F.D. Denia

Acoustic behavior of circular dual-chamber mufflers

The acoustic behavior of a circular dual-chamber muffler is investigated in detail by: (1) a two-dimensional (2-D) axisymmetric analytical approach based on the mode-matching technique for the concentric configurations; (2) the finite element method; and (3) experimental work. A number of effects is studied, including (1) the presence of a rigid baffle in the chamber; (2) the inner radius of the baffle; (3) the position of the baffle along the axial direction; and (4) the extended inlet/outlet and baffle ducts. Some of these effects are shown to modify the acoustic behavior drastically, suggesting potential means to improve the acoustic performance.

A CFD APPROACH TO THE COMPUTATION OF THE ACOUSTIC RESPONSE OF EXHAUST MUFFLERS

The study of the three-dimensional acoustic field inside an exhaust muffler is usually performed through the numerical solution of the linearized equations. In this paper, an alternative procedure is proposed, in which the full equations are solved in the time domain. The procedure is based on the CFD simulation of an impulsive test, so that the transmission loss may be computed and compared with measurements and other numerical approaches. Also, the details of the flow inside the muffler may be studied, both in the time and the frequency domains. The results obtained compare favorably with a conventional FEM calculation, mostly in the ability of the procedure to account for dissipative processes inside the muffler.

Analytic mode matching for a circular dissipative silencer containing mean flow and a perforated pipe

An analytic mode matching scheme that includes higher order modes is developed for a straight-through circular dissipative silencer. Uniform mean flow is added to the central airway and a concentric perforated screen separates the mean flow from a bulk reacting porous material. Transmission loss predictions are compared with experimental measurements and good agreement is demonstrated for three different silencers. Furthermore, it is demonstrated that, when mean flow is present, the axial kinematic matching condition should equate to that chosen for the radial kinematic boundary condition over the interface between the airway and the material. Accordingly, if the radial matching conditions are continuity of pressure and displacement, then the axial matching conditions should also be continuity of pressure and displacement, rather than pressure and velocity as previously thought. When a perforated screen is present the radial pressure condition changes, but the radial kinematic condition should always remain equivalent to that chosen for the axial kinematic matching condition; here, results indicate that continuity of displacement should be retained when a perforated screen is present.

A finite element approach for the acoustic modeling of perforated dissipative mufflers with non-homogeneous properties

Abstract In this work, a finite element approach is presented for modeling sound propagation in perforated dissipative mufflers with non-homogeneous properties. The spatial variations of the acoustic properties can arise, for example, from uneven filling processes during manufacture and degradation associated with the flow of soot particles within the absorbent material. First, the finite element method is applied to the wave equation for a propagation medium with variable properties (outer chamber with absorbent material) and a homogeneous medium (central passage). For the case of a dissipative muffler, the characterization of the absorbent material is carried out by means of its equivalent complex density and speed of sound. To account for the spatial variations of these properties, a coordinate-dependent function is proposed for the filling density of the absorbent material. The coupling between the outer chamber and the central passage is achieved by using the acoustic impedance of the perforated central pipe, that relates the acoustic pressure jump and the normal velocity through the perforations. The acoustic impedance of the perforated central duct includes the influence of the absorbent material and therefore a spatial variation of the impedance is also taken into account. A detailed study is then presented to assess the influence of the heterogeneous properties and the perforated duct porosity on the acoustic attenuation performance of the muffler.

Determination of the resonance response in an engine cylinder with a bowl-in-piston geometry by the finite element method for inferring the trapped mass

Cylinder resonance phenomenon in reciprocating engines consists of high-frequency pressure oscillations excited by the combustion. The frequency of these oscillations is proportional to the speed of sound on pent-roof combustion chambers and henceforth the resonance frequency can be used to estimate the trapped mass, but in bowl-in-piston chambers a geometrical factor must be added in order to deal with the bowl disturbance. This paper applies the finite element method (FEM) to provide a resonance calibration for new design combustion chambers, which are commonly dominated by the bowl geometry near the top dead centre. The resonance calibration does not need any sensor information when it is solved by a FEM procedure, and consequently, is free from measurement errors. The calibration is proven to be independent of the chamber conditions and the results obtained are compared with experimental data by using spectral techniques and measuring precisely the trapped mass.

Acoustic characteristics of circular dissipative reversing chamber mufflers

In this work, a three-dimensional analytical model is presented for the propagation of sound in circular dissipative reversing chamber mufflers with perforated frontal plates and bulk-reacting fibrous material. The procedure is based on the mode matching method that couples the acoustic pressure and particle velocity at each geometrical discontinuity, and includes the presence of axisymmetric as well as asymmetric modes in the air regions and also within the absorbent material. The complex characteristic impedance and wavenumber are taken into account to model the sound propagation in the bulk-reacting fibrous material. To validate the procedure, the analytical muffler transmission loss predictions are compared with finite element results, showing a good agreement. A detailed study is then performed to assess the acoustical effect of the resistivity of the absorbent material, the porosity of the perforated plates and the main geometrical parameters, such as the location of inlet/outlet ducts, the thickness of the dissipative region and its axial location.

Point collocation scheme in silencers with temperature gradient and mean flow

This work presents a mathematical approach based on the point collocation technique to compute the transmission loss of perforated dissipative silencers with transversal temperature gradients and mean flow. Three-dimensional wave propagation is considered in silencer geometries with arbitrary, but axially uniform, cross section. To reduce the computational requirements of a full multidimensional finite element calculation, a method is developed combining axial and transversal solutions of the wave equation. First, the finite element method is employed in a two-dimensional problem to extract the eigenvalues and associated eigenvectors for the silencer cross section. Mean flow as well as transversal temperature gradients and the corresponding thermal-induced material heterogeneities are included in the model. In addition, an axially uniform temperature field is taken into account, its value being the inlet/outlet average. A point collocation technique is then used to match the acoustic fields (pressure and axial acoustic velocity) at the geometric discontinuities between the silencer chamber and the inlet and outlet pipes. Transmission loss predictions are compared favorably with a general three-dimensional finite element approach, offering a reduction in the computational effort. Sound attenuation in perforated dissipative silencers including temperature gradients and mean flow is computed.A two-dimensional finite element eigenvalue problem is solved for a silencer cross section with transversal thermal variations.A point collocation scheme is presented to match the acoustic pressure and axial velocity at the silencer geometrical discontinuities.A significant reduction in the computational requirements is obtained compared to a full three-dimensional finite element approach.

3D Acoustic Modelling of Dissipative Silencers with Nonhomogeneous Properties and Mean Flow

A finite element approach is proposed for the acoustic analysis of automotive silencers including a perforated duct with uniform axial mean flow and an outer chamber with heterogeneous absorbent material. This material can be characterized by means of its equivalent acoustic properties, considered coordinate-dependent via the introduction of a heterogeneous bulk density, and the corresponding material airflow resistivity variations. An approach has been implemented to solve the pressure wave equation for a nonmoving heterogeneous medium, associated with the problem of sound propagation in the outer chamber. On the other hand, the governing equation in the central duct has been solved in terms of the acoustic velocity potential considering the presence of a moving medium. The coupling between both regions and the corresponding acoustic fields has been carried out by means of a perforated duct and its acoustic impedance, adapted here to include absorbent material heterogeneities and mean flow effects simultaneously. It has been found that bulk density heterogeneities have a considerable influence on the silencer transmission loss.

Description and measurement of the acoustic characteristics of two-tailpipe mufflers

Two-tailpipe mufflers have become a usual solution for flow noise abatement in turbocharged Diesel engines, since the mean flow velocity at the exhaust outlet may be reduced without increasing the tailpipe diameter, which would give rise to an increase of noise associated with engine orders. In this communication, the acoustic characteristics of mufflers with two tailpipes are studied. First, a global representation of the muffler acoustics in terms of three reflection coefficients and six transmission coefficients is presented, and a procedure to obtain, from such representation, a conventional two-port transfer matrix by setting the reflection coefficient at one of the outlet pipes is provided. A procedure based on the extension of the impulse method as applied to single inlet-single outlet mufflers was devised for the experimental determination of the transmission and reflection coefficients, and experiments on a nonsymmetric oval expansion chamber allowed one to check the suitability of the representa...

Method for obtaining the wheel-rail contact location and its application to the normal problem calculation through ‘CONTACT’

ABSTRACT This work presents a robust methodology for calculating inter-penetration areas between railway wheel and rail surfaces, the profiles of which are defined by a series of points. The method allows general three-dimensional displacements of the wheelset to be considered, and its characteristics make it especially suitable for dynamic simulations where the wheel–rail contact is assumed to be flexible. The technique is based on the discretisation of the geometries of the surfaces in contact, considering the wheel as a set of truncated cones and the rail as points. By means of this approach, it is possible to reduce the problem to the calculation of the intersections between cones and lines, the solution for which has a closed-form expression. The method has been used in conjunction with the CONTACT algorithm in order to solve the static normal contact problem when the lateral displacement of the wheelset, its yaw angle and the vertical force applied in the wheelset centroid are prescribed. The results consist of smooth functions when the dependent coordinates are represented as a function of the independent ones, lacking the jump discontinuities that are present when a rigid contact model is adopted. Example results are shown and assessed for the normal contact problem for different lateral and yaw positions of the wheelset on the track.