Aeroacoustic Interaction Research Articles

Modeling aeroacoustic interaction of rotating propeller with porous treatments using LBM (Lattice Boltzmann method) simulations

Porous treatments are more and more employed in presence of flow as passive noise control means to mitigate the generated noise. They are generally designed according to their acoustic dissipation properties under no-flow conditions. Nevertheless, in-flow porous treatments can alter the aerodynamic properties, modify the main acoustic source itself or even generate secondary acoustic sources. While most of the modelling methods (analytical, FEM, ...) are able to model the acoustic dissipation, they do not allow to model the interaction with the main source or the possible generation of spurious acoustic sources. The Lattice Boltzmann Method (LBM) is a computational fluid dynamics method able to run Direct Numerical Simulations (DNS) as well as Large Eddy Simulations (LES). The LES method resolves the macroscopic scale of turbulence and uses a sub-grid model for the small scales of turbulence. The low numerical dissipation of the LBM enables to compute both fluid flow and acoustics quantities in a single run. In this work, the LBM implemented in ProLB software is used to model the airborne noise generated by an electric propeller installed on an aircraft wing and the effects of sound absorbing treatments. Various treatments are investigated numerically and compared to experimental data when available.

Aeroacoustic Effects on the Forcing of Fan and Compressor Blades Due to Distortion

Abstract A distorted air stream entering an aeroengine fan or compressor leads to harmonic forces on the rotating blades. These aerodynamically induced forces are well-known causes of blade vibration and associated fatigue problems. Significant levels of distortion arise from different sources like side wind and high angles of attack that occur at specific operating conditions. For aircraft with the engine closely integrated with the fuselage, the engine will be exposed to distortion during the entire flight cycle. With a focus on understanding the aeroacoustic interaction the computational fluid dynamics (CFD) analyses used here consider harmonics of the distortion. Harmonic responses are calculated from low to transonic speeds for a range of cases. Major phenomena and driving parameters affecting the forcing strength and pressure amplitudes in the blade passage are identified from the analyses. It is demonstrated that the forcing strength is strongly affected by the cut-on/cutoff conditions upstream and downstream of the blades. Also, depending on design parameters of the blade, the aeroacoustics of the blade passage is important for the resulting forcing. All analyses are made in two-dimensional over a wide range of flow conditions as well as geometric variations. The results of the study provide an increased understanding of the harmonic forcing of blades. A simple model is proposed that can identify conditions where increased pressure amplitudes in the blade passage may be expected. The sensitivities to parameters may also give some guidance in how design and operation can be adapted to reduce the aerodynamic forcing.

Numerical analyses of aeroacoustic characteristics of tiltrotor considering the aerodynamic interaction by the fuselage in hover

Numerical investigations are performed to investigate the interaction effect between the rotor and fuselage on the aerodynamic and aeroacoustic characteristics of the XV-15 tiltrotor during hovering flight. A hybrid method, which combines a Computational Fluid Dynamics (CFD) solver and an aeroacoustics simulation code, has been developed in order to accurately capture the complex interaction flowfield and acoustic pressure of the tiltrotor. To simulate the interaction of the fuselage on the aeroacoustic source of the rotor, a CFD solver based on a sliding mesh algorithm and unsteady Reynolds-Averaged Navier-Stokes equations is utilized. Additionally, to enhance the accuracy of the interaction simulation, the Lower-Upper Symmetric Gauss-Seidel (LU-SGS) method is adopted. A specialized code, based on the Ffowcs Williams-Hawkings (FW-H) equations and propagation time, is developed to simulate the rotor noise. Then, the numerical investigation on the aerodynamic and aeroacoustic interaction between a full-scale tiltrotor and fuselage (including the wing) is conducted. In addition, the study explores various tiltrotor configurations to analyze mechanisms for suppressing interaction noise. The results indicate that the aerodynamic loading of tiltrotor fluctuates severely interacted with the fuselage. This, in turn, causes an increase in noise by approximately 5 dB. What's more, decreasing the fountain effect would have a positive effect on reducing the range of variation in loading, and it could ultimately suppress the interaction noise.

Linear investigation of sound-flow interaction along a corrugated plate

The experimental investigation of the aeroacoustic behavior of a corrugated plate with small cavities in presence of a grazing flow is discussed. In particular, the linear regime of the aeroacoustic interaction has been investigated. The obtained Strouhal number corresponds to those used in the recent literature for nonlinear investigations. Even if acoustic feedback was eliminated in the experimental setup, loss and gain frequency bands can still be identified. Then, the acoustic and Laser Doppler Velocimetry (LDV) measurements were used to analyze the acoustic power at four different frequencies: the four components that compose the acoustic power have been characterized and their relative importance identified. Finally, the difference behind gain and loss mechanisms are spotted and explained.

Reduced-order modelling of thermoacoustic instabilities in can-annular combustors

Thermoacoustic instabilities in stationary gas turbines may cause high-amplitude limit cycles, leading to damaged components and costly down-time. To better understand the physical origin of such instabilities in a can-annular combustor configuration, we study the properties of the spectrum of a reduced-order can-annular thermoacoustic system. Increased focus is placed on representing the aeroacoustic interaction between the longitudinal eigenmodes of the individual cans with physically relevant models. To represent the acoustic pressure dynamics in the combustor, we combine an analytical, experimentally validated model for the can-to-can impedance with a frequency-dependent model of the flame response in the cans to acoustic perturbations. By using this approach, we perform a parametric study of the linear stability of an atmospheric can-annular thermoacoustic system, and emphasize general features of the structure and properties of the eigenvalues and the eigenvectors of can-annular combustors. Lastly, we emphasize the differences in the can-to-can coupling that arise when considering open-end boundary conditions – representative of atmospheric set-ups – or closed-end boundary conditions – representative of real gas turbine combustors.

Aeroacoustic interaction between owl-inspired trailing-edge fringes and leading-edge serrations

The silent flight achieved by owls is attributed to their unique wing morphologies, characterized by leading-edge (LE) serrations, trailing-edge (TE) fringes, and a velvet-like surface. The specific morphological effects of LE serrations and TE fringes on aeroacoustic performance have been widely studied, but the LE–TE aeroacoustic interaction remains poorly understood. This paper describes a simulation-based study of the aeroacoustic characteristics of owl-inspired TE fringes and their interplay with LE serrations by combining large-eddy simulations of unsteady near-field flow structures with the Ffowcs Williams–Hawkings equation for sound radiation. Using owl-inspired LE serrated and TE fringed wing models, it is verified that TE fringes enable a pronounced high-frequency sound reduction at angles of attack (AoAs) of 5–15° while achieving comparable aerodynamic performance to a clean model. The near-field vortex dynamics, pressure distributions, and velocity spectra reveal that TE fringes suppress flow separation and vortex shedding in the vicinity of the TE, consequently reducing local velocity fluctuations and far-field overall sound pressure levels. Furthermore, the combination of TE fringes and LE serrations enables a remarkable reduction in overall sound pressure levels at all AoAs, and their aeroacoustic interplay is responsible for stabilizing velocity fluctuations over the suction surface, which suppress both low- and high-frequency sound. Our results demonstrate that TE fringes are a robust sound reduction device in resolving the trade-off between aerodynamic force production and sound reduction, while LE serrations and TE fringes complement one another as an effective noise-reducing biomimetic design.

Numerical Analysis of Side-loads Reduction in a Sub-scale Dual-bell Rocket Nozzle

A calibrated delayed detached eddy simulation of a sub-scale cold-gas dual-bell nozzle flow at high Reynolds number and in sea-level mode is carried out at nozzle pressure ratio NPR = 45.7. In this regime the over-expanded flow exhibits a symmetric and controlled flow separation at the inflection point, that is the junction between the two bells, leading to the generation of a low content of aerodynamic side loads with respect to conventional bell nozzles. The nozzle wall-pressure signature is analyzed in the frequency domain and compared with the experimental data available in the literature for the same geometry and flow conditions. The Fourier spectra in time and space (azimuthal wavenumber) show the presence of a persistent tone associated to the symmetric shock movement. Asymmetric modes are only slightly excited by the shock and the turbulent structures. The low mean value of the side-loads magnitude is in good agreement with the experiments and confirms that the inflection point dampens the aero-acoustic interaction between the separation-shock and the detached shear layer.

Design of metacontinua in the aeroacoustic spacetime

The effect of background flows on the response of acoustic metamaterials is a key aspect that prevented the full disclosure of their potential in those applications where an aerodynamic velocity field strongly influences the propagation of acoustic disturbances. Indeed, the classic approaches for metamaterial design do not consider the aeroacoustic interaction, and the resulting metamaterials cannot preserve their response when operating in flows. So far, only few authors have addressed the problem, mostly focusing on understanding the phenomenon or identifying corrective techniques with limited usability in practical applications. The present study proposes a general method for the modification of the mechanical properties of acoustic metacontinua to preserve their response in presence of a background flow. The method is based on the application of spacetime coordinate transformations exploiting the spacetime formal invariance of the generalised d’Alembertian. This methodology applies to the equation governing the propagation of acoustic disturbances in a metamaterial having arbitrary constitutive equations independently on the method used for its original design. The approach is validated through numerical simulations, using as a benchmark the problem of the acoustic cloaking of a cylinder impinged by a perturbation generated by an isotropic point source within a flowing medium. Numerical results are obtained for an asymptotic Mach number M_infty le 0.35.

Numerical Characterisation of the Aeroacoustic Signature of Propeller Arrays for Distributed Electric Propulsion

This paper presents an investigation of the aerodynamic and aeroacoustic interaction of propellers for distributed electric propulsion applications. The rationale underlying the research is related to the key role that aeroacoustics plays in the establishment of the future commercial aviation scenario. The sustainable development of airborne transportation system is currently constrained by community noise, which limits the operations of existing airports and prevents the building of new ones. In addition, the substantial saturation of the existing noise abatement technologies inhibits the further development of the existing fleet, and imposes the adoption of disruptive configurations in terms of airframe layout and propulsion technology. Simulation-based data may help in clarifying many aspects related to the acoustic impact of such innovative concepts. Blended-wing-body equipped with distributed electric propulsion is one of the most promising, due to the beneficial effect of the substantial shielding induced by its geometry. Nevertheless, the novelty of the layout requires a thorough investigation of specific aspect for which no previous experience is available. Herein, the interaction between propellers is analysed for a fixed propeller geometry, as a function of their mutual distance and compared to the acoustic pattern of the isolated one. The aerodynamic results have been obtained using a boundary integral formulation for unsteady, incompressible, potential flows which accounts for the interaction between free wakes and propellers. For the aeroacoustic analyses, the Farassat 1A boundary integral formulation for the solution of the Ffowcs Williams and Hawkings equation has been used. These results provide an insight into the minimum distance between propellers to avoid aerodynamic/aeroacoustic interaction effects, which is an important starting point for the development of distributed propulsion systems.

Experimental investigation of the aero-acoustic interaction at an area-expansion

Experimental results on the aero-acoustic interaction at an area expansion are scarce. In this study, new experimental data is acquired of the aero-acoustic interaction at an area expansion and compared with recent modeling efforts. The aero-acoustic interaction at an area expansion with expansion ratio η = 0.309 is investigated by measuring the scattering matrix for plane waves at 5 different flow speeds in the incompressible regime. The experimental results are complimented with a comprehensive uncertainty analysis to determine the precision of the data. The scattering coefficients together with derived quantities such as absorption coefficients and dimensionless end corrections are compared against recent analytical results. It is shown that there is consistent deviation between the models and measurement results for certain scattering coefficients. The measured end correction on Strouhal number close to the critical Strouhal number shows a relatively abrupt change. This in comparison with the models, where the change is predicted to be more gradual. Similar observations are made for the absorption coefficient for waves incident upstream of the area expansion, although a qualitative comparison is difficult to make in this case because of the strong influence of the flow downstream of the area-expansion on the results.

Measurement of the Local Sound Pressure on a Bias-Flow Liner Using High-Speed Holography and Tomographic Reconstruction

Bias-flow liners (BFL) are used as part of the cooling mechanism in airplane turbines and they also enable the reduction of noise emission. The attenuating principle is based on the interaction between sound waves and air flow. This turbulent aeroacoustic interaction occurs on a microscopic level and under harsh conditions. Therefore, in order to investigate local aeroacoustic phenomena, the distribution of the local sound field impedance, defined as sound pressure over acoustic particle velocity, is measured non-invasively. This work focuses on the non-invasive measurement of the local sound pressure. A fast camera-based holographic measurement system for the two dimensional tomographic measurement of the local sound pressure with high spatial resolution is presented. It features a spatial resolution of 52 μm with a simultaneous acquisition of up to 1600 measuring points. This enables the observation of sub-mm aeroacoustic effects, which are expected in the near field over the BFL's surface, while reducing the measurement duration by three orders of magnitude compared to single point measurement systems. An experimental validation of the camera-based tomographic system is carried out by reference measurements. The system was applied at an acoustical model and an aeroacoustic model. The measured local sound pressure variations are discussed for both models. This work seeks to enable the non-invasive investigation of sound pressure, in order to further understand aeroacoustic effects, their sound attenuating properties and increase the attenuation capability of a BFL without degrading the overall performance of turbines.

Experimental study of the aeroacoustic interaction between two supersonic hot jets

The first stage of space launchers often use multiple engines. The supersonic jet noise at lift-off is a major source of vibrations for the launcher’s equipments and payloads. In parallel with the development of the Ariane 6 launcher and its launch pad ELA4, the CNES MARTEL test bench have been improved in order to study experimentally the aeroacoustic interaction between two hot supersonic jets (mach 3, 2000K). In collaboration with the PPRIME laboratory of the University of Poitiers, acoustic and PIV measurements have been made in the free jets configurations. Further test campaigns will study the interaction of jets inside the flame duct, and their impingement on the launch table.

Numerical Investigation on Vortex-Structure Interaction Generating Aerodynamic Noises for Rod-Airfoil Models

In past several decades, vortex-structure interaction generated aerodynamic noise became one of the main concerns in aircraft design. In order to understand the mechanism, the acoustic analogy method combined with the RANS-based nonlinear acoustics solver (NLAS) is investigated. The numerical method is firstly evaluated by the experiment data of the classic rod-airfoil model. Compared with the traditional analogy methods, the RANS/NLAS can capture the nonlinear aerodynamic noise more accurately with lower gird requirements. Then different rod-airfoil configurations were simulated to investigate the aeroacoustic interaction effects. The numerical results are in good agreement with those of the earlier experimental research. It is found that the vortex-shedding crash to the airfoil is the main reason for the noise generation which is dependent on the configurations, distance, and flow conditions.

Impact of turbulence on the prediction of linear aeroacoustic interactions: Acoustic response of a turbulent shear layer

A powerful model to predict aeroacoustic interactions in the linear regime is the perturbed compressible linearized Navier–Stokes equations. Thus far, the frequently employed derivation suggests that the effect of turbulence and its associated Reynolds stresses is neglected and a quasi-laminar model is employed. In this paper, dynamic perturbation equations are derived incorporating the effect of turbulence and its interaction with perturbation quantities. This is done by employing a triple decomposition of the instantaneous variables. The procedure results in a closure problem for the Reynolds stresses for which a linear eddy-viscosity model is proposed. The resulting perturbation equations are applied to a grazing flow in a T-joint for which strong shear layer instabilities at certain frequencies are experimentally observed. Passive scattering properties of the grazing flow are validated against the experiments performed by Karlsson and Åbom and perturbation equations being quasi-laminar. We find that prediction models must include the effect of Reynolds stresses to capture the aeroacoustic interaction effects correctly. Neglecting its effect naturally results in the over prediction of vortex growth at the frequencies of shear layer instability and therewith in an over prediction of aeroacoustic interactions.

Analysis of frication noise modulation from a physical model

A physical model, built to investigate the aeroacoustic properties of voiced fricative speech, was used to study the amplitude modulation of theturbulence noise it generated. The amplitude and fundamental frequency of glottal vibration, relative positions of the constriction and obstacle, and the flow rate were varied. Measurements were made from pressure taps in the duct wall and the sound pressure at the open end. The high-pass filtered sound pressure was analyzed in terms of the magnitude and phase of the turbulence noise envelope. The magnitude and phase of the observed modulation was related to the upstream pressure. The effects of moving the obstacle with respect to the constriction are reported (representative of the teeth and the tongue in a sibilant fricative respectively). These results contribute to the development of a parametric model of the aeroacoustic interaction of voicing with turbulence noise generation in speech.

Aeroacoustic interaction in a corrugated duct

The sound generation by an air flow in a corrugated tube is studied experimentally for different values of the corrugation pitch and different tube lengths. The Strouhal numbers of sound generated in different tubes with different flow velocities lie within 0.4–0.6. As the flow velocity increases, the Strouhal number decreases. The effect of sound absorption by an air flow in a corrugated duct is described: in a corrugated tube with a flow, at frequencies below the generation frequency, the absorption of sound produced by an external source is observed. A semiempirical model of aeroacoustic interaction in a corrugated tube is proposed. The model provides a qualitative agreement with the experiment.

A LATTICE BOLTZMANN METHOD FOR COMPUTATION OF AEROACOUSTIC INTERACTION

This paper reports a study of the ability of an improved LBM in replicating acoustic interaction. With a BGK model with two relaxation times approximating the collison term, the improved LBM is shown not only able to recover the equation of state, but also replicates the specific heat ratio, the fluid viscosity and thermal conductivity correctly. With these improvements, the recovery of full set of unsteady compressible Navier-Stokes equations is possible. Two complex aeroacoustic interaction problems, namely the interaction of three fundamental aeroacoustic pulses and scattering of short wave by a zero circulation vortex, are calculated. The LBM solutions are compared with DNS results. In the first case it has been shown that the improved LBM is as effective as the DNS in simulating aeroacoustic interaction of three pulses. Both methods obtain essentially same results using same truncated domains. In the scattering problem, LBM is able to replicate the directivity of scattered acoustic wave from the vortex but it does not accurately reproduce the symmetry as calculated using DNS.

Instability of channel flow with fluid injection and parietal vortex shedding

This paper is devoted to the computation of the stability properties and the aeroacoustic interaction in a planar channel flow driven by air injection through porous wall. The purpose is to establish a simulation method which estimates the vortex-shedding and acoustic-wave interaction. To perform these unsteady computations, turbulent motion is taken into account by a first-order model based on a nonlinear relationship between time-dependent Reynolds stresses and velocity gradients. Model coefficients are explicit functions of both strain and rotation. This model is applied to the computation of parietal vortex shedding in a simplified configuration. It is a channel flow with fluid injection. Two cases are computed here. The first one is stable and the second one is unstable. To improve the evaluation of the turbulent effects, computations of the second case are presented with and without a turbulence model. In first configuration, a good agreement with experimental data is found. In the second case both with and without turbulence model the natural unsteadiness of the flow is captured. The parietal vortex shedding is described and the turbulence effects are characterized.

Computation of Vortex Shedding in Solid Rocket Motors Using Time-Dependent Turbulence Model

This paper is devoted to the computation of turbulent effects on the aeroacoustic interaction in solid rocket motors. The purpose is to establish a simulation method that estimates the vortex-shedding and acoustic-wave interaction. To perform these unsteady computations, turbulent motion is taken into account by a e rst-order model based on a nonlinear relationship between time-dependent Reynolds stresses and velocity gradients. Model coefe cients are explicit functions of both strain and rotation. This model is applied to the computation of two simplie ed cone gurations of a rocket booster. To better evaluate the turbulent effects, computations are presented with and without a turbulence model. In both cone gurations, the natural unsteadiness of the e ow is captured and the comparisons with numerical and experimental data are presented. The interaction between vortex shedding and acoustics is described and the turbulence effects are characterized.

Airframe Noise Measurements on a Supersonic Transport Small-Scale Model

Airframe noise has been measured on a 0.015 scale model of an advanced supersonic transport concept (AST100) in an anechoic flow facility. The model was equipped with leading- and trailing-edge flaps, nose and main landing gears, and engine nacelles. Each of these components was deployed, individually and collectively, to determine their contribution to the noise field. Results are presented which show that in the clean configuration the aircraft displays a symmetric dipole directivity, whereas in the more complex landing-approach configuration the directivity peaks in the forward quadrant. It was found that the landing-approach noise was due chiefly to the landing gear, the trailing-edge flaps, and the aeroacoustic interaction between the two.