Florent Margnat

An iterative algorithm for computing aeroacoustic integrals with application to the analysis of free shear flow noise

An iterative algorithm is developed for the computation of aeroacoustic integrals in the time domain. It is specially designed for the generation of acoustic images, thus giving access to the wavefront pattern radiated by an unsteady flow when large size source fields are considered. It is based on an iterative selection of source-observer pairs involved in the radiation process at a given time-step. It is written as an advanced-time approach, allowing easy connection with flow simulation tools. Its efficiency is related to the fraction of an observer grid step that a sound-wave covers during one time step. Test computations were performed, showing the CPU-time to be 30 to 50 times smaller than with a classical non-iterative procedure. The algorithm is applied to compute the sound radiated by a spatially evolving mixing-layer flow: it is used to compute and visualize contributions to the acoustic field from the different terms obtained by a decomposition of the Lighthill source term.

On compressibility assumptions in aeroacoustic integrals: a numerical study with subsonic mixing layers

Two assumptions commonly made in predictions based on Lighthills formalism are investigated: A constant density in the quadrupole expression and the evaluation of the source quantity from incompressible simulations. Numerical predictions of the acoustic field are conducted in the case of a subsonic spatially evolving two-dimensional mixing layer at Re = 400. Published results of the direct noise computation (DNC) of the flow are used as reference and input for hybrid approaches before the assumptions on density are progressively introduced. Divergence free velocity fields are obtained from an incompressible simulation of the same flow case, exhibiting the same hydrodynamic field as the DNC. For the tested values of the Mach number, hybrid predictions reproduce correctly the reference acoustic field, validating both assumptions in the source region. However, in the observer region, the inclusion of flow effects in the Lighthill source term is not preserved, which is illustrated through a comparison with the Kirchhoff wave-extrapolation formalism, and with the use of a convected Green function in the integration process.

Influence of the shape of 2D thick bodies on their aeolian noise

A hybrid noise prediction approach is proposed for the study of acoustic emission by laminar flows over 2D bodies of various sections. In a first step, the unsteady incompressible flow is simulated. The presence of the bluff body is modelled through an immersed boundary method. In a second step, the acoustic field is evaluated using Curle’s analogy and the compact source approximation. The validation is provided through the computation of the aelian tone in the flow over the circular cylinder at Re = 150 and M = 0.2. The method is then applied to investigate the influence of the angle of attack of a rectangular cylinder in a 2D flow at Re = 200. Ten values of the incidence are considered from 0 to 90. A change in the vortex street topology around 40 of incidence is exhibited, whose trace on the phase portrait of the aerodynamic force and on the acoustic field is investigated in details.

Analysis of acoustic source mechanisms in free shear flows

Specific tools devoted to the study of the source mechanisms related to the dynamic of free shear flows are developed. Our analysis methodology is based on a decomposition of the Lighthill source term into ten sub-terms, in which the velocity, vorticity, dilatation and density fields appear explicitly. This decomposition is applied to the source term involved in a 2D spatially evolving mixing-layer, computed by DNS. The compressible Navier-Stokes equations are solved in a computational domain which includes both aerodynamic and acoustic fields. The numerical code is based on sixth-order accurate compact finite difference schemes and a fourth-order Runge-Kutta time marching, while the characteristic analysis is used to specify non-reflecting boundary conditions. Acoustic results show that only five of the ten sub-terms of the decomposition contribute to the sound field.

Analysis of hydrodynamic and acoustic events in a turbulent boundary layer using a direct noise simulation database

An aeroacoustic analysis of a turbulent boundary layer numerical database is conducted, aiming at the identification of acoustically efficient flow structures. The free-stream Mach number is 0.5 while the initial Reynolds number based on the local momentum thickness of the laminar boundary layer is 480. The flow and its acoustic radiation was obtained in a previous study by direct numerical solution of the unsteady NavierStokes equations for a compressible, viscous flow. The post-treatment starts with data reduction, keeping only the first mode of a spanwise Fourier series expansion, selecting relevant parallel planes: one near the wall and one in the acoustic region, and focusing on the pressure field. The data thus reduces to two scalar fields of the streamwise coordinate and time. Space-time correlations then allows to quantify global properties of both fields, such as time and length scales and convection velocity of the hydrodynamic pressure, and main acoustic propagation angle, which are found in agreement with the qualitative and quantitative previous observations. Collection of flow and acoustic events is then conducted using amplitude criteria, and causality relationship is investigated assuming convected spherical radiation.

An Iterative Procedure For the Computation of Acoustic Fields Given by Retarded-Potential Integrals

An optimized procedure for the computation of acoustic fields given by retarded-time intgrals is provided. It is written in the time-domain and for fixed sources. It is devoted to applications in which there is a large amount of source data. Thus, as many observer points are required to build the acoustic image, the resulting number of source-observer pair may cause an issue in accessing to the source-observer distance, if this quantity can not be stored in a global variable. The algorithm is validated through comparisons with reference data in the case of a simple harmonic source and in the case of the aerodynamic noise generated by the cylinder flow. The implementation of the convected Green function is also presented.

Analysis of Free-Shear Flow Noise Through a Decomposition of the Lighthill Source Term

A decomposition of the Lighthill source term \(\nabla\cdot(\nabla\cdot(\rho \textbf{u}\otimes\textbf{u}))\) is performed with view to more clearly understanding the mechanisms which underlie the production of flow-induced noise. Observation of source fields and associated radiation patterns leads to 3 main observations: (1) the amplitude of subterms in the source field does not scale with the associated acoustic contributions; (2) balances and destructives interferences are present between subterms; (3) source behaviour associated with convection and sound-flow effects is highlighted.

Numerical strategies for investigation of gust‐airfoil interaction

The noise generated by the interaction between a gust and an airfoil in a uniform flow is investigated. This problematic is of major industrial interest, regarding fans, turbomachinery, or wind turbine applications. A two‐dimensional symmetric Joukowski‐type airfoil is immersed without incidence in a flow at Mach number 0.5, disturbed by a harmonic gust at 45° of incidence (4th CAA Workshop on Benchmark Problems, 2004). Our methodology is first to perform a high‐order direct resolution of Eulers equations of the disturbed flow over the airfoil and the associated acoustic emission, which is taken as a reference simulation. Second, the near aerodynamic field is simulated with Fluent 6.3 solver based on finite volume method with second‐order schemes. The aerodynamic data thus obtained are used for far field acoustic prediction, based on Ffowcs Williams and Hawkings analogy. Finally, following another hybrid approach, the noise is predicted by using integral formulations with source field from the DNS. The a...

Decomposition of the Lighthill source term and analysis of acoustic radiation from mixing layers

Acoustic radiation from jet flow has been studied extensively by means of theoretical, experimental and numerical approaches over the past decades. Unfortunately, the mechanisms responsible for the production of sound by unbounded turbulence in subsonic flows remain unclear. For advancing our fundamental understanding of these mechanisms, the development of specific analysis tools is needed. Our study is based on a decomposition of the Lighthill source term, which is known to contain all the existing links between the fluid flow and the acoustic field. Ten subterms are written with the help of physically meaningful quantities such as velocity, density, dilatation and vorticity. The methodology is tested through the two‐dimensional compressible mixing layer flow in spatial development, at a Reynolds number of 400 and a Mach number of 0.25. A direct numerical simulation of the flow and its acoustic radiation is performed and used as a reference solution. Acoustic field generated by each source terms is pred...