Reprint of: CAA Broadband Noise Prediction for Aeroacoustic Design

AbstractThe current status of a hybrid RANS/CAA approach for the simulation of broadband sound generation is presented. The method rests on the use of steady Reynolds Averaged Navier-Stokes (RANS) simulation to prescribe the time-averaged motion of turbulent flow. By means of synthetic turbulence the steady one-point statistics (e.g. turbulent kinetic energy) and turbulent length- and time-scales of RANS are translated into fluctuations of turbulent velocity (or vorticity), whose statistics very accurately reproduce the spatial target distributions of RANS. The synthetic fluctuations are used to prescribe sound sources which drive linear acoustic perturbation equations. The whole approach represents a methodology to solve statistical noise theories with state-of-the-art Computational Aeroacoustics (CAA) tools in the time-domain. A brief overview of the synthetic turbulence model and its numerical discretization in terms of the Random Particle-Mesh (RPM) and Fast Random Particle-Mesh (FRPM) method is given. Results are presented for trailing edge, slat, jet, and combustion noise. Some problems related to the formulation of vortex sound sources are discussed

Near-field sound radiation of fan tones from an installed turbofan aero-engine

The development of a distributed source model to predict fan tone noise levels of an installed turbofan aero-engine is reported. The key objective is to examine a canonical problem: how to predict the pressure field due to a distributed source located near an infinite, rigid cylinder. This canonical problem is a simple representation of an installed turbofan, where the distributed source is based on the pressure pattern generated by a spinning duct mode, and the rigid cylinder represents an aircraft fuselage. The radiation of fan tones can be modelled in terms of spinning modes. In this analysis, based on duct modes, theoretical expressions for the near-field acoustic pressures on the cylinder, or at the same locations without the cylinder, have been formulated. Simulations of the near-field acoustic pressures are compared against measurements obtained from a fan rig test. Also, the installation effect is quantified by calculating the difference in the sound pressure levels with and without the adjacent cylindrical fuselage. Results are shown for the blade passing frequency fan tone radiated at a supersonic fan operating conditio

Aeroacoustic tunnel effect in noise shielding problems

The question is addressed, under which conditions the commonly applied neglect of shear in the flow past an acoustically shielding object may not be justified, even at low Mach numbers, representative for take-off and landing of aircraft. The numerical results generated to quantify the engine noise shielding effect of a high lift airfoil in the EU project NACRE are taken as starting point. These results showed an unexpectedly significant influence of shear on the shielding when compared to a simplified approach neglecting the shear at a low Mach number of M = 0.3. The paper gives an explanation of this phenomenon based on total reflection and accompanying evanescent waves in the shear flow near the gap slot. The problem is stripped down to the essential physical ingredients and is then reproduced in a much more simplified setup. The investigation shows that evanescent waves below a reflecting shear layer may become propagational in the presence of an edge or slot, i.e. sound may "tunnel" through even a totally reflecting shearlayer and radiate into the acoustic shadow zones predicted with simple methods, which neglect shear

RANS / CAA based prediction of NACA0012 broadband trailing edge noise and experimental validation

The prediction quality of a fast Computational Aeroacoustics (CAA) approach is studied for noise generated at a NACA 0012 trailing edge. Broadband spectra, the power law underlying their Mach number scaling, and the effect of Reynolds Number on the spectra are juxtaposed against published data from measurements and results from a semi-empirical prediction tool. The CAA method rests on the use of Reynolds Averaged Navier-Stokes (RANS) solutions to describe the turbulent flow problem around the airfoil. Acoustic Perturbation Equations (APE) are solved in the time domain, using a vortex source term, which is a function of turbulent field quantities. The acoustic approach was sccessfully validated in other works by utilizing turbulence data from Large Eddy Simulation (LES) to prescribe the unsteady sound sources. For the fast CAA approach applied in this work unsteady vortex sound sources are determined by a stochastic method, which generates 4D spatio-temporal synthetic turbulence that very accurately accompishes a local realization of all statistical and mean-flow features provided by steady RANS. Based on these prerequisites it becomes now feasible to make an assessment as to which acoustic accuracy can be achieved with such a hybrid RANS / CAA prediction method, bearing in mind the approximative nature and limited turbulence resolution of RANS

Portevin-LeChatelier effect in Al-Mg alloys : influence of obstacles, experiments and modelling

International audienceDue to their low weight and high mechanical strength Al–Mg alloys provide a large variety of applications as material for lightweight construction. However, the practical benefit is limited due to instabilities and inhomogeneities caused by correlated movement of dislocations during plastic deformation (Portevin–LeChatelier effect, PLC). By addition of unshearable obstacles the formation of dislocation avalanches should be prevented or at least reduced. Uniaxial tension tests at various constant strain rates on the metal matrix composite (MMC) AA5754 with 2 or 5 vol.% Al2O3 particles showed hardly any influence of the particles added. This could be explained by an inhomogeneous distribution of the particles during manufacturing of this MMC. First results of FE simulations using a macroscopic PLC model are presented. The importance of local stress concentrations around the inclusions is emphasized