A. Mimani

Multiple line arrays for the characterization of aeroacoustic sources using a time-reversal method

This letter investigates the use of multiple line arrays (LAs) in a Time-Reversal Mirror for localizing and characterizing multipole aeroacoustic sources in a uniform subsonic mean flow using a numerical Time-Reversal (TR) method. Regardless of the original source characteristics, accuracy of predicting the source location can be significantly improved using at least two LAs. Furthermore, it is impossible to determine the source characteristics using a single LA, rather a minimum of two are required to establish either the monopole or dipole source nature, while four LAs (fully surrounding the source) are required for characterizing a lateral quadrupole source.

Acoustical behavior of single inlet and multiple outlet elliptical cylindrical chamber muffler

Transmission loss (TL) of an elliptical cylindrical chamber muffler having a single side/end inlet and multiple side/end outlet is analyzed by means of the 3-D semi-analytical formulation based upon the modal expansion (in terms of the angular and radial Mathieu functions) and the Greens function. The acoustic pressure response obtained in terms of Greens function is integrated over surface area of the side/end ports (modeled as rigid pistons) and upon subsequent division by the port area, yields the acoustic pressure response or impedance Z] matrix parameters due to the uniform piston-driven model. The 3-D semi-analytical results are found to be in excellent agreement with the results obtained by means of 3-D FEA (SYSNOISE) simulations, thereby validating the semi-analytical procedure suggested in this work. Parametric studies such as the effect of chamber length (L), angular and axial locations of the ports, interchanging the locations of inlet and outlet ports as well as the addition of an outlet port for double outlet mufflers on the TL performance are reported, thereby leading to the formulation of design guidelines for obtaining muffler configurations exhibiting a broad-band TL spectrum. One such configuration is an axially long chamber having side-inlet and side-outlet ports such that one of the side ports is located at half the axial length on themajor/minor axis and the other side port is located at three-quarters (or one-quarter) of the axial length on the minor/major axis

Enhancing the focal-resolution of aeroacoustic time-reversal using a point sponge-layer damping technique

This letter presents the Point-Time-Reversal-Sponge-Layer (PTRSL) technique to enhance the focal-resolution of aeroacoustic Time-Reversal (TR). A PTRSL is implemented on a square domain centered at the predicted source location and is based on damping the radial components of the incoming and outgoing fluxes propagating toward and away from the source, respectively. A PTRSL is shown to overcome the conventional half-wavelength diffraction-limit; its implementation significantly reduces the focal spot size to one-fifth of a wavelength for a monopole source. Furthermore, PTRSL reduces the focal spots of a dipole source to three-tenths of a wavelength, as compared to three-fifths without its implementation.

An experimental application of aeroacoustic time-reversal to the Aeolian tone

This paper presents an experimental application of the aeroacoustic time-reversal (TR) source localization technique for studying flow-induced noise problems and compares the TR results with those obtained using conventional beamforming (CB). Experiments were conducted in an anechoic wind tunnel for the benchmark test-case of a full-span circular cylinder located in subsonic cross-flow wherein the far-field acoustic pressure was sampled using two line arrays (LAs) of microphones located above and below the cylinder. The source map obtained using the signals recorded at the two LAs without modeling the reflective surfaces of the contraction-outlet and cylinder during TR simulations revealed the lift-dipole nature of aeroacoustic source generated at the Aeolian tone; however, it indicates an error of 3/20 of Aeolian tone wavelength in the predicted location. Modeling the reflective contraction-outlet during TR was shown to improve the focal-resolution of the source and reduce side-lobe levels, especially in the low-frequency range. The experimental TR results were shown to be comparable to (a) the simulation results of an idealized dipole at the cylinder location in wind-tunnel flow and (b) that obtained by monopole and dipole CB, thereby demonstrating the suitability of TR method as a diagnostic tool to analyze flow-induced noise generation mechanism.

Acoustic End-Correction in a Flow-Reversal End Chamber Muffler: A Semi-Analytical Approach

This work presents a semi-analytical technique based on the Greens function and uniform-piston driven model to determine the end-correction length l in an axially long flow-reversal end chamber muffler having an end-inlet and an end-outlet. The semi-analytical procedure is based on the 3D analytical uniform piston-driven model for obtaining the impedance Z] matrix parameters and numerically evaluating the frequency f(p) at which the imaginary part of the cross-impedance parameter Z(E2E1) crosses the frequency axis at the first instance. The frequency f(p) corresponds to the low-frequency peak in the transmission loss (TL) spectrum of the axially long flow-reversal end-chamber muffler obtained a priori to its computation by considering the influence of higher order evanescent transverse modes. The effective chamber length (and thence, the end-correction length) in the low-frequency range are determined by using the expression for resonance frequency of a classical quarter-wave resonator. This method is employed to determine the end-correction in axially long elliptical cylindrical end chambers and circular cylindrical end chambers (with or without a rigid concentric circular pass-tube). The TL graph predicted by the 1D axial plane wave model (incorporating the end-correction length) is shown to be in an excellent agreement with that obtained by the 3D analytical approach and an experimental result (from literature) up to the low-frequency limit, thereby validating the semi-analytical technique. Parametric studies are conducted using the proposed semi-analytical method to investigate and qualitatively explain the effect of angular location and offset distance of the end ports and the pass-tube diameter on the end-correction length, thereby yielding important insights into the influence of transverse evanescent modes on dominant axial plane wave modes of the axially long end-chamber. Development of an empirical end-correction expression in a flow-reversal circular end-chamber with offset inlet and outlet ports is a practically useful contribution of this work.

Enhancing the Resolution Characteristics of Aeroacoustic Time-Reversal Using a Point-Time-Reversal-Sponge-Layer

This paper presents a derivation of the mathematical model for implementing the Point-Time-Reversal-Sponge-Layer (PTRSL) technique to enhance the focal-resolution of aeroacoustic Time-Reversal (TR) and discusses its underlying theory. A PTRSL mimics a time-reversed acoustic source, i.e., an acoustic sink. It is implemented on a square domain centred at the predicted source location and is based on damping the radial components of the incoming and outgoing fluxes propagating towards and away from the source, respectively, whilst leaving the angular components unaffected. The effectiveness of PTRSL is demonstrated using test-cases of a tonal and broadband monopole and dipole sources located in a spatially-developing non-uniform mean flow field modeling a 2-D turbulent free-jet. A PTRSL is shown to overcome the conventional half-wavelength diffraction-limit; its implementation reduces the focal spot size of a monopole source to one-fifth of a wavelength in comparison to half-wavelength without its implementation. Furthermore, PTRSL reduces the focal spots of a dipole source to threetenths of a wavelength, as compared to three-fifths of a wavelength without its implementation.

An Experimental Comparison of Beamforming, Time-reversal and Near-field Acoustic Holography for Aeroacoustic Source Localization

Aeroacoustic source localization is an important experimental tool that uses an array of microphones to locate and quantify aeroacoustic sources. Obtaining such information is the first step towards reducing noise emissions. One emerging method of aeroacoustic source localization is aeroacoustic time-reversal. With a unique blend of numerical simulation and experimental data, aeroacoustic time-reversal has the potential to provide superior source resolution and characterization performance over other microphone array processing techniques. This paper presents an experimental comparison of three different aeroacoustic source localization methods: aeroacoustic time-reversal, beamforming and near-field acoustic holography. The source resolution performance of all three source localization methods is investigated via a wind tunnel experimental study using two line arrays of microphones for the test case of a circular cylinder in low Mach number flow. The experimental results show that all three source localization methods are able to satisfactorily locate the cylinder noise source at the aeolian tone frequency to within �/6. In addition, information about the directivity characteristics of the noise source are obtained with aeroacoustic time-reversal and beamforming.

Experimental Application of Aeroacoustic Time-Reversal

This paper presents an experimental application of the Time-Reversal (TR) source localization technique to investigate the flow-induced noise generation mechanisms due to a benchmark aeroacoustic problem of a flat-plate. Experiments were conducted in an Anechoic Wind Tunnel (AWT) for the test-case of a full-span symmetric flat-plate located in subsonic cross-flow. The far-field acoustic pressure was sampled using two Line Arrays (LAs) of microphones located above and below the test-case. It was shown that the Power Spectral Density (PSD) spectrum for a flat-plate exhibits a broadband nature. The TR simulations were implemented using the time-reversed signals at the two LAs and the superposition technique whereby the spatio-temporal evolution of the time-reversed acoustic pressure fields and the TR source maps indicate the dipole nature (with axis perpendicular to cross-flow) of the flow-induced noise source throughout the frequency range. In particular, the TR source maps reveal that beyond the low-frequency range (given by 1000 Hz c f  ), the predicted location of the dipole source is near the Trailing Edge (TE) of flat-plate; indeed, the TE dipole was shown to be the dominant noise source in the high-frequency range (given by 2500 Hz c f  ),

A computational flow-induced noise and time-reversal technique for analysing aeroacoustic sources

A simulation technique to analyse flow-induced noise problems that combines computational fluid dynamics (CFD), the boundary element method (BEM) and an aeroacoustic time-reversal (TR) source localisation method is presented. Hydrodynamic data are obtained from a high-fidelity CFD simulation of flow past a body and aeroacoustic sources are extracted based on Lighthills acoustic analogy. The incident pressure field on the body due to the aeroacoustic sources is combined with a BEM representation of the body to obtain the spectrum of the direct, scattered and total acoustic pressure fields at far-field microphone locations. The microphone data are then used as input for the time-reversal simulations which are implemented by numerically solving two-dimensional linearized Euler equations. Decomposing the far-field pressure enables the TR simulation of the direct, scattered and total acoustic fields to be performed separately which yields the location and nature of the corresponding aeroacoustic sources. To demonstrate the hybrid CFD-BEM-TR technique, the sound generated by a cylinder in low Mach number cross-flow is considered. The nature of the aeroacoustic sources at the vortex shedding frequency and its second harmonic for the direct, scattered and total fields are identified.