Phased microphone arrays have become a well-established tool for performing aeroacoustic measurements in wind tunnels (both open-jet and closed-section), flying aircraft, and engine test beds. This paper provides a review of the most well-known and state-of-the-art acoustic imaging methods and recommendations on when to use them. Several exemplary results showing the performance of most methods in aeroacoustic applications are included. This manuscript provides a general introduction to aeroacoustic measurements for non-experienced microphone-array users as well as a broad overview for general aeroacoustic experts.

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A review of acoustic imaging methods using phased microphone arrays

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

Aircraft noise prediction

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.

/pdf/towards-full-aircraft-airframe-noise-prediction-lattice-2oc2hsc2kt.pdf

Towards Full Aircraft Airframe Noise Prediction: Lattice Boltzmann Simulations

A series of fluid dynamic and aeroacoustic wind tunnel experiments are performed at the University of Florida Aeroacoustic Flow Facility and the NASA-Langley Basic Aerodynamic Research Tunnel Facility on a high-fidelity -scale model of Gulfstream G550 aircraft nose gear. The primary objectives of this study are to obtain a comprehensive aeroacoustic dataset for a nose landing gear and to provide a clearer understanding of landing gear contributions to overall airframe noise of commercial aircraft during landing configurations. Data measurement and analysis consist of mean and fluctuating model surface pressure, noise source localization maps using a large-aperture microphone directional array, and the determination of far field noise level spectra using a linear array of free field microphones. A total of 24 test runs are performed, consisting of four model assembly configurations, each of which is subjected to three test section speeds, in two different test section orientations. The different model assembly configurations vary in complexity from a fully-dressed to a partially-dressed geometry. The two model orientations provide flyover and sideline views from the perspective of a phased acoustic array for noise source localization via beamforming. Results show that the torque arm section of the model exhibits the highest rms pressures for all model configurations, which is also evidenced in the sideline view noise source maps for the partially-dressed model geometries. Analysis of acoustic spectra data from the linear array microphones shows a slight decrease in sound pressure levels at mid to high frequencies for the partially-dressed cavity open model configuration. In addition, far field sound pressure level spectra scale approximately with the 6th power of velocity and do not exhibit traditional Strouhal number scaling behavior.

/pdf/a-comparative-study-of-a-1-4-scale-gulfstream-g550-aircraft-nvkzwuoqc2.pdf

A Comparative Study of a 1/4-Scale Gulfstream G550 Aircraft Nose Gear Model

The NASA-Gulfstream joint Airframe Noise Flight Test program was conducted at the NASA Wallops Flight Facility during October, 2006. The primary objective of the AFN flight test was to acquire baseline airframe noise data on a regional jet class of transport in order to determine noise source strengths and distributions for model validation. To accomplish this task, two measuring systems were used: a ground-based microphone array and individual microphones. Acoustic data for a Gulfstream G550 aircraft were acquired over the course of ten days. Over twenty-four test conditions were flown. The test matrix was designed to provide an acoustic characterization of both the full aircraft and individual airframe components and included cruise to landing configurations. Noise sources were isolated by selectively deploying individual components (flaps, main landing gear, nose gear, spoilers, etc.) and altering the airspeed, glide path, and engine settings. The AFN flight test program confirmed that the airframe is a major contributor to the noise from regional jets during landing operations. Sound pressure levels from the individual microphones on the ground revealed the flap system to be the dominant airframe noise source for the G550 aircraft. The corresponding array beamform maps showed that most of the radiated sound from the flaps originates from the side edges. Using velocity to the sixth power and Strouhal scaling of the sound pressure spectra obtained at different speeds failed to collapse the data into a single spectrum. The best data collapse was obtained when the frequencies were left unscaled.

https://ntrs.nasa.gov/api/citations/20080018649/downloads/20080018649.pdf

Preliminary analysis of acoustic measurements from the nasa-gulfstream airframe noise flight test

Computational Aeroacoustics Validation and Analysis of a Nose Landing Gear

This paper describes a numerical procedure for the prediction of aircraft noise certi cation metrics starting from the aircraft trajectory. The procedure is applied to the nose landing gear of a Gulfstream business jet. The numerical core of the procedure is a hybrid aeroacoustic method based on a lattice Boltzmann ow simulation and a Ffowcs-Williams & Hawkings noise propagation computation. The hybrid method is initially validated by computing the noise generated by a geometrically simpli ed model of the same landing gear installed on a at plate and comparing wall pressure and fareld noise spectra with wind-tunnel measurements. The same numerical method and a similar discretized model are then employed to compute the unsteady ow eld past the real landing gear deployed under the aircraft. The upstream ow conditions are the same occurring during a ight along a \nominally" constant descent trajectory at the nearest point to the ground microphone. Comparisons between the predicted noise levels and the measured ones during a ight test, with only the nose landing gear deployed and other airframe and engine sources kept at their admissible minimum, are in good agreement.

Towards Numerical Aircraft Noise Certification: Analysis of a Full-Scale Landing Gear in Fly-Over Configuration

In conjunction with the NASA-Gulfstream and Gulfstream-University of Florida wind tunnel testing of a generic nose landing gear model, Computational Aero-Acoustics research was conducted at Gulfstream using the commercial Navier-Stokes Finite Volume CFD solver STAR-CCM+. The simulation modeled the ¼ scale G550 nose landing gear mounted in the NASA Basic Aerodynamics Research Tunnel (BART) wind tunnel test section. The model was run at the 0.166 Mach Number condition on the simplified nose landing gear with the gear cavity closed. Prior to simulation execution, dynamic pressure probes were inserted onto the surface of the model at the exact locations of the dynamic pressure transducers on the nose landing gear wind tunnel model. Delayed Detached Eddy Simulation (DDES) (1) was run with a 58 million cell unstructured grid on 47 3.0 GHz quad-core processors for approximately two weeks to obtain a total of 0.05 seconds of statistically limit-cycled pressure data was processed. Results of the simulation were compared to data from the NASA BART and University of Florida anechoic wind tunnel results at both surface mounted steady and unsteady pressure transducers. The simulation showed good agreement in comparisons of power spectral density up to 5 kHz at mesh/time step dependent frequencies at all locations captured. In addition, the simulation was in somewhat good agreement with the measured mean turbulence levels at the wheel hub height and good agreement at static pressure taps located on the starboard wheel.

Thomas Van de Ven

Papers

A Comparative Study of a 1/4-Scale Gulfstream G550 Aircraft Nose Gear Model

Preliminary analysis of acoustic measurements from the nasa-gulfstream airframe noise flight test

Computational Aeroacoustics Validation and Analysis of a Nose Landing Gear

Towards Numerical Aircraft Noise Certification: Analysis of a Full-Scale Landing Gear in Fly-Over Configuration

Computational aeroacoustic analysis of a ¼ scale g550 nose landing gear and comparison to nasa & ufl wind tunnel data