Narrow Results

In the high-speed railway wireless communication, a joint channel estimation and signal detection algorithm is proposed for the orthogonal frequency division multiplexing (OFDM) system without cyclic prefix in the doubly-selective fading channels. Our proposed method first combines the basis expansion model (BEM) and the inter symbol interference (ISI) cancellation to overcome the situation that exists with the fast time-varying channel and the normalized maximum multipath channel exceeding the length of the cyclic prefix (CP). At first, the channel estimation and signal detection can be approximated without considering the ISI. Then, the channel parameters and signal detection are updated through ISI cancellation and circular convolution reconstruction from the frequency domain. The simulations show the algorithm can improve the performance of channel estimation and signal detection.

The calculation efficiency of the conventional single-domain acoustic BEM (SBEM) is important for analyzing large scaled or complicated acoustic cavity systems. Although the multi-domain BEM (MBEM) has been developed to deal with such complex shaped and large size acoustic systems effectively, it is generally known that the MBEM requires more computation time and computer memory space than the SBEM. However, if the proper division were applied on a single cavity, it is thought that the MBEM could be the better method than the SBEM in the viewpoint of accuracy and efficiency. This might be possible when one reminds the fact that the effort in the MBEM calculations also depends strongly on the shape of the total acoustic cavity. In this article, the general computational characteristics of the MBEM for analyzing the interior acoustic fields are investigated to provide a guideline in the division of a single cavity into several subdomains for having better computational performance than using the SBEM. A two-dimensional long duct comprised of a number of linear elements is taken as a demonstration example. It is clearly shown that the modification of MBEM model through the present guidelines achieves more accurate and efficient computation than using the SBEM.

The process of mesh coarsening describes the reduction and simplification of a highly detailed mesh in such a way that the original geometry is best possible preserved. The earliest coarsening algorithms were developed for real time visualizations in the upcoming field of computer graphics and later on adopted for some element topologies of the finite element method (FEM). An algorithm is presented in this article which applies the mesh decimation process to the requirements of both the FEM and the Boundary Element Method (BEM) in acoustics. The capabilities of the algorithm in order to significantly reduce the CPU times for both numerical methods are shown.

The present article studies what effect the shape of a rigid acoustic barrier has on the acoustic insertion loss provided by the barrier. The Boundary Element Method (BEM), formulated in the frequency domain, is used to evaluate the sound propagation around acoustic screens in the vicinity of a tall building. The acoustic screen is assumed to be non-absorbing, and the building is modeled as an infinite barrier. Signals in the time domain are obtained from the frequency domain computations by applying inverse Fourier transforms. In the examples provided, the height of the acoustic barrier remains constant, but different geometric shapes are modeled. The results obtained for a vertical barrier are used as a reference.

The fast multipole method is employed to accelerate the Galerkin boundary element method for acoustics. Important parameters as the nearfield size and the expansion length are discussed and numerical examples are provided. An approximate inverse preconditioner and a preconditioner based on spectral properties of the boundary integral operators are presented. For the example of sound radiation from a brake disk, both approaches prove suitable to reduce the iteration count and to improve the efficiency of the method.

In this paper, we proposed a unified formulation to explain the reason why spurious eigensolution occurs in the eigenproblem of interior acoustics using the real-part and imaginary-part BEMs and why fictitious frequency occurs in exterior acoustics using the complex-valued BEM. Both the two problems stem from the rank deficiency of the influence matrix. Based on the circulant properties and degenerate kernels, an analytical study in a discrete system for a circular cavity is conducted. The Fredholm alternative theorem is employed to study the rank-deficiency problems in conjunction with singular value decomposition updating technique. The spurious and fictitious boundary modes are found to locate in the column vectors of left unitary matrix. Also, the effects of different types of boundary condition on the spurious and fictitious solutions using direct and indirect methods are discussed. The mathematical and physical meanings for the nontrivial boundary solution in spurious eigensolution and fictitious frequency are explained. Numerical experiments are found to agree with the analytical predictions.

This paper suggests the use of rigid reflectors to provide additional information for source reconstruction in near-field acoustical holography based on the inverse boundary element method. The additional field pressure and transfer matrix equations introduced provide a virtual increase in the measurement data without increasing the number of sensors or altering their arrangement, which could cost more than using reflectors. In order to validate this method, we successfully reconstruct a vibrating ellipse.

The one-dimensional analytical solutions are derived and three-dimensional substructure boundary element approaches are developed to predict and analyze the acoustic attenuation characteristics of straight-through perforated tube silencers and folded resonators without mean flow, as well as to examine the effect of nonplanar waves in the silencers and resonators on the acoustic attenuation performance. Comparisons of transmission loss predictions with the experimental results for prototype straight-through perforated tube silencers demonstrated that the three-dimensional approach is needed for accurate acoustic attenuation performance prediction at higher frequencies, while the simple one-dimensional theory is sufficient at lower frequencies. The BEM is then used to investigate the effects of geometrical parameters on the acoustic attenuation characteristics of straight-through perforated tube silencers and folded resonators in detail.

Green functions of Helmholtz's equation in an impedance half space (IHS) can be computed using various methods, e.g. the method presented by S. N. Chandler-Wilde and D. C. Hothersall.¹ The approach of W. L. Li, T. W. Wu and A. F. Seybert² is simpler, but it can only be used in a mass-like IHS. In this paper, the method is reformulated so that it can also be used in a stiffness-like IHS. Numerical experiments indicate that the method yields reliable results.

This paper deals with the behavior of acoustic cavities in the mid-high frequency range. The method proposed here is based on an energy flow method named Simplified Energy Method (MES). MES method is quite efficient in the mid-high frequency range but the directivity of the boundary sources is not well estimated. We propose a hybrid method which couples MES and the Boundary Element Method (BEM). Thus, the BEM method is used to estimate the direct field, considering a "correct" directivity. As a complete calculation is not adapted to BEM in mid-high frequency range because of the calculation costs, we only apply BEM on the domains including boundary sources. Other parts of the system and the reverberated field are estimated by the mean of MES method. This hybrid method leads to a consistent prediction of injected power densities. Numerical comparisons prove the efficiency of the proposed reformulation.

Traffic noise barriers are one of the most important ways of environmental noise pollution control. Profiled barriers are one of the most successful noise control screens. A simple mathematical model representation of the zones behind rigid and absorbent barriers with the highest insertion loss using the destructive effect of the indirect path via barrier image is introduced. The results are compared with the results of a verified two-dimensional (2D) BEM in a wide field behind those barriers. Field measurements for the model near a highway in a megacity were also tested. A very good agreement between the results has been achieved. In this method, effective height is used for any different profile barrier. Since the highway dominant noise frequency was found to be at 125 Hz 1/3-octave band center frequency, the presented model in this study showed that the best place for installing a T-shaped barrier is 10.5m from the target receiver. The introduced model is very simple and fast and could be used for choosing the best location of profiled barriers to achieve the highest performance with no additional cost.

A direct approach of coupling the element-free Galerkin method (EFGM) to both the finite element method (FEM) and the boundary element method (BEM) is applied to study dynamic soil-structure interactions. The structure and the soil body are assumed to be two-dimensional systems and discretized by EFG nodes, while the boundary of the soil region is modeled either by FEM or BEM to impose the boundary conditions in an easy way. Essential parameters in the EFG domain are chosen in a way where stable and reliable results at a relatively low cost are obtained. The accuracy and efficiency of the two new methodologies are compared to those of the conventional mesh-based approaches.

The presentation is mainly devoted to the research on the regularized boundary integral equations (BIEs) with indirect unknowns for torsion problem of the anisotropic uniform bar. Based on a new view and idea, a novel regularization technique is pursued, in which the nonsingular indirect BIE (IBIE) excluding the CPV and HFP integrals is established. Such torsion problems can be solved directly by using the presented technique without transforming them into isotropic ones, for this reason, no inverse transform is required. Moreover, a unique feature of the shear stress BIEs expressed by density functions is that they are independent of the warp BIEs and, as such, can be collocated at the same locations as the warp BIEs. This provides additional and concurrently useable equations for various purposes. Besides, in the numerical implementation, the boundary geometric is depicted by exact elements, while the distribution of the boundary quantity on each element is approximated by a discontinuous quadratic element. Some numerical examples will be applied to validate the current scheme. It is shown that a better precision and high-computational efficiency can be achieved by the presentation.

In this paper, a general technique for coupling two arbitrary methods is presented. The problem domain is decomposed into two sub-domains. Afterwards, a sensitivity analysis at the interface of each sub-domain is carried out. Sensitivity matrices of the two sub-domains are used to find the coupling matrix equation. Unknowns at the interface are then found by solving the equations. The size of the matrix equation is very small in comparison with coefficient matrices of each sub-domain. The present method allows the black box coupling of different methods, even commercial software without having access to the matrices created by the methods.

This paper presents a simple strategy allowing to adapt well-established isotropic BEM approach for the solution of multi-physics problems with anisotropic material parameters. The method is based on the partition of the primary kinematical fields into complementary and particular parts. The isotropic linear equations for the complementary fields are solved by the conventional boundary element method. The particular fields are obtained by a point collocation of a strong form differential equation. Adopting local radial point interpolation, the effectiveness of the approach is proved by considering various examples of stationary thermal conduction, thermos-elasticity and thermos-piezoelectricity.

A recently proposed criterion is used to study the behavior of debonds produced at a fiber–matrix interface. The criterion is based on the Linear Elastic–(Perfectly) Brittle Interface Model (LEBIM) combined with a Finite Fracture Mechanics (FFM) approach, where the stress and energy criteria are suitably coupled. Special attention is given to the discussion about the symmetry of the debond onset and growth in an isolated single fiber specimen under uniaxial transverse tension. A common composite material system, glass fiber–epoxy matrix, is considered. The present methodology uses a two-dimensional (2D) Boundary Element Method (BEM) code to carry out the analysis of interface failure. The present results show that a non-symmetrical interface crack configuration (debonds at one side only) is produced by a lower critical remote load than the symmetrical case (debonds at both sides). Thus, the non-symmetrical solution is the preferred one, which agrees with the experimental evidences found in the literature.