1,721,063 research outputs found
Wave envelope and infinite elements for acoustical radiation
No full textFinite element models are presented for the calculation of near and far field acoustical radiation. These models are applied to the specific problem of fan noise radiation from axisymmetric turbofan inlets. In all cases conventional acoustic finite elements are used within an inner region close to the inlet. The far field is represented by infinite elements or wave envelope elements. Theory and results are presented for the case with zero mean flow. Comparisons of computed data with analytic solutions and measured values establish the utility of both the infinite element and wave envelope element schemes in determining the near field values of acoustical pressure. The wave envelope scheme is shown to be effective also in the far field. Both schemes use meshes an order of magnitude more sparse that would be required in conventional numerical discretizations, and may consequently be applied at modest computational cost.<br/
Finite elements in solids and structures. An introduction
No full textThis is very much a teaching text intended as an accompaniment to an advanced undergraduate engineering course. In content, the book primarily deals with static problems in solids and structures, but also leads into dynamics, while focusing unequivocally on the needs of students rather than researchers and professionals
A finite element, wave envelope formulation for acoustical radiation in moving flows
No full textAn axisymmetric finite element model is presented for the prediction of radiation patterns generated by stationary acoustical sources in moving flows. The acoustical field is represented in an outer region by wave envelope elements which incorporate some features of ray acoustical behaviour. These are compatibly matched to a conventional finite element mesh in a region close to the generating mechanism. Results are presented for problems involving vibrating cylinders and spheres in subsonic mean flows. Good agreement is established between computed and exact solutions.<br/
Letter to the editor. A comparative note on the effects of local versus bulk reaction models for air moving ducts lined on all sides
No full textFE mode-matching schemes for the exterior Helmholtz problem and their relationship to the FE-DtN approach
No full textFinite element (FE) mode-matching procedures for the solution of Helmholtz' equation on an unbounded domain are reviewed and a symmetric general formulation is presented. This is a formal restatement of procedures applied previously to computations involving scattering of shallow water waves, acoustic transmission in non-uniform ducts and acoustic radiation from prismatic sheet metal ducts. An essential feature of the method is the use of a Galerkin procedure, rather than collocation, to match a finite computational model to a truncated modal expansion with the desired radiation characteristics. The method produces a symmetric set of linear equations which can be solved to give the unknown nodal values of the dependent variable and the modal coefficients of an outer expansion. Either of these sets of variables can be eliminated prior to solution to yield a reduced set of equations in the remaining parameters. The reduced equations obtained by eliminating the modal coefficients are shown to be identical to those obtained by applying a truncated Dirichlet-to-Neumann (DtN) boundary condition. If applied in this form, mode-matching can therefore be regarded as an alternative to the DtN method for generating this common set of discrete equations while permitting simultaneous solution for the modal coefficients in the outer region.<br/
Transient wave envelope elements for wave problems
No full textA novel family of infinite wave envelope elements is described which can be used in conjunction with conventional finite elements to model the transient wave equation in unbounded regions. The elements are obtained by applying an inverse Fourier transformation to a mapped wave envelope formulation in the frequency domain. The discrete transient equations obtained in this way can be applied to two-dimensional and three-dimensional problems without restriction, being valid over a full range of excitation frequencies. The effectiveness and accuracy of the method is demonstrated in application to simple test cases which involve the calculation of transient sound fields generated by pulsating spheres and cylinders excited from rest in an unbounded region. Test solutions are compared to analytic solutions and to finite element solutions obtained by using large computational grids which extend beyond the region influenced by the transient disturbance.<br/
Transient spheroidal elements for unbounded wave problems
No full textA wave-envelope element numerical scheme is applied to the solution of unbounded wave problems. The scheme is based on a Fourier transformation of a discrete model formulated in the frequency domain. This yields a discrete system of ordinary differential equations in time which are local in space. Oblate and prolate spheroidal element geometries are used. The accuracy of the scheme is demonstrated by a comparison of computed and analytic solutions for axisymmetric test cases. Time-harmonic and transient solution are presented. An indirect solution procedure is also presented which permits the sparsity of the transient equations to be utilised more effectively.<br/
A finite element scheme for acoustic transmission through the walls of rectangular ducts: Comparison with experiment
No full textPrevious work on modelling acoustic transmission through the walls of rectangular ducts has left some open questions about structural damping, radiation damping and the way in which the acoustical radiation should be treated. In an attempt to provide answers to these questions a new numerical theory for the problem is described in this paper. It is both complementary to, and more accurate than, the simpler models described elsewhere. The results are gratifying, being in good agreement with measurements, both in detailed and in overall descriptions of the transmission phenomena. The numerical results are generally in close agreement with those of the previous theoretical approaches, providing some justification for several of the approximations implicit in those models.<br/
A three-dimensional boundary element scheme for acoustic radiation in low mach number flows
No full textA boundary element scheme is proposed for acoustical radiation in moving flows. The scheme relies upon a transformation, valid at low Mach numbers, which permits the case with flow to be restated as an equivalent problem, in the absence of flow but with modified boundary conditions. This restatement of the original problem is then amenable to boundary integral representation and boundary element solution. Results obtained by using this approach are presented for the test case of a pulsating or juddering sphere of finite dimensions in a uniform low Mach number flow. A fully three-dimensional boundary element model is used. The validity of the results is established by comparison with an analytic perturbation solution (where applicable) and by comparison with results from an alternative, axisymmetric numerical scheme.<br/
- …
