Chao-Nan Wang

Experimental study of the absorption characteristics of some porous fibrous materials

Abstract The sound absorption characteristics of a porous fibrous material, that is manufactured and used in Taiwan, is investigated. The flow resistivity and the absorption coefficient are the important parameters that are determined. The measured flow resistivity of the glass and rock wool samples were scattered in a wider range and usually greater than previously reported in the literature. This means that the manufacturing quality control needs further improvement. Additionally, the characteristic impedance of the porous material is estimated by the model of Allard et al and consequently the normal impedance and sound absorption coefficient is obtained.

Numerical decoupling analysis of a resonator with absorbent material

Abstract The purpose of this paper is to analyze the acoustic performance of resonators with porous material inside the expansion chamber. Sound wave propagation in the porous media is an extremely complicated phenomenon. In the present study the dynamic density and bulk modulus of air in the porous media are used to describe the viscous effects and the thermal effects in an air-saturated rigid frame porous material. With this assumption the absorbent material in the expansion chamber can be equivalent to a fluid with complex sound speed and wave number. Then the coupled equations for sound wave propagation in air (in straight tube) and the equivalent fluid (in expansion chamber) are solved by the numerical decoupling method. Finally, the effects of porous material on the performance of a resonator are discussed. ©

A boundary element analysis of a concentric-tube resonator

Abstract A method for modeling the acoustic system of a perforated component based upon the boundary element analysis is presented. It applies to systems with complex shaped boundaries, and is not confined to the configuration which has one acoustically long dimension assumed from the traditional one-dimensional analyses. The technique can also apply to either a short or long resonator with a fully or partially perforated center tube, and has been proved by experimental measurement. The influence of mean flow, without including the convective effect, is also investigated by using the flow-induced perforated parameter.

Analysis of three dimensional muffler with boundary element method

Abstract A numerical scheme for solving the three dimensional interior acoustic problem governed by the Helmholtz equation has been developed. The main feature of this formulation is that the singularity can be removed analytically. The boundary integral equation is solved for a specific geometry by using second-order quadrilateral surface elements and does not have the difficulty of non-uniqueness associated with the boundary integral equation formulation in an exterior problem. The transmission loss of a muffler is computed by using the derived four-pole parameters. The effects of higher-order modes due to the area discontinuity as well as the various inlet/outlet alignment on the acoustic performance of a muffler are also studied. The case of a muffler with elliptic cross-section, which is troublesome in analytical solution, can be treated easily by the present scheme. Finally, the transmission loss characteristics of a muffler with extended inlet/outlet are investigated. All the numerical results are compared with experimental measurement and agreement is good.

A numerical scheme for the analysis of perforated intruding tube muffler components

Abstract A numerical scheme for analyzing perforated intruding tube muffler components has been developed. In the present study, the main difficulty that the boundary conditions cannot be specified at the tube open ends in the plane wave theory analysis is overcome. The performance prediction is conducted on the perforated intruding tube muffler and also on the plug muffler with or without mean flow. Comparisons of different porosity and mean flow velocity are investigated. However, only the results for zero mean flow are compared with the experimental measurements, and agreement is good.

The application of boundary element evaluation on a silencer in the presence of a linear temperature gradient

Abstract A boundary element method for analyzing the acoustic performance of a muffler in the presence of a linear temperature gradient is developed. In order to simulate the temperature gradient, the muffler is divided into segments with different constant temperatures. The boundary element approach is applied to each sub-volume to establish the relationship between pressure and its gradient on the surface. Then combining all the sub-volumes by matching the continuity of pressure and velocity on the contact surface to obtain the relationship of the inlet and outlet of a muffler Consequently, the transmission loss of a muffler can be evaluated. In the present study, the performance of mufflers with different temperature and temperature gradients is investigated.

Boundary element evaluation on the performance of sound absorbing wedges for anechoic chambers

In the present study, the acoustic performance analysis for wedges used in an anechoic chamber by the boundary element method is conducted. In a wedge formed by porous material, mechanisms of dissipated processes due to viscosity and heat conduction are very complicated and are not easy to handle. For simplicity, the laws of Delany and Bazley are used to replace the porous material by an equivalent fluid. With this assumption, the agreement between numerical results and experimental measurements reported elsewhere is still good. Influences on the pressure reflection coefficient due to the dimensions of wedge, such as the length of the wedge, the thickness of base, the thickness of air gap, for fiber and foam materials with different flow resistances are studied. In addition, the hybrid wedge and the wedge with cutting head are also investigated.

Estimating the Elastic Modulus through the Thickness Direction of a Uni-direction Lamina which Possesses Transverse Isotropic Property:

This investigation presents three formulae of the material constants. A simple method is applied to determine the material properties of a unidirectional lamina (UD) in the through-thickness direction. The mechanical characteristics of an orthotropic composite material generally require nine parameters: three Youngs moduli; three shear moduli; and three Poissons ratios. Most studies concentrate on two orthogonal axial moduli, a shear modulus and a Poissons ratio in the plane of the lamina. Four material parameters were adopted in those works instead of nine. Some cases in many studies are based on three-dimension modeling, making the nine material constants necessary, as the four material parameters cannot satisfy such cases. However, the material constants through the thickness direction cannot easily be measured experimentally. In this work, the relationships between the nine mechanical properties are revealed by the wave equations of an orthotropic material. Two restrictions in the study are considered: first, the suitable cases are limited to UD possessing the transverse isotropic property; second, the distribution of the fibers in the cross-section of UD laminate must be uniform. On the base, the shear modulus and the Poisson ratio through the thickness direction of the UD material can be computed from the simple relationships.

The scattering of an acoustic wave incident on the rigid floating body

Abstract In the present study, the sound scattering of acoustic waves incident on a rigid floating body is analyzed by the boundary element method. In the first instance, a plane wave incident on a finite cylinder and spherical body are considered. To simulate the noise produced by a propeller, a spherical incident wave generated by a point source located just behind the rigid floating body is used in the analysis. Furthermore, a sound wave incident on a floating body at different depths is also investigated.

A numerical study of the effects of a winglet on airfoils

The purpose of this research is to investigate the effects of a winglet on aerodynamic noise of an airfoil. For simplicity, Reynolds Averaged Navier-Stokes equations combined with Realizable turbulence model are used to solve the turbulent flow. In order to verify the accuracy of flow field analysis, a uniform flow past a three-dimensional rectangular airfoil is analyzed and tracks the center line of the tip vortex. The agreement between the simulated and measured center line is good. For the sound field analysis, the flow induced noise around a rectangular airfoil is computed by the Broadband Noise Source (BNS) model. Proudmans formula was used to evaluate acoustic power per unit volume of aerodynamic noise. This study focuses on the sound power of aerodynamic noise generated by tip vortex when the flow passes through an airfoil and an airfoil with winglet. In order to understand the effects of the winglet on the aerodynamic noise, the different winglet characteristics are investigated and discussed. It...