David Freed

Lattice-Boltzmann Method for Macroscopic Porous Media Modeling

An extension to the basic lattice-BGK algorithm is presented for modeling a simulation region as a porous medium. The method recovers flow through a resistance field with arbitrary values of the resistance tensor components. Corrections to a previous algorithm are identified. Simple validation tests are performed which verify the accuracy of the method, and demonstrate that inertial effects give a deviation from Darcys law for nominal simulation velocities.

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.

Multi-speed thermal lattice Boltzmann method stabilization via equilibrium under-relaxation

The lattice Boltzmann method (LBM) is extended to allow stable and consistent thermal evolution. Two multi-speed schemes are investigated; a three-speed, 34-state system that contains an artifact in the energy evolution equation and a four-speed, 52-state system that removes this artifact. A dual-rate collision operator is used in order to achieve variable Prandtl number (Pr). The schemes are tested on the decay of a sinusoidal thermal perturbation that excites only the purely decaying eigen-mode of this fully coupled system. Results over a range of transport coefficients, velocities, Pr and the allowable temperature range agree with theory for both schemes. However, under most flow conditions, the schemes eventually go unstable, a previously noted problem with multi-speed thermal models. We stabilize the scheme by identifying a temperature-dependent factor in the equilibrium function that leads directly to the removal of the Galilean-invariance artifact, and relax the requirement of instantaneous accuracy of this factor. This results in a stable scheme but introduces artificial thermal diffusion strongly dependent on the bulk velocity. Empirically, we develop a scheme to push this error to higher order. The resultant multi-speed schemes are also used to simulate Couette flow with a temperature gradient and produce results that agree well with theory.

Fundamental Aeroacoustic Capabilities of the Lattice-Boltzmann Method

The Lattice Boltzmann Method (LBM) was used to model four canonical problems in acoustics. The goal was to show that the LBM, which recovers the transient, compressible, and viscous Navier-Stokes equations, allows the accurate capture of time-dependent acoustic phenomena. The first case was that of a planar propagating sound wave. The dispersion of an initial Gaussian pulse in a two-dimensional domain was then investigated. Grid resolution requirement for minimizing numerical dispersion were determined for these two cases. The influence of noise and that of the numerical bulk viscosity was investigated and is discussed. The case of a driven standing-wave tube was then used to investigate the possibility of implementing sound-absorbing boundaries. Finally the case of a Helmholtz resonator was investigated. Results that are consistent with basic acoustic theory were obtained for all cases. The results illustrate the capability of the LBM to model acoustic problems accurately. Since these numerical schemes are already utilized for the modeling of external fluid flows, they are useful for the modeling of strongly coupled fluid-acoustic interactions.

Simulation of Wall Pressure Fluctuations on Simplified Automobile Shapes Using a Lattice Based Method

A comparison of experimental data and CFD simulation results of wall pressure fluctuations on simplified geometries that generate flow structures similar to an automobile are presented. The numerical results have been obtained using the commercial software PowerFLOW 3.4p4a. The simulation kernel of this software is based on the numerical scheme known as the Lattice Boltzmann Method (LBM), combined with an RNG turbulence model. This scheme accurately captures time-dependent aerodynamic behavior of high Reynolds number flows over complex geometries, together with the acoustics. The geometries considered for this study represent the green house and the side mirror of a car. Spectral analysis is performed on the simulation data and the results are compared to the experimental data. This comparison provides good correlation between the simulation and experiment, and demonstrates the capability of this numerical scheme in predicting turbulent fluctuations due to complex flow phenomena.Copyright

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.

NREL wind turbine aerodynamics validation and noise predictions using a Lattice Boltzmann Method

The 3-D design and optimization of wind-turbine blades remains a challenging task for computational fluid simulations both in term of aerodynamics and aeroacoustics. In this paper, flow and noise simulations are performed on a full-scale 2-blade wind-turbine tested at the National Renewable Energy Laboratory. The Lattice Boltzmann Method is used to predict the unsteady flow around the wind-turbine at different operating conditions corresponding to various inflow speeds. From these unsteady calculations, the normal force coefficient, the mean torque and the pressure coefficient at multiple locations on the blades are extracted and compared to experiments. The far-field noise is estimated by coupling the blade and tower wall-pressure fluctuations to a Ffowcs-Williams and Hawking’s equation solver. The analysis of the surface pressure fluctuations in the frequency domain and the turbulent flow in the vicinity of the blade gives an insight on the noise sources responsible for the tonal noise radiation.

Numerical Simulation of Leakage Effects on Sunroof Buffeting of an Idealized Generic Vehicle

Sunroof buffeting is a flow-acoustic resonance phenomenon that causes high interior noise leading to discomfort for the passengers. In order to make a priori predictions about the tendency of a given vehicle to experience buffeting with a high degree of reliability, it is important to understand the sensitivity of this phenomenon to various noise parameters. The current investigation studies the mechanism and the effect of leakage on an SAE Type 4 body. For this, the CAA tool PowerFLOW was used and good agreement of the peak SPL over a wide range of velocities between experiment and simulation was observed for the baseline configurations and the leakage configurations. This allows the analysis and identification of the mechanism for how the artificial leakage affects the buffeting behavior. I. Introduction UNROOF and side-window buffeting in passenger vehicles results in high sound-pressure levels in the interior and can cause considerable discomfort for the passengers. It is well understood that this phenomenon is a result of an unsteady shear layer in the sunroof or window opening which induces an acoustic resonance in the passenger compartment. Pragmatic design solutions for suppressing sunroof buffeting at various wind speeds and geometric configurations are well-known. However, a complete solution to this problem without resorting to expensive design measures has not been achieved in the industry. Recent numerical investigations 9,10 on real cars exhibiting real world effects have successfully predicted the effect of a deflector on buffeting suppression in an SUV. In a validation study 9 , the efficiency of a deflector system was studied on a sedan and an SUV. It was shown conclusively that a sunroof system which works well to mitigate buffeting on one vehicle cannot be assumed to be universally effective. Ref. 16 provided a detailed review of previous experimental and numerical investigations of this problem and concluded that many open questions remained in previous experimental and numerical studies and attempted a systematic investigation of the phenomenon of buffeting in automotive applications. Experiments were carried out on an SAE Type 4 body, a geometrically simple and structurally rigid vehicle model. This removed experimental uncertainties associated with geometric details and structural and acoustic properties of real vehicles. Two different wind tunnels were used to measure the acoustic response of the body at various wind speeds, in order to address wind tunnel affects. Ref. 15 which covered only the experimental portion of the study also investigated the effect of leakage on the overall buffeting behavior at various wind speeds. The leakage was represented by a round hole of 200 mm in diameter in the rear wall of the SAE Body. This study validates the buffeting behavior of the leakage configuration using numerical predictions over the relevant range of velocities.