Direct Aeroacoustic Simulation Research Articles

Deep cavity noise suppression by exploiting aeroacoustic–structural interaction of multiple elastic panels

This paper reports a numerical study of a novel methodology for passive suppression of deep cavity noise by means of strategically designed and arrangements of multiple elastic panels and examines its underlying aeroacoustic–structural interaction physics. The study is conducted with a freestream, at Mach number 0.09 and Reynolds number of 4 × 104 based on the cavity length, past a two-dimensional cavity by means of direct aeroacoustic simulation coupled with a panel dynamic solver in monolithic fashion. For each cavity-panel configuration, the fluid-loaded panel natural frequencies are harmonized with the characteristic aeroacoustic processes of the original/modified cavity aeroacoustic feedback loop. This promotes panel aeroacoustic-structural resonance for absorption of feedback flow and acoustic fluctuation energy for achieving less eventual cavity noise. The most effective configuration gives a remarkable noise power reduction by 15 dB from a rigid cavity. Inadvertently, it reduces cavity drag by almost 15%. Simultaneous reduction of both cavity noise and drag is unprecedented among similar attempts in the literature. In-depth spatiotemporal analyses of aeroacoustic–structural interaction results elucidate the intricate interplay between cavity flow, panel vibration responses, and cavity acoustic modes, leading to noise reduction in all cavity-panel configurations studied. Essentially, the vertical panel acts to curtail the efficacy of coupling between growing shear layer and cavity acoustic modes whose sustenance is further impeded by an acoustically induced resonant panel at the cavity bottom. The proposed methodology is confirmed to be feasible yet effective, which holds great potential for fluid-moving applications in which a quiet and energy-efficient cavity configuration is desired.

Utilization of Embedded Surface Compliance for Suppression of Deep Cavity Flow Noise

Cavity noise suppression is one of the extensively debated aeroacoustic research problem due to its application over the wide range of mechanical systems. This study presents a scheme of embedding surface compliance in form of elastic panels on cavity walls as a mean to achieve significant tonal noise reduction. A deep cavity of aspect ratio D/L = 0.4 operating at Mach number 0.09 is considered. Using the Direct Aeroacoustic Simulation, the rigid cavity case is investigated to uncover the underlying physical processes driving the aeroacoustic coupling phenomenon between the shear layer and the cavity mode. The analysis suggests that each cavity wall supports a certain coupling-associated activity and therefore provides a platform to modify the shear layer - cavity mode interaction in a tacit way through the well-designed positioning of elastic panels. The accordantly mounted elastic panels are then excited with the flow perturbation energy to significantly reduce the pressure fluctuations magnitude of the cavity mode which strongly affects its synchronization with shear layer and hence presents the possibility of significant noise reduction.

Direct aeroacoustic simulation of a cross-flow fan with Helmholtz resonator using lattice Boltzmann method

It is necessary to reduce the aerodynamic sound generated by fans since they are often used for air-conditioners, which are placed in human living areas, such as homes and offices. To reduce the aerodynamic sound, we focused on the Helmholtz resonator. To develop a resonator-type silencer for the aerodynamic sound of air-conditioners, the present study investigated the flow and acoustic fields around a cross-flow fan with a resonator and the effectiveness of the resonator for blade-passing frequency (BPF) sound using aeroacoustic simulations. As a numerical method, we used the lattice Boltzmann method (LBM), which can simultaneously simulate aerodynamically sound generation in the fan and the acoustic resonance in the resonator. Consequently, the simulation results showed that a reduction of 2.9 dB for the BPF sound can be realized by installing a resonator in the tongue part of the fan casing.

Direct aeroacoustic simulation of a flow through an expanding pipe with orifice plates

Expanding pipes with orifice plates are utilized as silencers for fluid machinery. However, tonal sounds can be generated from flows through the silencers with such a configuration. To clarify the mechanism of tonal sound generation from a flow over a circular expanding pipe with orifice plates, flow and acoustic fields were directly solved on the basis of the compressible Navier-Stokes equations. Prediction results demonstrated that tonal sounds occurred at multiple frequencies. The phase-averaged flow fields for each frequency indicated that vortex rings or spiral vortices were periodically shed in the flow around the orifice plates. Acoustic radiation occurred owing to the collision of these vortices with the orifice plates, which led to the occurrence of acoustic resonance in the expanding pipe. The predicted circumferential phase distributions of pressure fluctuations demonstrated the coexistence of different acoustic modes in the expanding pipe. The relationship between acoustic modes and vortical behaviors was presented. The effects of inflow Mach number on the vortex structures and acoustic modes for the resonance were also discussed.

Reduction of Flow-Induced Trailing Edge Noise of Semi-Infinite Flat Plate by Structural Resonance

A unique method for the reduction in flow-induced trailing edge noise scattering for a semi-infinite thin plate is proposed by utilizing short flexible panels mounted on the surface of the plate. The proposed configuration differs from conventional cantilevered elastic trailing edges due to the limitations of their structural integrity and applicability. The key idea is to design short panels that vibrate in structural resonance under the fluid loading for absorbing the energy from the flow to sustain their vibration. A time-domain direct aeroacoustic simulation coupled with structural dynamics is carried out at a low Reynolds number of 50,000 to explore the aeroacoustic-structural interactions. The effectiveness of noise reduction of the designed configuration is analyzed by comprehensive aeroacoustic analysis where a significant noise reduction is observed for the plate mounted with three flexible panels without any adverse effects on the plate aerodynamics. The structural analysis shows a systematic vibration pattern for all the panels which clearly indicates the presence of complex fluid-structural interaction under resonance conditions. Design strategy of the proposed configuration along with its limitations are also discussed.

Control of acoustic scattering of trailing edge flow by distributed compliance

In this paper, an approach for the reduction of trailing edge noise due to flow scattering from a semi-infinite splitter plate is proposed. It utilizes the fluid–structure interactions of well-designed multiple compliant elastic panels to suppress the flow instabilities within the boundary layers over the splitter plate to reduce overall trailing edge noise scattering. The approach is studied numerically using high-fidelity direct aeroacoustic simulation at low Reynolds numbers based on a panel length of 5×104. The noise reduction efficacy of the approach is analyzed by studying two different cases, and their underlying physical mechanisms are explored. First, the boundary layer over one side of the plate is subjected to a weak monochromatic acoustic excitation to produce laminar instabilities. Second, the boundary layer is subjected to a weak broadband excitation within the boundary layer. For each case, the panel system is uniquely designed with thorough consideration of the flow characteristics of the boundary layer instabilities of the problem. Comprehensive aeroacoustic analyses reveal that a significant sound power level reduction of 4.2 and 7.4 dB can be achieved by designed configurations for both kinds of excitation without any drag penalty. Nonlinear fluid–structure interactions of carefully designed elastic panels result in a weak correlation between the near-field flow instabilities and far-field noise. The flow-induced panel structural resonance is proven to effectively absorb the energy of boundary layer instabilities and their scattering at the trailing edge. Key characteristics for the design of compliance systems under different flow conditions are discerned and discussed.

Suppression of deep cavity aeroacoustics at low Mach number by localized surface compliance

A unique concept of utilizing localized surface compliance is proposed to suppress deep cavity aeroacoustics at a low Mach number. The core idea is to provide local absorption of the energy of aeroacoustic processes supporting cavity flow self-sustained feedback loop responsible for tonal noise generation. The concept is studied with a flow past cavity of length-to-depth ratio of 0.4 at freestream Mach number 0.09 and Reynolds number based on cavity length 4 × 104 using high-fidelity, two-dimensional direct aeroacoustic simulation. Having confirmed the replication of key aeroacoustic processes in the numerical solution through careful validation, localized surface compliance in the form of an elastic panel is strategically introduced to modify every process for cavity noise suppression. The panel natural frequency is set equal to the feedback loop characteristic frequency to facilitate its flow-induced structural resonance for energy absorption. Suppression of cavity noise pressure and power levels by 3.8 and 4.8 dB, respectively, is successfully achieved, together with an unforeseen cavity drag reduction by almost 19%. Comprehensive wavenumber–frequency analyses of the coupled aeroacoustics and flow-induced panel vibration are conducted to uncover the physical mechanism of noise suppression. The results show that the same type of aeroacoustic feedback loop occurs, but its efficacy is significantly reduced due to the exhaustion of aeroacoustic process energy to the flow-induced vibrating panel. The proposed concept is confirmed to be feasible in terms of giving remarkable cavity noise and drag suppression, yet it retains the basic problem geometry intact, which are considered important in many practical applications.

A computational study of trailing edge noise suppression with embedded structural compliance.

A unique concept for suppression of trailing edge noise scattering from a splitter plate in a low Reynolds number flow is proposed. The key idea of the concept is the adoption of a structural compliance system embedded with a finite number of elastic panels. Specific compliance system designs are devised for promotion of panel structural resonance that effectively absorbs broadband flow/acoustic fluctuation energy responsible for noise scattering. The concept is examined using high-fidelity direct aeroacoustic simulation together with spatiotemporal aeroacoustic-structural interaction analysis. The concept is confirmed feasible and outperforms many similar trailing edge noise reduction approaches reported in the literature.

Fluid-Structure Interactions and Aeroacoustic coupling of Airfoil with Flexible Membrane(s)

In this paper, fluid-structure interactions of a NACA 0012 airfoil mounted with short flexible membrane(s) and its coupling effect on airfoil aeroacoustics are presented. A time-domain direct aeroacoustic simulation coupled with structural dynamics is carried out at a low Reynolds number of 50,000 to explore the aeroacoustic-structural interactions. Two different airfoil configurations based on single and dual membranes are analyzed. The membrane deflections and their impact on the flow field are characterized in wavenumber-frequency domain to analyze the structural dynamics due to flow unsteadiness within the laminar boundary layer and the resulting acoustic waves emanating from the airfoil trailing edge. A strong correlation between the membrane displacement and downstream propagating flow is observed for all configurations whereas the correlation is considerably weakened between the membrane displacement and upstream acoustic waves which ultimately results in the airfoil self-noise reduction without affecting the airfoil aerodynamics. The extent of noise reduction for dual membrane airfoil configuration is observed to be considerably higher than the single membrane airfoil configuration which corresponds to a much lower correlation among the upstream propagating acoustic waves and membrane deflection for both the membranes and redistribution of upstream flow energy into different frequencies.

Distributed surface compliance for airfoil tonal noise reduction at various loading conditions

A novel concept of utilizing distributed surface compliance to achieve airfoil tonal noise reduction at various loading conditions is proposed. The aeroacoustics of airfoil configuration subjected to different loading conditions at angles of attack (AoAs) from 3° to 7° are numerically studied using high-fidelity two-dimensional direct aeroacoustic simulation at Reynolds and Mach numbers of 5×104 and 0.4, respectively. Initially, airfoil configurations mounted with single elastic panel (SEP) at individual AoA are designed with the knowledge of respective rigid airfoil flow characteristics. Stemming from the analysis of noise reduction potential of SEP configurations using a reduced-order modeling approach, a distributed surface compliance (DSC) airfoil configuration utilizing three resonating panels is designed to attain airfoil tonal noise reduction over entire range of AoA. Comprehensive acoustic analyses establish that the DSC airfoil could provide a maximum noise reduction ranging from 3 to 7 dB without any sacrifice in airfoil aerodynamics. The extent of noise reduction with DSC airfoil is found dependent on the flow-induced modal responses of the panels. At lower AoA, the panel(s) resonate in their designed structural modes, which remarkably weaken the flow instabilities convecting over the airfoil suction surface and eventually airfoil noise radiation. At higher AoA, the panel responses deviate from their designed structural mode shapes but could still give less noise reduction. Therefore, the designed DSC airfoil shows a feasible concept for tonal noise reduction over a wide range of operational AoA, which substantiates its applicability for aerodynamic devices at low Reynolds numbers.

Coupled structural resonance of elastic panels for suppression of airfoil tonal noise

In this paper the aeroacoustic characteristics of coupled flow-induced vibration of elastic panels mounted on an airfoil is numerically studied to uncover its potential for suppression of airfoil tonal noise. The proposed approach utilizes the inter-dynamical structural coupling between two elastic panels which maintain a synchronized flow-induced vibration pattern to weaken the flow fluctuations responsible for tonal noise generation. In the study, a NACA0012 airfoil at a low Reynolds number of 5×104 and an angle of attack of 5° is taken as the basic structure on which various panel configurations are attempted. The results of preliminary analysis of aeroacoustic-structural interactions of various panel configurations, using a reduced-order modeling approach, reveal that an airfoil-panel configuration with two elastic panels separated by approximately one convective wavelength and mounted in a region exposed to high boundary layer flow instabilities yields optimal noise suppression. Comprehensive aerodynamic and acoustic analyzes of airfoil-panel configurations, using the results of high fidelity direct aeroacoustic simulation, reveal that an average and maximum noise suppression up to 7.6 dB and 7.9 dB respectively can be achieved with negligible sacrifice on overall airfoil aerodynamics when strongly coupled structural resonance between the panels prevails. The magnitude of noise suppression is higher than twice of that from the configuration mounted with a single panel. This fact firmly illustrates the synergy of coupled flow-induced structural resonance of the panels prevailing in noise suppression. Nonlinear fluid–structure interactions of coupled resonant panels in airfoil-panel configuration are characterized by extensive spectral analyzes in frequency and wavenumber domains as well as correlation analyzes. The panels exhibit highly synchronized dynamic behaviors and run into limit cycle oscillations. Their strongly coherent structural responses are proven able to provide a more effective suppression of the boundary layer instabilities to be scattered at airfoil trailing edge as noise. The upstream propagating component of the generated noise does not interfere the coupled structural resonance of the panels as revealed from their weak correlations. Key characteristics required for the design of strongly coupled structural resonant panels for effective absorption of flow fluctuation energy are discerned and discussed.

Interaction and acoustics of separated flows from a D-shaped bluff body

PurposeFor flow around elongated bluff bodies, flow separations would occur over both leading and trailing edges. Interactions between these two separations can be established through acoustic perturbation. In this paper, the flow and the acoustic fields of a D-shaped bluff body (length-to-height ratio L/H = 3.64) are investigated at height-based Reynolds number Re = 23,000 by experimental and numerical methods. The purpose of this paper is to study the acoustic feedback in the interaction of these two separated flows.Design/methodology/approachThe flow field is measured by particle image velocimetry, hotwire velocimetry and surface oil flow visualization. The acoustic field is modeled in two dimensions by direct aeroacoustic simulation, which solves the compressible Navier–Stokes equations. The simulation is validated against the experimental results.FindingsSeparations occur at both the leading and the trailing edges. The leading-edge separation point and the reattaching flow oscillate in accordance with the trailing-edge vortex shedding. Significant pressure waves are generated at the trailing edge by the vortex shedding rather than the leading-edge vortices. Pressure-based cross-correlation analysis is conducted to clarify the effect of the pressure waves on the leading-edge flow structures.Practical implicationsThe understanding of interactions of separated flows over elongated bluff bodies helps to predict aerodynamic drag, structural vibration and noise in engineering applications, such as the aerodynamics of buildings, bridges and road vehicles.Originality/valueThis paper clarifies the influence of acoustic perturbations in the interaction of separated flows over a D-shaped bluff body. The contribution of the leading- and the trailing-edge vortex in generating acoustic perturbations is investigated as well.

Investigation on a Duct Noise Control Method through Membranes in Tandem

Duct noise control is an important practical problem. This paper explores the vibro-acoustic mechanism of duct noise through the membrane in tandem. Validity and feasibility of the proposed analytical model is demonstrated by comparing existing simplified models and full direct aeroacoustic simulation solved with the CE/SE model. It is shown that the coupling effect between two membranes in tandem is not negligible to predict system response. Moreover, introduction of multimembranes is important or even the only efficient way to apply this passive control method in practice.

Three-dimensional numerical analysis of acoustic energy absorption and generation in an air-jet instrument based on Howe's energy corollary.

The sounding mechanism of a recorder-like air-jet instrument at low Strouhal number is numerically investigated by three-dimensional direct aeroacoustic simulation and acoustic simulation. Howe's energy corollary is applied to estimate the acoustic energy generation and absorption induced by an oscillating jet and vortex shedding. The quantitative results show that the main acoustic energy generation occurs in the jet downstream, and the absorption occurs in the jet upstream. It is found that the region defined by the Q-criterion identifies the main acoustic energy generation (absorption) region in the downstream (upstream) region of the jet. The results indicate that the vortex shedding mainly induced by the jet deflection gives additional contributions to the acoustic energy absorption. The shed vortices affect the temporal structure of the acoustic energy transfer, in particular, the timing of the double peaks with respect to the jet displacement. If we focus only on the air-jet, the dominant peak is observed when the jet crosses the edge from the inside to the outside of the pipe, as reported in previous experimental works. However, when we include the contributions of shed vortices, the dominant peak appears when the jet dives under the edge, which is consistent with the jet-drive model.

Direct aeroacoustic simulation with a cumulant Lattice-Boltzmann model

This study presents a direct aeroacoustic simulation using a cumulant lattice Boltzmann method that provides very high stability without modifying the bulk viscosity compared to the Bhatnagar–Gross–Krook model. It is shown with the Chapman–Enskog analysis that the numerical model recovers the compressible Navier–Stokes equations for low Mach number flows to second-order accuracy. The low dissipation and low dispersion properties required for aeroacoustics are verified by simulating a planar wave propagation test case. The predictive accuracy of the method is investigated for a complex test case by performing a direct aeroacoustic simulation for a highly turbulent flow in a channel with obstructive walls and an orifice leading the flow into an ambient environment. The acoustic sound pressure level is evaluated for microphone positions inside the turbulent flow where flow induced sources are located, and also for positions in the far field of the channel orifice. The agreement between simulation results of the cumulant lattice Boltzmann method and experimental data is excellent. The simulation predicts for all microphone positions the measured spectra over a wide frequency range and single dominant peaks are captured very well.

Effect of back cavity configuration on performance of elastic panel acoustic liner with grazing flow

This paper reports a comprehensive numerical study of noise mitigation performance of elastic panel liner comprising an elastic panel and a cavity beneath exposed to low Mach number grazing flow. A time-domain direct aeroacoustic simulation (DAS) seamlessly coupled with panel dynamics is adopted for its least assumptions taken on duct flow unsteadiness and acoustical behaviors so that both linear and nonlinear aeroacoustic-structural interactions of the problem can be fully explored. The numerical method is well validated with theoretical and experimental works on a liner with thick cavity reported in literature. The noise mitigation of liner with various combinations of cavity depth, panel length and cavity shape, are explored and the present numerical results show that back cavity configuration plays an important role in the liner problem. A decomposition method is applied to DAS acoustic solutions for uncovering the role of aeroacoustically induced panel vibration. The nonlinear effect due to aeroacoustic-structural interaction is found to be of secondary importance. Extensive cross spectral analyses between duct aeroacoustics and panel vibration reveal that the overall liner performance is largely determined by the liner elastic panel whose aeroacoustic and vibration responses are greatly modified by the variations of back cavity acoustics of the back cavity with different shapes. Based on these understandings a new configuration with acoustic absorption materials placed on a cavity wall is proposed. Detailed analysis of its numerical results shows that the introduction of acoustic absorption effectively relieves the cavity acoustics and modifies the panel responses in such a way that an enhanced liner mitigation performance over a broadband can be achieved.

Passive airfoil tonal noise reduction by localized flow-induced vibration of an elastic panel

In this paper a novel passive control method is explored for airfoil tonal noise reduction using localized flow-induced vibration of a short elastic panel flush mounted on suction surface of a NACA 0012 airfoil at low Reynolds number. The key idea is to provide local absorption of energy of natural instabilities evolving in the laminar boundary layer by self-sustained flow-induced vibration of a properly designed panel. The panel ultimately acts to weaken the aeroacoustic feedback loop responsible for airfoil tonal noise radiation. Perturbation evolution method with acoustic broadband excitation is utilized to assess the tonal noise reduction potential with various combinations of elastic panel design parameters and optimal resonating and non-resonating elastic panel configurations are determined. Subsequently, direct aeroacoustic simulation of airfoil flow with optimal panel configurations are carried out to uncover the mechanism of tonal noise reduction using localized flow-induced vibration in a quantitative manner. Extensive analysis of numerical results reveals that a resonating panel located just ahead of the sharp growth of boundary layer instability within the airfoil separation bubble provides the strongest reduction of flow instabilities and an overall tonal noise reduction up to 3 dB. Such significant noise reduction is achieved without any sacrifice in the original aerodynamic characteristics of the airfoil. The outcomes of the study evidently suggest that the proposed passive control method with a localized flow-induced panel vibration is effective in suppressing the fundamental mechanism responsible for tonal noise generation of airfoil flow, making it a promising approach for modifying the acoustics of existing aerodynamic or wing profile operating at low Reynolds numbers.

Direct aeroacoustic simulation of acoustic radiation in recorders with different windway geometries

To clarify the effects of the windway geometry on the aeroacoustic feedback in the jet fluctuations in recorders, direct aeroacoustic simulations were performed along with experiments. The simulations were based on the compressible Navier-Stokes equations to predict the fluid-acoustic interactions. The volume penalization method was used to reproduce the flow and acoustic fields around the complex shape of the recorders. Two recorders with straight and arch-shaped windway were explored. The occurrence of mode change was observed at the higher velocity for an arch-shaped windway model compared with the straight windway model. The modified formulation of the negative displacement model (N.H. Fletcher et al., 1976, J. Acoust.) was proposed based on the predicted jet fluctuations, where the jet fluctuations were divided into hydrodynamic and acoustic components. The ratio of the hydrodynamic component to the acoustic component near the windway exit was lower in the archshaped windway model than that in the straight windway model, whereas the amplification factor of the jet fluctuations was larger in the arch-shaped windway model.The relevance of these results and the mode change along the jet velocity is to be discussed.To clarify the effects of the windway geometry on the aeroacoustic feedback in the jet fluctuations in recorders, direct aeroacoustic simulations were performed along with experiments. The simulations were based on the compressible Navier-Stokes equations to predict the fluid-acoustic interactions. The volume penalization method was used to reproduce the flow and acoustic fields around the complex shape of the recorders. Two recorders with straight and arch-shaped windway were explored. The occurrence of mode change was observed at the higher velocity for an arch-shaped windway model compared with the straight windway model. The modified formulation of the negative displacement model (N.H. Fletcher et al., 1976, J. Acoust.) was proposed based on the predicted jet fluctuations, where the jet fluctuations were divided into hydrodynamic and acoustic components. The ratio of the hydrodynamic component to the acoustic component near the windway exit was lower in the archshaped windway model than that in the st...

Direct Aeroacoustic Simulation with Volume Penalization Method

Flow and acoustic fields around a circular cylinder and a cascade of flat plates are clarified by direct simulations with a volume penalization method, which is a useful method to predict flow and acoustic fields around a complex geometry in a flow. The predicted results are compared with our experimental results or those of literatures. It is also clarified that the acoustic resonance occurs between the plates for a cascade of flat plates. The phase-averaged predicted flow fields present that the vortices shed from neighboring plates is synchronized in an anti-phase mode. As a result, the intense standing waves are generated between plates.

One-step simulation of thermoacoustic waves in two-dimensional enclosures

Abstract This paper reports on a one-step simulation study of the generation and propagation of thermoacoustic waves in a two-dimensional enclosure using a finite-difference lattice-Boltzmann-type method (FDLBM) with a single relaxation time and one equilibrium particle distribution function (EPDF), and a direct aeroacoustic simulation (DAS) technique that solves the primitive Navier–Stokes (N–S) equations. Conventional expansion of the EPDF is not adopted; instead, it is expanded in terms of the particle velocity alone, and the expansion coefficients are determined by requiring the FDLBM to fully recover the N–S equations. The expansion coefficients are found to depend on the flow and thermal properties and their nonlinear interactions. Thus formulated, physical boundary conditions can be specified for the FDLBM. This EPDF has been validated against simple aeroacoustic problems and good agreement with DAS results is obtained. In this paper, the EPDF is further validated against three thermoacoustic cases: 1) a sudden increase of temperature on the left vertical wall of the enclosure; 2) a sudden increase of temperature on the left vertical wall coupled by a sudden decrease of temperature on the right vertical wall in the enclosure; 3) a gradually heated up left vertical wall of the enclosure. Comparisons with DAS simulations show that FDLBM and DAS give essentially identical results; hence, the validity and extent of the EPDF is ascertained for the calculation of these thermoacoustic waves. The FDLBM results are also compared with known theoretical and numerical results. These known solutions are found to be less accurate compared to the FDLBM results. The discrepancy could be attributed to the partially linearized equations solved in the theoretical analysis, and the inadequacy of the numerical scheme to replicate the nonlinear effects accurately in the generation and propagation of thermoacoustic waves.