ABSTRACT
A multimodal method for computing the potential base flow and propagating acoustic perturbations inside axisymmetric ducts is presented. Instead of using the standard modal basis, a polynomial basis is used in the radial direction to reduce the computational cost of the method, but this introduces non-physical high-order modes. The impact of these modes on the stability of the calculation is examined, and for the acoustic computation, a modification of the axial integration is proposed to improve the conditioning of the matrices involved. The flow computation is achieved by applying the method (initially devoted to acoustics) at a zero frequency without convective effects, by modifying the definition of the admittance at the exit of the duct, and by performing an induction process on the density. The method is validated against a finite element method for ducts with hard walls or lined walls. The results show that the proposed multimodal method is very efficient in computing the mean flow and propagating the sound disturbances inside axisymmetric ducts.
ABSTRACT
The scattering of the acoustic waves generated by a monopolar source propagating through a two-dimensional elliptic vortex, fixed or convected by a uniform flow, is studied by solving the Linearized Euler Equations in Cartesian coordinates using the Discontinuous Galerkin Method. For a fixed vortex position, the number, amplitudes, and angular spreads of the acoustic interference beams resulting from the sound scattering are found to significantly depend on the orientation of the vortex major axis with respect to the direction of the incident waves and on the vortex maximum tangential velocity. In particular, additional interference beams are obtained at large observation angles for a more elliptical vortex. For a convected elliptic vortex, the interference beams are curved as the angle between the incident acoustic wave and the vortex major axis varies when the vortex travels in the downstream direction. As expected, the scattering of the acoustic waves leads to spectral broadening in this case. Moreover, the widths and the frequencies of the lateral lobes obtained in the spectra on both sides of the peak at the source frequency are different for elliptic and round vortices.
ABSTRACT
In the linear regime and in the absence of mean flow, the impedance of perforated liners is driven by visco-thermal effects. In this paper, two numerical models are employed for predicting these visco-thermal losses. The first model is the linearized compressible Navier-Stokes equations (LNSE), solved in the frequency domain. The second model is the Helmholtz equation with a visco-thermal boundary condition, accounting for the influence of the acoustic boundary layers. These models are compared and validated against measurements. The quantitative analysis of the dissipation rate due to viscosity, computed from the LNSE solutions of four perforated plates, highlights significant differences between the edge effects of a macro- and a micro-perforated plate. In the latter case, a jet is present at the entrances of the perforation. In contrast, the proposed numerical method to calculate the impedance of perforated liners, based on the Helmholtz equation and a visco-thermal boundary condition, is found to be computationally cheaper and to provide reliable predictions.
ABSTRACT
This paper reports a theoretical study of the radiation of sound through jet exhausts. It focuses on the transition from near field to far field by considering the features of the near-field solution and how these features translate to the far field. The main focus of this work is the importance in some cases of lateral waves radiating from the jet. While the presence of lateral waves has long been recognized, there has been no systematic investigation of the practical consequences of these waves in the prediction of sound propagation through round jets. The physical mechanisms involved in the generation of these waves are presented as well as the conditions under which they become significant. Another issue is the possibility of "channeled waves" inside the jet associated with strong sound radiation in the forward arc. This paper also discusses the validity of the far-field approximation when lateral waves are present. It is shown that the standard far-field approximation can be improved by adding correction terms that account for the presence of the lateral waves and channeled waves. The challenge posed to computational aeroacoustics by these near-field effects is also discussed.
Subject(s)
Acoustics , Air , Aircraft , Sound , Humans , NoiseABSTRACT
The analysis of 6153 accidents reported to the insurance company of the French Rhône-Alpes Soccer Association, for the 1980-81 season was undertaken, providing a survey of acute pathology in French soccer accidents and an estimation of the cost of this pathology to French society. Findings from this study include: injuries--ankle sprain is the most common; fractures prevail in the young players pathology; exposure--the average risk is one accident for 20 matches; the highest risk is for the senior category; collisions with opponents is the main cause of accidents; the first 5 minutes of the second half have a peak of accidents; the players exposure is roughly the same whatever their position on the ground; risk--winter should not increase the risk if the matches are played under good conditions; the risk is unevenly distributed according to the level of practice; cost--the cost for France over 1 year was estimated at US$20,000,000 and the total number of sick leave days at 2000 years; games with several accidents are very common in January and for the adult category. Consequently, tightening up the safety measures would be a very good investment.
