Michel Tournour

Pseudostatic corrections for the forced vibroacoustic response of a structure-cavity system

The analysis of the coupled vibroacoustic behavior of a structure-cavity system is often performed using the in vacuo structure modes and the rigid cavity modes. Unfortunately, the use of such a modal basis can result in a poor convergence when the high-frequency modes of one of the two subsystems are coupled to the low-frequency modes of the other subsystem. This problem is made critical by the lack of a reliable criterion for selecting the number of kept modes for each subsystem. This paper shows that the effect of modal truncation is critical for the convergence of the method and that the convergence can be greatly improved using pseudostatic corrections for both the structure and the cavity. The theory behind the proposed technique is presented together with two generic problems exhibiting strong coupling: (i) an elastic cylindrical cavity filled with air; and (ii) an elastic plate coupled to a rectangular cavity filled with water.

Validation, performance, convergence and application of free interface component mode synthesis

Abstract During the past three decades the component mode synthesis methods were extensively presented in the literature. Unfortunately, it seems that no convergence criterion was clearly defined in the literature. A free interface method is presented in this paper and six test cases are used to evaluate the number of retained substructure modes necessary to ensure convergence. The approach is validated numerically and experimentally. Also, an application of the proposed free interface method to optimization design is presented.

Free and scattered acoustic field predictions of the broadband noise generated by a low-speed axial fan

Broadband noise generated by a low-speed industrial axial fan and its scattered field by a benchmark obstacle have been addressed. Amiets theory on turbulence-interaction noise has been extended in order to predict the acoustic response of a fan in its geometrical near-field. A segmentation technique has been applied for spanwise varying flow conditions. The improved model has been combined with boundary element method (BEM) for acoustic scattering. The validation of the broadband scattering technique has been performed through comparisons with an analytical model considering acoustic scattering from an infinite plate and with measurements of a low-speed axial fan operating nearby a flat scattering screen.

Sound transmission through multilayer structures with isotropic elastic porous materials

This paper discusses the prediction of transmission loss through multilayer structures with porous materials. First, a brief review of recent formulations for multilayer structures with poroelastic materials is presented. Second, the transmission loss problem is described and formulated using a coupled finite element and boundary element procedure. Issues such as poroelastic‐elastic coupling, poroelastic‐fluid coupling, and radiation from a poroelastic material will be addressed. The developed model is validated through numerical examples. Its advantages and limitations are discussed. Finally, typical results showing the vibroacoustic effects of several parameters such as the multilayer configuration, the types of porous materials, and the mounting conditions are presented. [Work supported by Bombardier, Inc., Canadair and N.S.E.R.C.]

Fan noise predictions using scale-resolved, statistical, stochastic and semi-analytical models

The recently concluded EC FP7 IDEALVENT project has seen the application of a variety of simulation methods by the consortium partners to an industrial blower unit such as used in aircraft Environmental Control Systems. The blower was subjected to different inflow conditions in order to better comprehend the importance of installation effects, and assess the capabilities of the proposed numerical tools to quantify those effects. The prediction tools include scale-resolved, statistical, stochastic and semi-analytical approaches. The predicted acoustic fields were validated by comparison with experimental data obtained on a dedicated test rig, permitting the necessary decontamination of the microphone signals from the reflections at the duct terminations, and turbulent boudary layer related pseudo-sound. A quite good agreement was found between the predicted and measured acoustic data. The tools showed also a good potential for estimating installation effects. Finally, the paper demonstrates the benefits of applying different methods, characterized by different modelling assumptions, for a more thorough understanding of the sound generation n mechanisms.

Application of the fast multipole method for solving very large acoustic radiation problems

Boundary element method is well known and extensively used to solve acoustic radiation problems. It is especially appropriated for exterior radiation since the fluid domain does not need to be meshed, as opposed to the finite element method. However, the mathematical formulation leads to a dense matrix system of equations. Therefore, the size of the model increases drastically as the frequency of analysis increases and huge computer resources are required to solve complex models in the mid‐frequency range. The fast multipole method can be used to extend the boundary element model and solve such problems. For a model with N nodes, this technique brings the number of operations down to O(N*LogN) instead of O(N**3) for conventional boundary elements. This new methodology has been applied here to study the acoustic radiation of a complete car in the mid‐frequency range. The accuracy of the results as well as the computation time demonstrate the great potential of this new method to solve very large acoustic r...

A modal approach for the acoustic and vibration response of an elastic cavity with absorption treatments and mechanical/acoustical excitation

The calculation of the acoustic and vibration response of an elastic cavity is considered. The presented technique couples a modal decomposition for the structure in vacuo and the cavity with a coordinate transformation to allow for efficient determination of the coupled modes. Using these coupled modes, it rewrites the coupled equations accounting for the impedance condition at the treated elastic walls, internal acoustic sources, and mechanical excitations in terms of a compact forced system. The theory behind the method is presented together with validations and numerical examples.

A novel acceleration method for the variational boundary element approach based on multipole expansions

The acoustic radiation of general structures with Neumann’s boundary condition using the variational boundary element method (VBEM) is considered. The classical numerical implementation of the VBEM suffers from the computation cost associated with double surface integrals. To circumvent this limitation a novel acceleration method is proposed. It is based on the expansion of the cross influence matrices in terms of multipoles using the expansion of the Green’s function in terms of spherical Bessel functions. Since the resulting multipoles are not dependent on the elements locations, the technique results in large computation time savings for homogeneous meshes. The theory behind the approach, its convergence, and its numerical implementation in a VBEM code will be presented. It will be shown that by accounting for the monopole, dipole, and quadrupole terms in the multipoles expansion, the classical convergence criteria usually used for quadratic element hold. Numerical applications to acoustic radiation fr...