Showing papers by "Gwenael Gabard published in 2008"

Influence of mean flow gradients on fan exhaust noise predictions

Abstract: Aft fan noise is becoming a more dominant source as engine bypass ratio is increased n this paper an assessment of the effect of the mean flow gradients on fan exhaust noise propagation is carried out using both analytical models for simplified problems and numerical methods for realistic configurations. Fan exhaust noise can be significantly refracted by the mean flow gradients in the jet mixing layer, especially at high operating conditions (i.e. during take off). The refraction effect is predicted using either Lilley’s equation or the linearized Euler equations. For parallel base flows, an issue with these linear models is the presence of Kelvin-Helmholtz instabilities whose unlimited exponential growth is unphysical and problematic for computational methods. This problem is less critical for developing mixing layer for instance where the growth of the vorticity thickness reduces the growth of the instability waves [1]. Various techniques have been used for suppressing the instability; these include adding non-linear terms to saturate the growth of the instability [2], using frequency domain analysis [3], or removing the mean flow gradient terms [4]. It is the last approach, termed Gradient Term Suppression (GTS), which is investigated in the present work.

Near- to far-field characteristics of acoustic radiation through plug flow jets.

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.

On the separation of hydrodynamic and acoustic waves in linear free‐shear flows

Abstract: The governing equations for sound propagation through free‐shear flows, like jets and mixing layers, are the linearized Euler equations. These equations support both hydrodynamic and acoustic waves. For an aeroacoustician wishing to study the refraction effects of a sound source by shear flows, it is important to distinguish the acoustic solution from the hydrodynamic waves. Agarwal et al. (AIAA J., Vol. 42, No. 1, 2004) presented a technique to achieve this in the frequency domain. In this talk, we present a time‐domain technique to separate the hydrodynamic and acoustic waves. The idea is to implement a filter that filters out only the acoustic wave solution from the linearized Euler equations. Some sample solutions are presented for two‐dimensional free‐shear flows and comparisons are made against known analytical solution for parallel flows and from solution obtained by approximate methods, which have a limited range of applicability. The advantage for the present technique is that it is applicable to...

Wave-based numerical methods for acoustics

Abstract: Recent developments in wave-based numerical methods are reviewed in application to problems in acoustics where small perturbations of pressure and velocity propagate on a steady compressible mean flow. In the most general case, such wave fields are represented by the solution of the linearized Euler equations. In the case of steady time-harmonic solutions in a homogeneous medium and in the absence of mean flow, these reduce to the solution of the Helmholtz equation. When irrotational mean flow is present, they can be represented by a convected Helmholtz-like equation. When rotational mean flow effects are significant, the time harmonic, coupled, first order linearized Euler equations must be solved. Wave-based numerical methods have been applied to all three categories of problem. The solution of the time-harmonic acoustic equations, particularly for large three-dimensional domains, is computationally challenging. When traditional finite element methods are used with polynomial basis functions within each element, approximability arguments alone require the use of many nodes per wavelength to resolve the resulting spatially harmonic solutions. It is moreover well known that these requirements are exacerbated when the computational domain spans many wavelengths of the solution. In such cases the ’pollution’ effect [1] means that the global error increases with frequency, irrespective of the number of nodes per wavelength that are used to resolve the solution. This effect is particularly acute for problems which involve exterior scattering by objects whose geometric lengthscale is large compared to a typical wavelength. A problem this type which is of particular interest to the authors is acoustic radiation from aero-engine nacelles where the length scale of the acoustic disturbance at peak frequencies is an order of magnitude smaller than the diameter of the nacelle. Another problem which exhibits the same disparity of lengthscales and where wave-based methods have been recently been applied is in the calculation of Head Related Transfer Functions for the human head and torso. Here a similar relationship holds between the wavelength of the disturbance and the dimensions of the torso over much of the audible range. To resolve either of these problems in three dimensions by using conventional numerical methods requires many millions of node or grid points. The use of non-polynomial, wave-like bases to represent such solutions more effectively with fewer degrees of freedom, can be traced back to infinite element schemes developed over several decades, in which a single outwardly propagating wave direction is used [2,3]. Such methods are now routinely implemented in a number of commercial codes. More recently, the Partition of Unity method [4,5,6] has