Vincent Clair

CAA methodology to simulate turbulence-airfoil noise

Turbulent wakes generated by turbofan blades and interacting with the outlet guide vanes are known to be mainly contributing to broadband noise emission of aero-engines at approach conditions. Analytical approaches, such as the well-known Amiet model can be adopted to estimate the noise generated by turbulent flows impinging thin airfoils, but they are limited by the flat-plate assumptions. The development of numerical methods allowing to consider more complex geometries and realistic flows is required. The method described in the present paper, is based on a CAA code solving the nonlinear Euler equations. The upstream turbulence is synthesized from a stochastic model and injected into the computational domain through an adapted boundary condition. It is first validated in 2D and 3D against academic flat plate configurations by comparison with Amiet solutions (exact in such cases). Then, 3D computations are applied to simulate the effect of a passive treatment (leading edge serrations) aiming at reducing turbulence interaction noise of an isolated airfoil studied in the framework of European project FLOCON. First calculations on baseline conditions are shown to be able to reproduce the measured spectra and far-field directivities, and the acoustic performances of the serrations (3-4 dB PWL reduction) are fairly well assessed too

Numerical assessment of the scattering of acoustic waves by turbulent structures

The scattering of harmonic sound waves by a single vortex is studied to provide some insight into the more complex problem of sound scattering by a turbulent layer. Two different methods are used to model the scattering by a steady vortex or by a vortex convected in a uniform mean flow. The first method is based on the linearized Euler equations in the frequency domain and is used to study the scattering by a steady vortex. The speed of this method allows to perform a study of the spatial scattering of a plane wave over a wide range of frequencies, where analytical models previously developed are restricted either to low or high frequencies. The second method uses a finite difference solver, also based on the linearized Euler equations but in the time domain. This method is used to study the scattering by a vortex convected in a uniform mean fow, where a spectral scattering occurs in addition to the spatial scattering. A parametric study is realized, focusing on four parameters including the source frequency and the convection velocity. The spectra deduced from this study show a pattern similar to the haystacking observed in existing studies of the scattering by a turbulent shear layer. Some of the trends deduced from the parametric study are also in agreement with these previous observations. Thus, the detailled study of the scattering by a single eddy is able to provide further understanding of the turbulence scattering.

CAA study of airfoil broadband interaction noise using stochastic turbulent vorticity sources

The interaction of the turbulent wakes of the rotor with the outer guide vanes is one of the main broadband noise source in turbofan engines at approach conditions. Hence its prediction and reduction is a priority for engine manufacturers. The development of numerical methods is required as analytical approaches are limited to simple geometries and simplified flow configurations. The linearized Euler equations are solved in the time-domain to model the response of an isolated airfoil interacting with turbulence that is stochastically synthesized and injected in the computational domain through vorticity sources. This new method of injection has the advantages of being easy to implement and parallelize in an existing solver, whilst the generated turbulence is frozen. The method is firstly validated on a 2D free-field configuration. It is then applied, in the framework of the Fan Stage Broadband Noise Benchmarking Programme, to a two-dimensional NACA 65(12)-10 airfoil with no angle of attack and the results are validated through comparisons with experimental data. Afterwards, the effect of the angle of attack is studied and the results suggest that a one-component turbulent model is not satisfactory to perform accurate acoustic predictions with an angle of attack, as it overestimates the rate of decay of the acoustic spectra at high frequencies. The study of the influence of the integral length scale of the turbulence confirms that the airfoil leading edge response is only modulated by the incoming turbulence characteristics. Finally, the acoustic spectra predicted for different velocities show a better agreement with a flat plate analytical model when the velocity is increased.

Numerical Predictions of Turbulence-Cascade Interaction Noise Using CAA with a Stochastic Model

Turbulent flow interactions with the outlet guide vanes are known to be mainly contributing to broadband noise emission of aeroengines at approach conditions. This paper presents a 3D CAA hybrid method aiming at simulating the aeroacoustic response of an annular cascade impacted by a prescribed homogeneous isotropic turbulent flow. It is based on a time-domain Euler solver coupled to a synthetic turbulence model implemented in the code by means of a suited inflow boundary condition proposed by Tam. The fluctuating pressure over the airfoil surface provided by CAA is used as an input to a FWH integral to calculate the radiated sound field. The method is first validated against an academic CAA benchmark in the case of a harmonic gust interacting with an annular flat plate cascade. Then, simulations are applied to turbulence-cascade interactions for annular configurations, in uniform and swirling mean flows, and numerical results in terms of sound power spectra in the outlet duct are compared to semi-analytical and numerical solutions, and to an available experiment

A numerical study of inflow turbulence distortion in the vicinity of blade leading edges

The interaction of the rotor with inflow turbulence is a source of broadband noise, dominant at high wind speeds. In the vicinity of the leading edge of realistic blades, the mean flow distorts the turbulence, resulting in an attenuation of the high-frequency part of the radiated noise compared to zero-thickness blades. In this paper, to study this turbulence distortion by the mean flow, two different numerical approaches are considered. First, the linearized Euler equations are solved in the time-domain using a finite difference code to model the response of an isolated blade interacting with synthetic turbulence. Second, a vorticity approach is applied, using the Biot-Savart law combined with a vortex panel method. Using these approaches on multiple configurations, the up-wash velocity fluctuations along a streamline which goes to the stagnation point, show a decrease of the turbulent levels, from a threshold distance independent of the wavenumber. This decay is then inverted for low frequencies after a...