100,412 research outputs found
Finite element simulation of noise radiation through shear layers
No full textPredicting sound propagation through the jet exhaust of an aero-engine presents the specific difficulty of representing the refraction effect of the mean flow shear. This is described in full in the linearised Euler equations but this model remains rather expensive to solve numerically. The other model commonly used in industry, the linearised potential theory, is faster to solve but needs to be modified to represent a shear layer. This paper presents a way to describe a vortex sheet in a finite element model based on the linearised potential theory. The key issues to address are the continuity of pressure and displacement that have to be enforced across the vortex sheet, as well as the implementation of the Kutta condition at the nozzle lip. Validation results are presented by comparison with analytical results. It is shown that the discretization of the continuity conditions is crucial to obtain a robust and accurate numerical model
Reflection of an acoustic line source by an impedance surface with uniform flow
No full textAn exact analytic solution is derived for the 2D acoustic pressure field generated by a time-harmonic line mass source located above an impedance surface with uniform grazing flow. Closed-form asymptotic solutions in the far-field are also provided. The analysis is valid for both locally-reacting and nonlocally-reacting impedances, as is demonstrated by analysing a nonlocally reacting effective impedance representing the presence of a thin boundary layer over the surface. The analytic solution may be written in a form suggesting a generalization of the method of images to account for the impedance surface. The line source is found to excite surface waves on the impedance surface, some of which may be leaky waves which contradict the assumption of decay away from the surface predicted in previous analyses of surface waves with flow. The surface waves may be treated either (correctly) as unstable waves or (artificially) as stable waves, enabling comparison with previous numerical or mathematical studies which make either of these assumptions.The computer code for evaluating the analytic solution and far-field asymptotics is provided in the supplementary material. It is hoped this work will provide a useful benchmark solution for validating 2D numerical acoustic codes
A comparison of wave-based discontinuous Galerkin, ultra-weak and least-square methods for wave problems
No full textSeveral numerical methods using non-polynomial interpolation have been proposed for wave propagation problems at high frequencies. The common feature of these methods is that in each element, the solution is approximated by a set of local solutions. They can provide very accurate solutions with a much smaller number of degrees of freedom compared to polynomial interpolation. There are however significant differences in the way the matching conditions enforcing the continuity of the solution between elements can be formulated. The similarities and discrepancies between several non-polynomial numerical methods are discussed in the context of the Helmholtz equation. The present comparison is concerned with the ultra-weak variational formulation (UWVF), the least-squares method (LSM) and the discontinuous Galerkin method with numerical flux (DGM). An analysis in terms of Trefftz methods provides an interesting insight into the properties of these methods. Second, the UWVF and the LSM are reformulated in a similar fashion to that of the DGM. This offers a unified framework to understand the properties of several non-polynomial methods. Numerical results are also presented to put in perspective the relative accuracy of the methods. The numerical accuracies of the methods are compared with the interpolation errors of the wave bases
CAA study of airfoil broadband interaction noise using stochastic turbulent vorticity sources
No full textThe interaction of the turbulent wakes of the rotor with the outer guide vanes is one of the main broadband noise source in turbofan engines at approach conditions. Hence its prediction and reduction is a priority for engine manufacturers. The development of numerical methods is required as analytical approaches are limited to simple geometries and simplified flow configurations. The linearized Euler equations are solved in the time-domain to model the response of an isolated airfoil interacting with turbulence that is stochastically synthesized and injected in the computational domain through vorticity sources. This new method of injection has the advantages of being easy to implement and parallelize in an existing solver, whilst the generated turbulence is frozen. The method is firstly validated on a 2D free-field configuration. It is then applied, in the framework of the Fan Stage Broadband Noise Benchmarking Programme, to a two-dimensional NACA 65(12)-10 airfoil with no angle of attack and the results are validated through comparisons with experimental data. Afterwards, the effect of the angle of attack is studied and the results suggest that a one-component turbulent model is not satisfactory to perform accurate acoustic predictions with an angle of attack, as it overestimates the rate of decay of the acoustic spectra at high frequencies. The study of the influence of the integral length scale of the turbulence confirms that the airfoil leading edge response is only modulated by the incoming turbulence characteristics. Finally, the acoustic spectra predicted for different velocities show a better agreement with a flat plate analytical model when the velocity is increased
Letter, [Author unclear] to Paulina T. Merritt
No full textHandwritten letter to Paulina Merritt from an unknown author, October 1, 1876.
Calculated line broadening parameters for methane perturbed by diatomic molecules
No full textInternational audienceWe report semiclassical line broadening calculations for methane perturbed by diatomic molecules: nitrogen, N2, oxygen, O2 and hydrogen, H2, at room temperature. For this, we have developed a symmetrized version of the Robert and Bonamy theory. The interaction potential was built from electrostatic (octopole and hexadecapole for methane, quadrupole for the diatomic molecules) and atom-atom contributions. The relative (classical) trajectories of the molecules were computed in the frame of the usual parabolic model, through analytical formula. High orders of developments had to be used for the short range molecular interactions in the case of N2 and O2. For H2, a lower order of development was found to be convenient. We have compared our calculations to some of the available experimental data for hydrogen and oxygen. For nitrogen, we have already reported some comparisons to measurements [T. Gabard, V. Boudon, J. Quant. Spectrosc. Radiat. Transfer 111 (2010) 1328-1343]. Thus, we report here new extensive calculations for the dyad (near 1300 cm-1) and the pentad (near 3000 cm-1 ) spectral regions
Comparison of 2D boundary curving methods with modal shape functions and a piecewise linear target mesh
No full textt is well known that high-order simulation techniques demand an accurate geometric representation and a coarse mesh. To fulfill both requirements, curved meshes are generated. In most cases, curving methods assume that the exact geometry is known. But it can be useful to develop curving methods with only a limited knowledge of the target geometry. In this paper, three curving methods are described that take a piecewise fine linear mesh as input: a least squares approach, a direct optimisation in the H1-seminorm, and a H1-seminorm optimisation in a reference space. Hierarchic, modal shape functions are used as basis for the geometric approximation. The methods are compared on two test geometries, a unit circle and a distorted ellipse. Considering both test cases, the direct optimisation approach shows the most promising results. Finally, the main steps for the extension to 3D are outlined
DATABASES AND PROGRAMS FOR THE SPECTROSCOPY OF SOME GREENHOUSE GASES: AND
No full text Ch. Wenger and J.-P. Champion, J. Quant. Spectrosc. Radiat. Transfer, 59, 471-480 (1998). Ch. Wenger V. Boudon, J.-P. Champion and G. Pierre, J. Quant. Spectrosc. Radiat. Transfer, 66, 1-16 (2000). V. Boudon, J.-P. Champion, T. Gabard, G. Pierre, M. Lo\""{e}te and Ch. Wenger, Env. Chem. Lett., in press (2003).Author Institution: Laboratoire de Physique de l'Universit\'{e} de BourgogneHighly symmetrical molecules such as or are known to be atmospheric pollutants and greenhouse gases. High-resolution spectroscopy in the infrared is particularly suitable for the monitoring of gas concentration and radiative transfer in the Earth's atmosphere. This technique requires prior extensive theoretical studies for the modeling of the spectra of such molecules (positions, intensities and shapes of absorption lines). We have developed powerful tools for the analysis and the simulation of absorption spectra of highly symmetrical molecules. These tools have been implemented in the Spherical Top Data System and Highly-spherical Top Data System software available at http: //www.u-bourgogne.fr/LPUB/shTDS.html. They include a compilation of modeled data obtained during the last 20 years. An overview of our latest results in this domain will be . We will especially focus on the recent advances concerning the high polyads of methane and the combination and hot bands of sulfur hexafluoride
Handwritten biographical information on Paulina T. McClung Merritt
No full textA handwritten biography of Paulina T. McClung Merritt by an unknown author, 1892.
- …
