Biomedical subjects

A Selamet

Publications and source records attributed to A Selamet.

7 recordsLinked to original sources

PubMed2005

Helmholtz resonator lined with absorbing material.

A closed-form, two-dimensional analytical solution is developed to investigate the acoustic performance of a concentric circular Helmholtz resonator lined with fibrous material. The effect of density and the thickness of the fibrous material in the cavity is examined on the resonance frequency and the transmission loss. With the expressions for the eigenvalue and eigenfunction in the cavity, the transmission loss is obtained for a piston-driven model by applying a pressure/velocity matching technique. The results from the analytical methods are compared to the numerical predictions from a three-dimensional boundary element method and the experimental data obtained from an impedance tube setup. It is shown that the acoustic performance of a Helmholtz resonator may be modified considerably by the density and thickness of the fibrous material without changing the cavity dimensions.

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PubMed2005

Analytical approach for sound attenuation in perforated dissipative silencers with inlet/outlet extensions.

The acoustic attenuation performance of perforated dissipative circular expansion chambers with inlet/outlet extensions is investigated. The eigenvalues and eigenfunctions of the sound field are analytically determined in the extended inlet/outlet circular ducts, upstream/downstream end annular dissipative chambers, and the central perforated dissipative expansion chamber. Utilizing the continuity conditions of velocity/pressure at the interfaces the transmission loss is predicted by a two-dimensional analytical approach. For a specific configuration, such predictions are compared with both experiments and a three-dimensional computational solution based on the substructure boundary element technique, showing a reasonable agreement. The analytical results for the effect of the absorbent resistivity, duct porosity, and geometryon the acoustic attenuation performance are discussed in detail.

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PubMed2001

Wave reflections from duct terminations.

The reflection coefficients and inertial end corrections of several duct terminations, including finite length duct extensions perpendicular to an infinite wall, as well as at a number of angles, curved interface surfaces, and annular cavities, are determined and analyzed in the absence of flow by employing the boundary element method. Predictions for the classical unflanged and flanged circular ducts show good agreement with analytical and computational results available in the literature. The predictions for curved interface surfaces (bellmouth or horn) are also consistent with the available experimental data. In view of its high reflection coefficient, the duct termination with an annular cavity may be suggested for the suppression of noise radiation in a specific frequency band or for an effective wave reflection from the termination.

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PubMed2001

A computational approach for flow-acoustic coupling in closed side branches.

The quarter-wave resonator, which produces a narrow band of high acoustic attenuation at regularly spaced frequency intervals, is a common type of silencer used in ducts. The presence of mean flow in the main duct, however, is likely to promote an interaction between these acoustic resonances and the flow. The coupling for some discrete flow conditions leads to the production of both large wave amplitudes in the side branch and high noise levels in the main duct, thereby transforming the quarter-wave silencer into a noise generator. The present approach employs computational fluid dynamics (CFD) to model this complex interaction between the flow and acoustic resonances at low Mach number by solving the unsteady, turbulent, and compressible Navier-Stokes equations. Comparisons between the present computations and the experiments of Ziada [PVP-Vol. 258, ASME, 35-59 (1993)] for a system with two coaxial side branches show that the method is capable of reproducing the physics of the flow-acoustic coupling and predicting the flow conditions when the coupling occurs. The theory of Howe [IMA J. Appl. Math. 32, 187-209 (1984)] is then employed to determine the location and timing of the acoustic power production during a cycle.

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PubMed2001

An experimental study of the impedance of perforated plates with grazing flow.

An experimental investigation of the linear impedance of perforated interfaces exposed to grazing fluid flow is presented, including a description of the branch-type setup used in the study. The experimental setup employs a movable microphone in the branch duct that improves the error characteristics of the system. The impedance parameters of three perforated samples have been measured including one circular, square-edged orifice and two production muffler louver geometries. The data are presented in terms of the equivalent length and resistance of the samples, with the friction velocity being used to characterize the grazing flow. At higher grazing flow velocities, the equivalent length of each sample decreased by an amount representative of the Rayleigh end correction for one side of an orifice. Resistance for the louvers appears to be somewhat less sensitive to grazing flow than the circular orifice, though all of the samples showed significant increases. While the experimental results for all three samples showed similar basic trends, distinct and substantial differences were observed between each perforate geometry.

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PubMed2000

The effect of high-amplitude sound on the attenuation of perforated tube silencers

A time-domain computational approach is applied to investigate the behavior of perforated tube silencers at high sound levels. The one-dimensional computational technique employs a lumped parameter model for the perforate flows. The lumped parameter perforate model is based on time-invariant approximations for the equivalent length l(eq) and resistance R, consistent with the use of a nonlinear perforate impedance. Empirical expressions for l(eq) and R are developed experimentally using nondimensional scaling parameters. The model is applied to geometries representative of automotive resonators and multiple-duct mufflers. Conditions are simplified from those in an actual automotive system by considering single-frequency excitation and zero mean flow. Simulations with linear perforate behavior are compared to experimental data obtained with an extended impedance tube system. Simulations with nonlinear perforate behavior for one concentric tube resonator are compared to published experimental data.

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PubMed2000

Circular asymmetric Helmholtz resonators

A three-dimensional (3D) analytical approach is developed to account for the nonplanar wave propagation in the cavity and neck of "piston-driven" circular asymmetric Helmholtz resonators. The present 3D analytical results are compared with (1) the numerical predictions from the boundary element method (BEM) to evaluate the analytical approach; and (2) the one-dimensional (1D) solution to examine the effect of nonplanar waves at area discontinuity between the neck and the cavity. In order to improve the 1D solution, the end correction is also determined by using the 3D analytical approach. The effect of neck offset on the resonance frequency of circular asymmetric Helmholtz resonators is investigated. Predictions of resonance frequency and transmission loss from the present 3D and corrected 1D analytical approaches are, respectively, identical and close to the BEM results, while the corrected 1D approach provides a better accuracy compared to the 1D solutions with Ingard's correction. Finally, the boundary element method is employed to determine the wave attenuation performance of the "pipe-mounted" Helmholtz resonators to examine the effect of multidimensional waves in the vicinity of the main duct and neck junction.

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