Damiano Casalino

An advanced time approach for acoustic analogy predictions

Abstract This paper deals with a new interpretation of the retarded time approach that is widely used in the prediction of acoustic fields from moving sources. A hierarchical inversion between the emission time and the reception time leads to advanced time approach. This consists in projecting the current status of a source in the observer time domain where the received signal is progressively built. The practical relevance of this methodology lies on two statements: no retarded time equations must be solved; an aerodynamic noise prediction can be processed parallelly to the aerodynamic computation. Theoretically, the advanced time approach differs from the retarded time approach only in one aspect. A signal emitted at a given instant by a point source, moving at subsonic as well as supersonic velocity, is received only one time by an observer moving at subsonic velocity. Consequently, only one value of the advanced time corresponds to a value of the emission time. The advanced time approach is herein applied to a retarded time solution of the Ffowcs Williams and Hawkings equation proposed by Farassat. The noise radiated by elementary acoustic sources in complex motion is then computed and checked against analytical solutions.

Prediction of Rod-Airfoil Interaction Noise Using the Ffowcs-Williams-Hawkings Analogy

SoundgeneratedatlowMachnumberbyanairfoilin thewakeofarodisinvestigatednumerically.TheGaussian spanwise loss of coherence of the vortex shedding is shown to have a signie cant ine uence on the broadband noise. Spanwise effects are successfully introduced into a time-domain formulation of the Ffowcs-Williams ‐Hawkings analogy, which is applied to aerodynamicdata computed on various contoursaround the sourceregion. Itisshown that a careful choice of these contours is required. The e owe eld is obtained from a two-dimensional Reynolds averaged Navier ‐Stokes calculation. Computed far-e eld spectra compare very well to measurements obtained in an accompanying experiment. Nomenclature Cp = pressure coefe cient c = airfoil chord d = rod diameter f = frequency g = integration surface, 0 k = turbulent kinetic energy Lg = Gaussian correlation length l = span length Mi = Mach number of g D0 Moi = observer Mach number vector Mref = reference Mach number M1 = ine ow Mach number

Towards Full Aircraft Airframe Noise Prediction: Lattice Boltzmann Simulations

Computational results for an 18%-scale, semi-span Gulfstream aircraft model are presented. Exa Corporation’s lattice Boltzmann PowerFLOW® 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 39o (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.

Lattice–Boltzmann Aeroacoustic Analysis of the LAGOON Landing-Gear Configuration

The unsteady flowfield about the ONERA–The French Aerospace Lab/Airbus SAS LAGOON two-wheel landing-gear configuration and the associated aerodynamic noise generation are computed using a hybrid approach in which the flowfield is provided by a lattice–Boltzmann simulation, and the noise radiation is computed using the Ffowcs-Williams–Hawkings analogy. A detailed validation study is carried out, following the guidelines of the second AIAA workshop on Benchmark Problems for Airframe Noise Computations and using the complete experimental database for detailed comparisons. The effect of grid resolution on both near- and far-field results is investigated, showing the physical consistency of the numerical model. In addition, an assessment of the numerical prediction is carried out by computing the maximum perceived noise level along a nominal approach trajectory. Finally, an unsteady flow mechanism involving the onset of cavity modes in the two facing rim cavities is analyzed in detail and correlated with the g...

Prediction of Sound Propagation in Ducted Potential Flows Using Green's Function Discretization

The sound propagation in ducted mean potential flows is computed by using a Greens function discretization (GFD) technique. Linear combinations of the free-space Greens functions of the locally uniform convected Helmholtz problem are analytically differentiated to build shape functions for the derivatives of the acoustic potential. These are used to discretize both the field governing equation and the boundary conditions. The GFD approach is validated by computing the sound propagation in annular ducts with hard/soft walls and uniform flow. Acoustic modes of increasing wave number are computed without changing the computational mesh. A good level of accuracy is ensured up to three points per wavelength. As a first step toward relevant applications, the propagation in nonconstant annular ducts, with/without wall treatment and with/without flow, is computed. The numerical solutions compare favorably with the well-known analytical multiscale solutions.

Turbofan Aft Noise Predictions Based on Lilley's Wave Model

To model the sound propagation through the discontinuous flow in a turbofan bypass configuration, a homogeneous linearized Lilleys equation is discretized in the frequency domain by using the Greens function discretization scheme. The main advantage of this third-order wave model, with respect to a conventional second-order model, is that it describes the refraction of an acoustic pressure field without restrictions on the ratio between the acoustic wavelength and the spatial nonuniformity scale of the mean flow. The resulting numerical model can be also applied in the vortex-sheet limit without any treatment of the wake shed from a trailing edge under sound excitation. Three-dimensional numerical solutions of the sound radiation from a bypass duct configuration are compared with analytical results available in the literature. Different cut-on duct modes are considered in the presence of different flow conditions, also including a developing mixing layer instead of an infinitesimal vortex sheet. The influence of an acoustically treated centerbody is also evaluated, as well as the noise scattering effects due to the presence of rigid splices on the afterbody. Additional key aspects of the present study are the use of a cylindrical formulation of the perfectly-matched-layer far-field condition, and the use of an iterative technique to solve the linear system without any observed occurrence of instability waves.

Green's Function Discretization Scheme for Sound Propagation in Nonuniform Flows

A new frequency-domain discretization technique that permits the numerical solution of the equation governing the propagation of small disturbances within nonuniform potential flows is described. The method can be applied successfully to the numerical solution of aeroacoustic problems because a good accuracy is preserved up to 3-4 points per period for three-dimensional unstructured meshes. The discretization scheme is based on a local interpolation formula that is strictly joined to the physics of wave propagation because It is constructed with the superimposition of elementary sources that are local solutions of the local convective wave equation. The method presents aspects in common with both finite difference and finite element methods, with the peculiarity that the local interpolation formula can be interpreted as a specific shape function introduced to take advantage of the physics of the problem. The method is applied to the potential equation linearized around an arbitrary aerodynamic mean flow, and several comparisons with theoretical results, as well as several convergence tests, are conducted to show that a good accuracy is preserved up to 3-4 points per period also for irregular meshes with both random and Systematic distortions. Numerical calculations are presented for three-dimensional problems with both uniform and nonuniform flow, and comparisons are made with theoretical and other available numerical results. With the appropriate generalizations, the method can be regarded as a new, efficient general approach for the discretization of generic partial differential equations.

Evaluation of Airframe Noise Reduction Concepts via Simulations Using a Lattice Boltzmann Approach

Unsteady computations are presented for a high-fidelity, 18% scale, semi-span Gulfstream aircraft model in landing configuration, i.e. flap deflected at 39 degree and main landing gear deployed. The simulations employ the lattice Boltzmann solver PowerFLOW to simultaneously capture the flow physics and acoustics in the near field. Sound propagation to the far field is obtained using a Ffowcs Williams and Hawkings acoustic analogy approach. In addition to the baseline geometry, which was presented previously, various noise reduction concepts for the flap and main landing gear are simulated. In particular, care is taken to fully resolve the complex geometrical details associated with these concepts in order to capture the resulting intricate local flow field thus enabling accurate prediction of their acoustic behavior. To determine aeroacoustic performance, the farfield noise predicted with the concepts applied is compared to high-fidelity simulations of the untreated baseline configurations. To assess the accuracy of the computed results, the aerodynamic and aeroacoustic impact of the noise reduction concepts is evaluated numerically and compared to experimental results for the same model. The trends and effectiveness of the simulated noise reduction concepts compare well with measured values and demonstrate that the computational approach is capable of capturing the primary effects of the acoustic treatment on a full aircraft model.

Tonal and Broadband Noise Calculations for Aeroacoustic Optimization of a Pusher Propeller

A multidisciplinary analysis and optimization is carried out for a propeller in a real pusher aircraft configuration with the goal of reducing the radiated noise power levels, while preserving the aerodynamic efficiency. The optimization process involves the shape of the blade and the position of the engine exhaust ducts. A coupling of the unsteady aerodynamic and structural-dynamic blade models provides the aeroelastic propeller model that drives a tonal and broadband aeroacoustic prediction. The tonal noise results from the periodic flow unsteadiness due to the nonaxial flight and to the impingement of the engine exhausts on the propeller disk. The broadband noise is mainly due to the interaction between the blade leading edge and the exhaust turbulence. It is shown that the tonal noise overwhelmsthebroadbandnoiseandthattheoptimizationaffectstheshapeofthebladeatthetipandinthespanwise segment hit by the exhausts. An overall sound pressure level reduction of 3.5 dB is achieved at the takeoff condition, while preserving the design propeller thrust and resulting in a small penalty on the propeller efficiency in cruise.

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.