Franck Perot

Properties of the Lattice-Boltzmann Method for Acoustics

Numerical simulations are performed to investigate the fundamental acoustics properties of the Lattice–Boltzmann method. The propagation of planar acoustic waves is studied to determine the resolution dependence of numerical dissipation and dispersion. The two setups considered correspond to the temporal decay of a standing plane wave in a periodic domain, and the spatial decay of a propagating planar acoustic pulse of Gaussian shape. Theoretical dispersion relations are obtained from the corresponding temporal and spatial analyses of the linearized Navier–Stokes equations. Comparison of theoretical and numerical predictions show good agreement and demonstrate the low dispersive and dissipative capabilities the Lattice–Boltzmann method. The analysis is performed with and without turbulence modeling, and the changes in dissipation and dispersion are discussed. Overall, the results show that the Lattice–Boltzmann method can accurately reproduce time-explicit acoustic phenomena.

A Ffowcs Williams-Hawkings Solver for Lattice-Boltzmann based Computational Aeroacoustics

This paper presents the development of an efficient far-field noiseprediction code using the near-field results from a Lattice-Boltzmann flow solver as input to an acoustic analogy solver. Two formulations, based on the Ffowcs Williams–Hawkings equation, are implemented to efficiently perform far-field prediction from large input data sets. For configuration where the noise source is moving through a fluid at rest (such as aircraft certification), the efficient and well-validated formulation 1A i s implemented. For windtunnel configurations where both the source and observer are stationary in a uniform flow, a formulation based on the Garrick Triangle, and referred to as GT, is used to increase the computational efficiency. Numerical simulations and far-field prediction are performed for three representative validation cases: a three-dimensional monopole source, a tandem cylinder flow, and a fan noise case. Comparisons of the results from the far-field solver show excellent agreement with the theoretical predictions and the available experimental data.

Flow and noise predictions for the tandem cylinder aeroacoustic benchmarka)

Flow and noise predictions for the tandem cylinder benchmark are performed using lattice Boltzmann and Ffowcs Williams–Hawkings methods. The numerical results are compared to experimental measurements from the Basic Aerodynamic Research Tunnel and Quiet Flow Facility (QFF) at NASA Langley Research Center. The present study focuses on two configurations: the first configuration corresponds to the typical setup with uniform inflow and spanwise periodic boundary condition. To investigate installation effects, the second configuration matches the QFF setup and geometry, including the rectangular open jet nozzle, and the two vertical side plates mounted in the span to support the test models. For both simulations, the full span of 16 cylinder diameters is simulated, matching the experimental dimensions. Overall, good agreement is obtained with the experimental surface data, flow field, and radiated noise measurements. In particular, the presence of the side plates significantly reduces the excessive spanwise coherence observed with periodic boundary conditions and improves the predictions of the tonal peak amplitude in the far-field noise spectra. Inclusion of the contributions from the side plates in the calculation of the radiated noise shows an overall increase in the predicted spectra and directivity, leading to a better match with the experimental measurements. The measured increase is about 1 to 2 dB at the main shedding frequency and harmonics, and is likely caused by reflections on the spanwise side plates. The broadband levels are also slightly higher by about 2 to 3 dB, likely due to the shear layers from the nozzle exit impacting the side plates.

Investigation of the Noise Generated by Cylinder Flows Using a Direct Lattice-Boltzmann Approach

During the last decade, the number of aeroacoustic simulations increased with the improvement of computational capabilities. Computational AeroAcoustics (CAA) and more particularly Direct Numerical Acoustics (DNA) became a powerful investigation tool and help toward the understanding of complex mechanisms related to flow induced noise. Flow and noise can be simulated at the same time and precisely studied. In this paper, results related to low Reynolds and Mach number flows in the range Re=[40,200] and M=[0.1,0.3] over a circular cylinder are presented. The numerical method used is based on a Lattice Boltzmann approach which is an alternative to the traditional Navier Stokes equations resolution. The goal of the present study is in one hand to propose an in depth validation of the numerical approach and in the other hand to study the effect of external parameters on the flow and on the acoustics radiation. The flow and the acoustics radiation are compared to results from the literature. Mathematical and numerical developments are presented to point out the effect of the viscosity on the pressure coefficient at the stagnation point for low Reynolds number flows. The effects of the Reynolds and Mach number on both the flow and the aeroacoustic radiation are presented at the end of the paper.

Advanced Noise Control Fan Direct Aeroacoustics Predictions using a Lattice-Boltzmann Method

A Lattice-Boltzmann Method (LBM) based approach is used to perform transient, explicit and compressible CFD/CAA simulations on the Advanced Noise Control Fan (ANCF) configuration. The complete 3-D ducted rotor/stator model including all the geometrical details and the truly rotating rotor is simulated. Detailed near and far-field measurements conducted at the NASA Glenn research center are used to validate the simulation results. The measured and predicted sound pressure levels at the far-field microphones are compared and both show the presence of broadband noise and sharp peaks which frequencies depend on the number of rotor blades and the angular velocity of the rotor. The 3-D duct acoustics modes observed in experiments are also captured in the 3-D transient CFD/CAA calculation and detailed analyses of the results are presented. The main circumferential modes predicted from the number of rotor blades and stator vanes are recovered in both experimental and simulation modal decompositions.

Investigation of the statistical properties of pressure loadings on real automotive side glasses

In modern vehicle, aerodynamic noise is now the major source of annoyance at highway speeds and for frequencies higher than 400 Hz. The origin of the aerodynamic noise in the cabin is the flow around the vehicle particularly in the neighborhood of the side glass. A complex and unsteady pressure field excites the glass panels which vibrates and radiates noise inside the cabin. The development of numerical methods related to wind noise prediction is closely associated to the understanding of the pressure field properties. Some quantities like local fluctuating pressure can be measured but other quantities like two points correlation are much more complicated to obtain. In this paper are presented one point pressure measurements performed on the side glass of a Sedan vehicle at two yaw angles. The experimental method is described and the results are analyzed and compared to unsteady CFD simulations. This study depicts general properties of the side glass loading and validates the numerical approach. The numerical results are then used to study two point statistics and are compared to semi-empirical models of the literature. The strong inhomogeneous and spatial dependency of the pressure field is underlined and detailed.

Towards Lattice-Boltzmann Prediction of Turbofan Engine Noise

The goal of the present paper is to report verification and validation studies carried out by Exa Corporation in the framework of turbofan engine noise prediction through the hybrid Lattice-Boltzmann/Ffowcs-Williams & Hawkings approach (LB)-(FW-H). The underlying noise generation and propagation mechanisms related to the jet flow field and the fan are addressed separately by considering a series of elementary numerical experiments. As far as fan and jet noise generation is concerned, validation studies are performed by comparing the LB solutions with literature experimental data, whereas, for the fan noise transmission through and radiation from the engine intake and bypass ducts, LB solutions are compared with finite element solutions of convected wave equations. In particular, for the fan noise propagation, specific verification analyses are carried out by considering tonal spinning duct modes in the presence of a liner, which is modelled as an equivalent acoustic porous medium. Finally, a capability overview is presented for a comprehensive turbofan engine noise prediction, by performing LB simulation for a generic but realistic turbofan engine configuration.

Direct self-noise simulation of the installed Controlled Diffusion airfoil

Broadband noise produced by the trailing-edge of a controlled di usion (CD) airfoil is directly simulated using a Lattice-Boltzmann method (PowerFlow) that resolves both the aerodynamic and acoustic eld around the airfoil. The proper DNS resolution is rst achieved in the vicinity of the airfoil on a quasi-2D slice of the mock-up. It is then extended to a 3D slice with a span of 12 % chord length. Two numerical setups of the anechoic openjet facility where both aerodynamic and acoustic data have been collected are investigated to capture the installation e ects: in a rst numerical setup (called free), the CD airfoil is set in an uniform ow, while in the second setup (lips) the real jet nozzle geometry is considered. While in the freeeld con guration the boundary layer rapidly detaches on the suction side, in the lips the jet shear layers modify the pressure load on the airfoil and the boundary layer keeps attached in the con guration with nozzle. In both setups a laminar recirculation bubble is captured on the suction side near the leading edge which triggered the development of the boundary layers along the suction side. The wall-pressure and noise spectra for the free con guration are spread over a large frequency band and agree with similar measurements at higher angle of attack for which the ow is detached. The spectra for the lips con guration better agree with the experimental data and has been selected for the 3D simulation. The vortex stretching along the span-wise direction that was missing in the previous investigated set-ups, allows to nely capture the turbulence length scales and accurately reproduce the experimental measurements. Both the boundary layer pro le and the wall pressure spectra near the trailing edge are nicely and accurately captured. The predicted fareld sound pressure levels also provide satisfactory agreement with noise measurements in the anechoic wind tunnel.

Characterization of Acoustic Liners Absorption using a Lattice-Boltzmann Method

The impedance of acoustic liners is predicted using lattice-Boltzmann fluid dynamics simulations. The complete three-dimensional geometry of liners, which corresponds to the combination of micro-perforated sheets, honeycomb cavities and porous materials are directly characterized through a numerical setup that reproduces a Kundts tube and realistic liner samples. In a first step, a mesh resolution study is performed for a One Degree of Freedom (1-DOF) liner in order to show the convergence of the deducted value of the acoustic impedance with the grid size. In a second step, the influence of all the geometric parameters of the liner is predicted for 1-DOF, Two Degree of Freedom (2-DOF) and Bulk Absorber (BA) liners. The predicted impedance is compared with two analytical impedance models available in the literature.

Acoustic absorption of porous materials using LBM

Porous materials are commonly used in various industrial systems such as ducts, HVAC, hood compartments, mufflers and acoustic liners in order to introduce acoustic absorption and to reduce radiated acoustics levels. For problems involving flow-induced noise mechanisms and explicit interactions between turbulent source regions, numerical approaches remained a challenging task involving on one hand the coupling between unsteady flow calculations and acoustics simulations and on the other hand the development of advanced and sensitive numerical schemes. In this paper, acoustic materials are explicitly modeled in Lattice Boltzmann simulations using equivalent fluid regions having porosity equal to one. Numerical simulations are compared to analytical derivations to validate the approach. In order to propose realistic acoustic models of more complex materials, simulations are compared to experiments and semi-empirical models.