Thomas Le Garrec

Trailing edge noise from an isolated airfoil at a high Reynolds number

A multi-size mesh multi-time step strategy is used to perform three dimensional direct noise computations (DNC) around airfoils at high Reynolds numbers. This method allows to realize local grid refinements in conjonction with high order numerical methods commonly used in Computational AeroAcoustics (CAA). In order to analyse broadband trailing edge noise, the configuration consists in a truncated NACA0012 airfoil at high Reynolds 2.32 × 10 6 with an angle of attack of 2.5 ◦ and with triggered turbulent boundary layers both sides. This challenging case implies a wide variety of scales, and tackle the limit of the current computational capabilities. Two grids have been designed to test Large Eddy Simulations based on explicit filtering. In the present work, two of the five self-noise mechanisms due to specific boundary-layer phenomena identified and modeled by Brooks et al.[NASA Tech. Rep., 1989] are studied numerically. Namely, the noise radiation comes from the diffraction of the kinetic energy of evanescent waves advected in the turbulent boundary layers passing in the vicinity of the trailing edge in conjonction with the vortex shedding from the truncated trailing edge. First comparisons are made with an experimental database realized by the french aerospace agency ONERA. Particular attention is drawn to the characteristics of the wall pressure, in order to relate it with the acoustic field, obtained directly in the present approach.

Evaluation of a Genuinely Multi-Dimensional High-Order Explicit Filters for Aeroacoustic Computations

The present paper addresses the evaluation for acoustic and aeroacoustic computations of a new multi-dimensional extension, recently developed at ONERA, for high-order explicit and implicit filters. The multi-dimensional extension, initially developed for Shapiro’s optimal filters, applies to many existing implicit and explicit filters that can be expressed as linear combination of Shapiro filters. The new method takes full advantage of the multidimensional nature of the field to reduce the stencil used for the filtering while maintaining the accuracy order and dissipating properties of the initial one-dimensional filters. Here, this new method is compared to classical ones through a set of acoustic and aeroacoustic test cases.

A study based on the sweeping hypothesis to generate stochastic turbulence

With the development of so-called ’electric’ aircraft, the integration of jet pumps to various systems of the engines could o er signi cant gains (mass, reliability,...) over the technologies currently used. Since jet pumps may directly contribute to ramp noise, an accurate predictive tool is therefore desired to propose adapted noise reduction solutions in an industrial context. A direct computation of the unsteady turbulent ow being too expensive, the main idea is to compute acoustic sources from a stochastic velocity eld and to inject it in Euler’s equations to model neareld acoustic propagation. A Kircho analogy is then used to reduce calculation cost in the far eld. In following previous works by Bailly and Juv e (AIAA Paper 99-1872) and Billson et al. (AIAA paper 2003-3282), a combined approach to generate a stochastic velocity eld is presented and validated in the present work. This methodology is based on the sweeping hypothesis or the fact that small scale turbulent structures are advected by energy containing eddies. The validation study is done on aerodynamic quantities of a cold free jet at Mach number M=0.72. The capability of the method to reproduce space-time velocity correlations in the shear layer is shown. The model is then tested on a cold free jet con guration to predict radiated acoustic levels using a volumic Lighthill solver.

Turbulence Generation from a Sweeping-Based Stochastic Model

Stochastic methods are widely used because they constitute a low-cost computational-fluid-dynamics approach to synthesize a turbulent velocity field from time-averaged variables of a flowfield. A new combined stochastic method based on the sweeping hypothesis is introduced in this paper. This phenomenon, stating that inertial range structures are advected by the energy containing eddies, is known to be an important mechanism of the turbulent velocity field decorrelation process. The proposed method presents the advantage of being easily implementable and applicable to any three-dimensional configuration as long as a steady Reynolds-averaged Navier–Stokes computation of the flow is available and assuming that the considered turbulence physics is compatible with the hypotheses made to build the current numerical model. The developed method is applied on a subsonic round cold free jet. The validation study shows that the synthesized turbulent velocity fields reproduce statistical features of the flow, such a...

Prediction of subsonic jet noise relying on a sweeping based turbulence generation process

Jet pumps, which are considered in the framework of this study as simple jets confined in ducts, could be a significant ally in the mission of the aeroacoustic community to make the aircraft more environmentally friendly. Despite the significant gains that it could bring for various functions of the systems of the engines, such devices could contribute directly to ramp noise. Therefore, a predictive code of the noise radiated by these configurations could be a useful tool to propose noise reduction solutions. A numerical tool, designed for acoustic prediction of confined jets, is presented in this paper. The proposed method is adapted to industrial requirements since it is easily implementable and applicable to any 3D configuration. The acoustic propagation is obtained by computing the response of the linearized Euler equations, enforced by a non-linear source term calculated with a stochastic velocity field synthetized from the sweeping based turbulence generation process proposed by Lafitte et al. [AIAA 2011-2088]. The chosen formulation is validated by computing test cases including simple dipole and quadrupole distributions. The predictive tool is then applied to a cold free jet configuration at Mach number M=0.72. Preliminary computations show that the forcing of the linearized Euler equations by a stochastic source term appears to be a complex operation since far-field acoustic spectra levels are overestimated. An accurate calibration method of the acoustic source term is therefore introduced in the present work.

Hybrid simulation combining two space–time discretization of the discrete-velocity Boltzmann equation

Abstract Despite the efficiency and low dissipation of the stream-collide scheme of the discrete-velocity Boltzmann equation, which is nowadays implemented in many lattice Boltzmann solvers, a major drawback exists over alternative discretization schemes, i.e. finite-volume or finite-difference, that is the limitation to Cartesian uniform grids. In this paper, an algorithm is presented that combines the positive features of each scheme in a hybrid lattice Boltzmann method. In particular, the node-based streaming of the distribution functions is coupled with a second-order finite-volume discretization of the advection term of the Boltzmann equation under the Bhatnagar–Gross–Krook approximation. The algorithm is established on a multi-domain configuration, with the individual schemes being solved on separate sub-domains and connected by an overlapping interface of at least 2 grid cells. A critical parameter in the coupling is the CFL number equal to unity, which is imposed by the stream-collide algorithm. Nevertheless, a semi-implicit treatment of the collision term in the finite-volume formulation allows us to obtain a stable solution for this condition. The algorithm is validated in the scope of three different test cases on a 2D periodic mesh. It is shown that the accuracy of the combined discretization schemes agrees with the order of each separate scheme involved. The overall numerical error of the hybrid algorithm in the macroscopic quantities is contained between the error of the two individual algorithms. Finally, we demonstrate how such a coupling can be used to adapt to anisotropic flows with some gradual mesh refinement in the FV domain.

A numerical insight into the effect of confinement on trailing edge noise

The flow and the acoustic field around a 3D NACA 0018 airfoil at Reynolds 160 000 with an angle of attack of 6° are investigated numerically by direct noise computation to make comparisons with the experimental results of Nakano et al. (Experiments in Fluids, 2005) and the numerical simulations of Kim et al. (Heat and Fluid Flow, 2006). The direct noise computation of the flow around a 3D airfoil reaches the limits of current computational capacities. Indeed the main difficulty of such simulations comes from the large disparities between the fine scales of turbulence and the large wavelengthes of acoustic radiation which impose severe constraints on the meshes. In order to limit the number of points and to reduce the calculation cost, a multisize‐mesh multitime‐step strategy is adopted. The main purpose of the paper is to study numerically the influence of the confinement due to the top and bottom surfaces of the wind tunnel used in the experiments of Nakano et al. on the flow around the airfoil. The nume...

Numerical prediction of rotor-stator interaction noise using 3D CAA with synthetic turbulence injection

Turbulent RSI (rotor-stator interaction) mechanism is a major broadband source contribution of turbofan noise generation. Acoustic prediction tools used by Industry are based on flat-plate cascade response models with restrictive assumptions on flow and geometry. Due to huge CPU memory and time cost required, Large Eddy Simulations of the complete fan-OGV stage are still out of reach (apart from recent impressive results obtained using the Lattice Boltzmann Method). This paper presents an alternative approach based on the use of a 3-D CAA (Computational Aeroacoustics) code solving the linearized-Euler equations applied to the disturbances and coupled with a synthetic turbulence injection model. The inflow turbulence is synthetized by means of a sum of harmonic gusts with random phases. The Fourier-mode amplitudes are trimmed by a 2 or 3-wave number Von-Karman or Liepmann turbulence spectrum. Swirling convection of the synthetic turbulence is provided by a 3D RANS mean flow solution and interpolated at the nodes of the CAA grid. In this paper, our methodology is first validated on a benchmark case (fully annular duct with swirling flow and a prescribed turbulence) and then applied for the first time to an industrial turbofan in the framework of a European project, TurboNoiseBB. Previous implemented 2D formulation (2-wave number spectrum) for turbulence generation is extended here to 3D (axial, radial, and angular modes) in order to study the sensitivity on cascade effects.

Open-Rotor Noise Assessment with CFD/CAA Chaining

A numerical strategy based upon a coupling of Computational Fluid Dynamics (CFD) and Computational AeroAcoustics (CAA) solvers is set up for open-rotor noise predictions. The method simulates the near field noise, and is possibly followed by an integral method calculation which leads to the far field noise. The goal of such a method is to take into account the mean flow and acoustical installation effects that a chaining restricted to the CFD and the integral method cannot predict at low computational costs. The chaining is assessed on the Airbus Clean Sky generic open rotor configuration, first on the front rotor only, as an isolated single propeller noise evaluation, second on the isolated CROR. In both configurations, the CFD/CAA results relying on freestream uniform flow are first compared to the CFD/Integral method ones for validation. Then, the chaining is performed considering the mean background flow, allowing to observe the effects of the latter on acoustics. In the case of the CROR, these results are compared to experimental measurements. Results are believed to be sensitive to the data injection into CAA, with more or less effects according to the injection technique.

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...