Denis Ricot

Comparison between lattice Boltzmann method and Navier-Stokes high order schemes for computational aeroacoustics

Computational aeroacoustic (CAA) simulation requires accurate schemes to capture the dynamics of acoustic fluctuations, which are weak compared with aerodynamic ones. In this paper, two kinds of schemes are studied and compared: the classical approach based on high order schemes for Navier-Stokes-like equations and the lattice Boltzmann method. The reference macroscopic equations are the 3D isothermal and compressible Navier-Stokes equations. A Von Neumann analysis of these linearized equations is carried out to obtain exact plane wave solutions. Three physical modes are recovered and the corresponding theoretical dispersion relations are obtained. Then the same analysis is made on the space and time discretization of the Navier-Stokes equations with the classical high order schemes to quantify the influence of both space and time discretization on the exact solutions. Different orders of discretization are considered, with and without a uniform mean flow. Three different lattice Boltzmann models are then presented and studied with the Von Neumann analysis. The theoretical dispersion relations of these models are obtained and the error terms of the model are identified and studied. It is shown that the dispersion error in the lattice Boltzmann models is only due to the space and time discretization and that the continuous discrete velocity Boltzmann equation yield the same exact dispersion as the Navier-Stokes equations. Finally, dispersion and dissipation errors of the different kind of schemes are quantitatively compared. It is found that the lattice Boltzmann method is less dissipative than high order schemes and less dispersive than a second order scheme in space with a 3-step Runge-Kutta scheme in time. The number of floating point operations at a given error level associated with these two kinds of schemes are then compared.

Measured wavenumber: Frequency spectrum associated with acoustic and aerodynamic wall pressure fluctuations

Direct measurements of the wavenumber-frequency spectrum of wall pressure fluctuations beneath a turbulent plane channel flow have been performed in an anechoic wind tunnel. A rotative array has been designed that allows the measurement of a complete map, 63×63 measuring points, of cross-power spectral densities over a large area. An original post-processing has been developed to separate the acoustic and the aerodynamic exciting loadings by transforming space-frequency data into wavenumber-frequency spectra. The acoustic part has also been estimated from a simple Corcos-like model including the contribution of a diffuse sound field. The measured acoustic contribution to the surface pressure fluctuations is 5% of the measured aerodynamic surface pressure fluctuations for a velocity and boundary layer thickness relevant for automotive interior noise applications. This shows that for aerodynamically induced car interior noise, both contributions to the surface pressure fluctuations on car windows have to be taken into account.

Measurements of the wavenumber-frequency spectrum of wall pressure fluctuations under turbulent flows

The prediction of the vibratory response of structures excited by turbulent flows implies a good knowledge of both aerodynamic and acoustic components of the wall pressure fluctuations. In the present work, experiments were aimed at measuring wall pressure fluctuations under turbulent flows, in order to separate the two exciting loadings. The experiments were conducted in the anechoic wind tunnel of Ecole Centrale de Lyon (France). Two configurations were more precisely studied : a turbulent boundary layer and a cylindrical bar in crossflow. A rotative array has been designed that allows the measurement of a complete map of cross-power spectral densities over a large area. A post-processing has been developed to transform the space-frequency data into wavenumber-frequency spectra. Results for the boundary layer are consistent with the Corcos model. Analysis of the spectra shows the presence of an acoustic pressure field, which magnitude is about 5% of the aerodynamic pressure field. Concerning the bar, the whistling frequencies do appear on wavenumber spectra, but not the broadband acoustic field previously observed for the boundary layer. Besides, the strong flow inhomogeneity makes quality of wavenumber spectra worse.

Aeroacoustic simulation of automotive ventilation outlets

In this work we have numerically studied aeroacoustics of automotive ventilation outlets. Simulations are performed with the CFD software PowerFLOW based on Lattice Boltzmann method (LBM). Low dissipative LBM scheme enables to compute aeroacoustic sources generated by turbulence fluctuations and to propagate them in the same simulation. In a first step we validate the ability of LBM for propagating acoustic waves in ducts and radiating them at open end terminations. In a second step, aeroacoustic simulations on automotive vents will be presented and compared with experimental data obtained from a DoE (Design of Experiment). This DoE is based on an idealized outlet with variing parameters (number and lenght of grid blades, grids spacing ...) which gives 18 distinct geometrical configurations. All these configurations have been simulated with PowerFLOW and measured with a new test facility (built in the Renault Research Department). The large number of tested geometries and the statistical analysis of the D...

Accuracy of Lattice Boltzmann Method for Aeroacoustic Simulations

The lattice Boltzmann method is used in fluid mechanics since the end of the 90’s. Recently some papers have been published about LBM used in flow acoustics. Because the LBM scheme is a weakly compressible one, we can access aerodynamics and acoustics information using one simulation. The purpose of this work is to study the behavior of this information through the LBM and the discrete velocity Boltzmann equation (DVBE). A von Neumann analysis leads us to a modal decomposition of the scheme providing the dispersion and dissipation relation for the shear and propagation modes. We show that in the limit of small Knudsen number, the DVBE dispersion relation and dissipative coefficients perfectly match the theoretical expressions found by Chapman-Enskog analysis. On the other hand, time and space discretization imposed by LBM, introduce a variation of the dispersion relation depending on the wavenumber. The above results are perfectly matched to numerical computations obtained with a 3D code based on the D3Q19 velocity model. An analysis is made on the MRT model with a compressible distribution function. We show that this model does not improve the dispersion relation but can modify the dissipation to improve stability.

Characterisations of force and pressure fluctuations of real vehicles

Both the unsteady aerodynamics force and base pressure distributions are investigated for four model cars in real flow conditions. The low pass cutoff frequency of the measurements is about 5 Hz which is sufficient to consider the global fluctuations of drag, lift and side forces. The drag fluctuations are very low, they never exceed 1% of the mean drag. A clear correlation is found with the base pressure distribution fluctuations. It is found that the regions of smallest pressure fluctuations on the vehicle base are the most correlated to the drag fluctuations or in other words, the regions of largest pressure fluctuations on the base are not associated with the drag fluctuations. Sideslip effect is studied, and one of the model presented a clear bistable behavior on the lift creating huge fluctuations as investigated in the academic experiment of Grandemange, Gohlke and Cadot, Physics of Fluids 25 (2013) 095103.

Direct aeroacoustic source identification based on Lattice Boltzmann simulation and beamforming technique

Lattice Boltzmann Method (LBM) is a powerful technique for Computational Fluid Dynamics that is now widely used to simulate flow problems interacting with complex geometries. This numerical scheme that was first developed for aerodynamics application gets properties (unsteadiness, low dissipative and weak compressibility) that makes it very interesting for aeroacoustic and acoustic applications. In this paper, the ability of LBM to capture acoustic waves generated by turbulent flows and propagate them over large distances is presented through a bibliographic review and academic tests on acoustic monopoles. Then, it is shown that a radiated acoustic far field can be analyzed with a classical experimental beamforming technique to localize and estimate the power of emitting sources. At last, this procedure is applied to study the aeroacoustic noise generated by various mirrors on a production car. Numerical results are compared with experimental data measured in an aeroacoustic wind tunnel.

Aeroacoustic analysis of the automotive ventilation outlets using Extended Proper Orthogonal Decomposition

Extended Proper Orthogonal Decomposition based on snapshot POD is used to investigate the correlation between the aerodynamic quantities of interest in an automotive ventilation outlet and the associated sound pressure field. A 3D simulation of the flow and the associated sound field of a real engineering application is obtained thanks to a Direct Noise Calculation. The flow field on a 2D measurement plane is decomposed on three components using the POD decomposition : The large and small scale coherent structures and the background quasi-Gaussian fluctuations. Based on this POD flow partitioning, the far-field acoustic pressure is used to calculate the extended modes. We exhibit that each part of the far-field acoustic pressure spectrum can be related to each POD aerodynamic flow partitioning. I. Introduction Noise is a major concern in modern automotive industry due to the car manufacturers wish to reduce the noise level for passengers. The purpose of this study concerns the analysis of noise generation mechanisms occuring in automotive ventilation outlets. Historically, fifty years ago, Lighthill 1 reformulated the momentum and the mass conservation equations in the form of a wave equation. While this equation provides an analytical expression for the aerodynamic sound generation, the understanding of the underlying physics and the noise production mechanisms still needs further investigations. Experimental studies have been contributing to a better understanding of the governing mechanisms but they are usually hindered by the complexity of the acoustic sources, requiring the measurement of the fluctuating quantities in a highly resolved space-time extent. The recent progresses in the numerical simulations facilitate the access to the reliable data in those zones where the sound generation is concentrated. The flow simulation and its related acoustic field could be simultaneously performed thanks to the Direct Noise Calculation (DNC). The DNC of compressible flows provides a spatio-temporal database for a particular case. This database can be employed to check the theoretical models and to investigate the phenomena involved in the sound production. The

Grid refinement for aeroacoustics in the lattice Boltzmann method: A directional splitting approach

This study focuses on grid refinement techniques for the direct simulation of aeroacoustics, when using weakly compressible lattice Boltzmann models, such as the D3Q19 athermal velocity set. When it comes to direct noise computation, very small errors on the density or pressure field may have great negative consequences. Even strong acoustic density fluctuations have indeed a clearly lower amplitude than the hydrodynamic ones. This work deals with such very weak spurious fluctuations that emerge when a vortical structure crosses a refinement interface, which may contaminate the resulting aeroacoustic field. We show through an extensive literature review that, within the framework described above, this issue has never been addressed before. To tackle this problem, we develop an alternative algorithm and compare its behavior to a classical one, which fits our in-house vertex-centered data structure. Our main idea relies on a directional splitting of the continuous discrete velocity Boltzmann equation, followed by an integration over specific characteristics. This method can be seen as a specific coupling between finite difference and lattice Boltzmann, locally on the interface between the two grids. The method is assessed considering two cases: an acoustic pulse and a convected vortex. We show how very small errors on the density field arise and propagate throughout the domain when a vortical flow crosses the refinement interface. We also show that an increased free stream Mach number (but still within the weakly compressible regime) strongly deteriorates the situation, although the magnitude of the errors may remain negligible for purely aerodynamic studies. A drastically reduced level of error for the near-field spurious noise is obtained with our approach, especially for under-resolved simulations, a situation that is crucial for industrial applications. Thus, the vortex case is proved useful for aeroacoustic validations of any grid refinement algorithm.

Characterisation of the flow past real road vehicles with blunt afterbodies

The flow past two different road vehicles with blunt afterbodies is studied at a Reynolds number based on the vehicle length of 10 7. The boundary layer thickness and the pressure distribution around the body are characterized. Then, the wake is investigated through static pressure and velocity measurements. Similar properties are obtained for both vehicles, in particular the lowest pressure on the after-body is reported on the lower part of the base. Hot-wire anemometry is also used to depict the dynamics of the flow. The detached shear from the roof behaves as free shear turbulent flows whereas the flow from the underbody rather corresponds to homogeneous shear turbulent flows. In addition, global mode dynamics is reported in the wake of one vehicle and is associated with an antisymmetric coupling of the lateral mixing layers. However, the intensity of this mode is limited and it may not be a contributor to the drag since its maximum of amplitude is downstream the recirculation bubble. Eventually, these results are analyzed to orient future drag reduction control strategies.