual BEM for exterior problems can be extended easily for half-space problems only if the infinite plane is either rigid or soft, since the necessary tailored Green’s function is available. The difficulties arise when the infinite plane has finite impedance. Numerous expressions for the Green’s function have been found which need to be computed numerically. The practical implementation of some of these formulas shows that their application depends on the type of impedance of the plane. In this work, several formulas in frequency domain are discussed. Some of them have been implemented in a BEM formulation and results of their application in specific numerical examples are summarized. As a complement, two formulas of the Green’s function in time domain are presented. These formulas have been computed numerically and after the applicatio...

Numerical simulation of the transmission loss of plates

Numerical simulations for estimating the transmission loss of plates can be an important alternative to measurements when there is no access to transmission loss test facilities. Furthermore, parametric studies and design changes can be made easily and faster. This work presents a method to calculate the transmission loss of plates placed between a source and a receiver room using an iterative approach. The sound radiation due to the vibration of the plates is solved with a Boundary Element formulation while the motion of the plate is determined using a Finite Element formulation with the sound pressure as the exciting force. The starting point is the blocked pressure approximation. The real pressure on the plate and its displacement are obtained after some iterations. If no damping in the plate is considered, poor or no convergence is expected at the resonant frequencies of the plate. This problem is avoided introducing some damping in the plate as well as in its fixation (boundary). With this approach, ...

Modelling of the Sound Radiation from Flames by means of Acoustic Equivalent Sources

This chapter addresses the calculation of the far field sound radiation of turbulent flames based on a coupling of a Large Eddy Simulation (LES) with two acoustic methods, the Equivalent Source Method (ESM) and the Boundary Element Method (BEM). The numerical aspects of the coupling, including the choice of a coupling interface, data sampling at coarsened grids and Fourier Transform of the coupling variables from time to frequency domain, are surveyed. The simulation results for the radiated sound power of an open turbulent non-premixed jet flame are compared to measurement data. The influence of ground effects on the sound radiation is briefly discussed. Considering a second configuration, where the exhaust of a combustion chamber opens to an adjacent temperature field, the Dual Reciprocity Boundary Element Method (DRBEM) is applied for the simulation of the sound propagation through a non-homogeneous medium. The sensitivity of the radiated sound field to the temperature profile of the surrounding field is investigated. As the results show, the presented hybrid approach is able to predict effectively the sound radiation of flames and the DRBEM still expands the possibilities of the hybrid methodology for the numerical simulation of combustion noise.