The Lattice-Boltzmann method (LBM) is extended to allow incorporation of traditional turbulence models. Implementation of a two-layer mixing-length algebraic model and two versions of the k-e two-e...

Incorporating Turbulence Models Into The Lattice-Boltzmann Method

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.

Properties of the Lattice-Boltzmann Method for Acoustics

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.

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

Computational results for an 18%-scale, semi-span Gulfstream aircraft model are presented. Exa Corporation's lattice Boltzmann PowerFLOW(trademark) solver was used to perform time-dependent simulations of the flow field associated with this high-fidelity aircraft model. The simulations were obtained for free-air at a Mach number of 0.2 with the flap deflected at 39 deg (landing configuration). We focused on accurately predicting the prominent noise sources at the flap tips and main landing gear for the two baseline configurations, namely, landing flap setting without and with gear deployed. Capitalizing on the inherently transient nature of the lattice Boltzmann formulation, the complex time-dependent flow features associated with the flap were resolved very accurately and efficiently. To properly simulate the noise sources over a broad frequency range, the tailored grid was very dense near the flap inboard and outboard tips. Extensive comparison of the computed time-averaged and unsteady surface pressures with wind tunnel measurements showed excellent agreement for the global aerodynamic characteristics and the local flow field at the flap inboard and outboard tips and the main landing gear. In particular, the computed fluctuating surface pressure field for the flap agreed well with the measurements in both amplitude and frequency content, indicating that the prominent airframe noise sources at the tips were captured successfully. Gear-flap interaction effects were remarkably well predicted and were shown to affect only the inboard flap tip, altering the steady and unsteady pressure fields in that region. The simulated farfield noise spectra for both baseline configurations, obtained using a Ffowcs-Williams and Hawkings acoustic analogy approach, were shown to be in close agreement with measured values.

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Towards Full Aircraft Airframe Noise Prediction: Lattice Boltzmann Simulations

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.

Flow and noise predictions for the tandem cylinder aeroacoustic benchmarka)

Computational Aeroacoustics Validation and Analysis of a Nose Landing Gear

A hybrid approach to predict the far-field noise generated by an airplane nose landing gear is presented in this paper. The approach consists of a Lattice-Boltzmann Method (LBM) for the calculation of the flow-field around the fully detailed geometry of the landing gear which provides the input for a Ffowcs Williams – Hawkings (FW-H) solver to calculate the far-field noise. Both parts have been validated independently. The method is applied to the nose landing gear of a Gulfstream G550 business jet, for comparison with experimental results obtained in the University of Florida UFAFF wind tunnel. The near-field flow simulation using the LBM method showed good correlation with the PIV measurements of the flow field as well as surface microphone measurements, up to frequencies of about 4kHz. The comparison to experimental far-field results shows good agreement in the midfrequency range of 1-3kHz. At both low and high frequencies the simulations underpredict the measured results more strongly than the near field and surface measurements would suggest, which may be due to experimental limitations. A comparison between the solid and porous formulation of the FW-H solver shows that both method provide nearly equivalent results. However, inclusion of additional surfaces such as part of the fuselage is critical to achieving good results with the solid formulation. The effects of resolution of the near-field simulations are also investigated and show the expected lower cut-off frequency for lower resolutions for both the near-field and the far-field. No differences between the cases with different resolutions are observed up to 1 kHz in the near-field and up 2-3kHz in the farfield.

Patrick Dethioux

Papers

Computational Aeroacoustics Validation and Analysis of a Nose Landing Gear

A Hybrid Lattice-Boltzmann/FW-H Method to Predict Sources and Propagation of Landing Gear Noise

CFD Validation Study for a Sedan Scale Model in an Open Jet Wind Tunnel