Showing papers on "Acoustic radiation published in 2009"

Acoustic radiation from turbulent premixed flames

Abstract: Turbulent combustion processes are inherently unsteady and, thus, a source of acoustic radiation. While prior studies have extensively characterized their total sound power, their spectral characteristics are not well understood. This work investigates these acoustic spectral features, including the flame's low- and high-frequency characteristics and the scaling of the frequency of peak acoustic emissions. The spatiotemporal characteristics of the flame's chemiluminescence emissions, used as a marker of heat release fluctuations, were measured and used to determine the heat release spectrum, spatial distribution and spatial coherence characteristics. These heat release characteristics were then used as inputs to an integral solution of the wave equation and compared to measured acoustic spectra obtained over a range of conditions and burners and at several spatial locations. The spectral characteristics of the flame's acoustic emissions are controlled by two processes: the underlying spectrum of heat release fluctuations that are ultimately the combustion noise source, and the transfer function relating these heat release and acoustic fluctuations. An important result from this work is the clarification of the relative roles of these two processes in controlling the shape of the acoustic spectrum. This transfer function is primarily controlled by the spatiotemporal coherence characteristics of the heat release fluctuations which are, in turn, strongly influenced by burner configuration/geometry and operating conditions. Low-frequency acoustic emissions are controlled by the heat release spectrum essentially independent of flame geometry. Both the heat release spectrum and heat release-acoustic transfer function are important at intermediate and high frequencies. An important feature of the investigated geometry that controls the heat release phase dynamics is the high-velocity flow relative to the flame speed and anchored character of the flame. This leads to convection of flame sheet disturbances (i.e. heat release fluctuations) along the front that dominates the high frequency and peak frequency scaling of the flame's acoustic emissions.

Langevin acoustic radiation force of a high-order bessel beam on a rigid sphere

Abstract: The acoustic radiation force of Langevin type resulting from the interaction of a high-order Bessel beam with a rigid immovable sphere in an ideal fluid is theoretically investigated. The analysis is based on applying the generalized Rayleigh series used in the near-field acoustic scattering problem to calculate the force. With appropriate selection of specific Bessel beam parameters, results for the rigid sphere unexpectedly reveal a negative radiation force caused by the Lagrangean energy density. Specifically, the negative force on the rigid sphere arises when the kinematic energy density is larger than the potential energy density. This condition provides an impetus for further designing acoustic tweezers operating with high-order Bessel beams of progressive waves for potential applications in particle entrapment and manipulation.

Application of an acoustic analogy to PIV data from rectangular cavity flows

Abstract: The present paper describes a method to derive information about the acoustic emission of a flow using particle image velocimetry (PIV) data The advantage of the method is that it allows studying sound sources, the related flow phenomena and their acoustic radiation into the far field, simultaneously In a first step the time history of two-dimensional instantaneous pressure fields is derived from planar PIV data In a successive step the Curle’s acoustic analogy is applied to the pressure data to obtain the acoustic radiation of the flow The test cases studied here are two rectangular cavity flows at very low Mach number with different aspect ratios L/H The main sound source is located at the cavity trailing edge and it is due to the impingement of vortices shed in the shear layer It is shown that the flow emits sound with a main directivity in the upstream direction for the smaller aspect ratio and the directivity is more uniform for the larger aspect ratio In the latter case the acoustic pressure spectra has a broader character due to the impact of the downstream recirculation zone onto the shear layer instabilities, destroying their regular pattern and alternating the main sound source

Behaviour of an air-assisted jet submitted to a transverse high-frequency acoustic field

Abstract: Acoustic instabilities with frequencies roughly higher than 1 kHz remain among the most harmful instabilities, able to drastically affect the operation of engines and even leading to the destruction of the combustion chamber. By coupling with resonant transverse modes of the chamber, these pressure fluctuations can lead to a large increase of heat transfer fluctuations, as soon as fluctuations are in phase. To control engine stability, the mechanisms leading to the modulation of the local instantaneous rate of heat release must be understood. The commonly developed global approaches cannot identify the dominant mechanism(s) through which the acoustic oscillation modulates the local instantaneous rate of heat release. Local approaches are being developed based on processes that could be affected by acoustic perturbations. Liquid atomization is one of these processes. In the present paper, the effect of transverse acoustic perturbations on a coaxial air-assisted jet is studied experimentally. Here, five breakup regimes have been identified according to the flow conditions, in the absence of acoustics. The liquid jet is placed either at a pressure anti-node or at a velocity anti-node of an acoustic field. Acoustic levels up to 165 dB are produced. At a pressure anti-node, breakup of the liquid jet is affected by acoustics only if it is assisted by the coaxial gas flow. Effects on the liquid core are mainly due to the unsteady modulation of the annular gas flow induced by the acoustic waves when the mean dynamic pressure of the gas flow is lower than the acoustic pressure amplitude. At a velocity anti-node, local nonlinear radiation pressure effects lead to the flattening of the jet into a liquid sheet. A new criterion, based on an acoustic radiation Bond number, is proposed to predict jet flattening. Once the sheet is formed, it is rapidly atomized by three main phenomena: intrinsic sheet instabilities, Faraday instability and membrane breakup. Globally, this process promotes atomization. The spray is also spatially organized under these conditions: large liquid clusters and droplets with a low ejection velocity can be brought back to the velocity anti-node plane, under the action of the resulting radiation force. These results suggest that in rocket engines, because of the large number of injectors, a spatial redistribution of the spray could occur and lead to inhomogeneous combustion producing high-frequency combustion instabilities.

Assessment of shear modulus of tissue using ultrasound radiation force acting on a spherical acoustic inhomogeneity

Abstract: An ultrasound-based method to locally assess the shear modulus of a medium is reported. The proposed approach is based on the application of an impulse acoustic radiation force to an inhomogeneity in the medium and subsequent monitoring of the spatio-temporal response. In our experimental studies, a short pulse produced by a 1.5-MHz highly focused ultrasound transducer was used to initiate the motion of a rigid sphere embedded into an elastic medium. Another 25 MHz focused ultrasound transducer operating in pulse-echo mode was used to track the displacement of the sphere. The experiments were performed in gel phantoms with varying shear modulus to demonstrate the relationship between the displacement of the sphere and shear modulus of the surrounding medium. Because the magnitude of acoustic force applied to sphere depends on the acoustic material properties and, therefore, cannot be used to assess the absolute value of shear modulus, the temporal behavior of the displacement of the sphere was analyzed. The results of this study indicate that there is a strong correlation between the shear modulus of a medium and spatio-temporal characteristics of the motion of the rigid sphere embedded in this medium.

Aircraft Fan Noise Absorption: DNS of the Acoustic Dissipation of Resonant Liners

Abstract: The acoustic dissipation mechanisms of resonant liners are strongly nonlinear. Usually set in the inlet nacelles to reduce fan noi se, perforated honeycomb sandwich panels are excited by grazing-incidence high sound pressure level (SPL) acoustic waves and air flows. The purpose of this paper is to stud y and quantify the acoustic absorption of a resonant liner, through direct numerical simul ations (DNS) performed with the Onera CEDRE code. The liner is modeled by a cylindrical Helmholtz resonator, with a 0.8 mm diameter opening. Its response to acoustic w aves is computed for different frequencies, SPL and incidences (normal and grazing). For the lowest SPL, the viscous wall friction and the acoustic radiation are identi fied as the “linear” dissipation mechanisms; for the highest SPL, the additional “no nlinear” vortex shedding mechanism is highlighted. Independently on the SPL, the flowf ields around the resonator hole are found to be mostly axisymmetric (along the resonator axis). As for the usual acoustic properties, the impact of a rise of the SPL is not as obvious as anticipated: for the frequencies of low linear absorption, the absorptio n gets better when the SPL increases, but for the frequencies of high linear absorption, this evolution is reversed beyond a given threshold. In the latter case, the absorption spectrum of the liner is flattened but wider. A simplified aeroacoustics excitation is fin ally approached: a grazing low Mach number inviscid air flow is coupled with the acoust ic waves. Several effects are pointed out, the main of which being the convection of the vortices downstream the resonator.

Material properties from acoustic radiation force step response.

Abstract: An ultrasonic technique for estimating viscoelastic properties of hydrogels, including engineered biological tissues, is being developed. An acoustic radiation force is applied to deform the gel locally while Doppler pulses track the induced movement. The system efficiently couples radiation force to the medium through an embedded scattering sphere. A single-element, spherically-focused, circular piston element transmits a continuous-wave burst to suddenly apply and remove a radiation force to the sphere. Simultaneously, a linear array and spectral Doppler technique are applied to track the position of the sphere over time. The complex shear modulus of the gel was estimated by applying a harmonic oscillator model to measurements of time-varying sphere displacement. Assuming that the stress-strain response of the surrounding gel is linear, this model yields an impulse response function for the gel system that may be used to estimate material properties for other load functions. The method is designed to explore the force-frequency landscape of cell-matrix viscoelasticity. Reported measurements of the shear modulus of gelatin gels at two concentrations are in close agreement with independent rheometer measurements of the same gels. Accurate modulus measurements require that the rate of Doppler-pulse transmission be matched to a priori estimates of gel properties.

Acoustic Radiation From an Infinite Laminated Composite Cylindrical Shell With Doubly Periodic Rings

Abstract: Acoustic radiation from a point-driven, infinite fluid-loaded, laminated composite shell, which is reinforced by doubly periodic rings, is investigated theoretically. The theory is based on the classical laminated composite shell theory, the Helmholtz equation, and the boundary conditions at the shell-fluid interface as well as at the junctions between the shell and the rings. The rings interact with the shell only through normal forces. The solution for the radial displacement in wave number domain is developed by using Mace’s method (1980, “Sound Radiation Form a Plate Reinforced by Two Sets of Parallel Stiffeners,” J. Sound Vib., 71(3), pp. 435‐441) for an infinite flat plate. The stationary phase approximate is then employed to find the expression for the far-field pressure. Numerical results are presented for discussion of the effects of lamination schemes, Poisson’s ratios, ply angles, and damping on the far-field acoustic radiation, which may lend themselves to better understanding the characteristics of acoustic radiation from the laminated composite shells. In addition, the helical wave spectra of the stiffened cylinders are presented, in which the effects of wave number conversion due to the periodic rings are obviously identified as additional bright patterns. DOI: 10.1115/1.2980376

Coupled Structural-Acoustic Response Prediction with Complex Modal Models

Abstract: The design of hypersonic vehicles to perform future Air Force missions will require the accurate prediction of the response of stiffened shell structures to extreme thermal-acoustic loading. The random nature of the high-intensity acoustic loading will necessitate time integration of nonlinear models. Considerable progress has been made recently in the development of reduced-order models for acoustic response prediction which greatly reduce the computational burden. The acoustic loading is treated in an uncoupled fashion, as an applied load. Recent work has shown that neglecting the coupling between the structure and the acoustic medium can result in significant errors in predicted response. This paper presents a novel improvement to the response prediction methods in which complex modes of the coupled structural-acoustic system are used to reduce the model to low order. The complex modes preserve the coupling of the structure and acoustic medium while also accurately modeling acoustic radiation. Predictions of random response from a complex reduced-order model (CROM) of a clamped beam in an unbounded acoustic domain are compared to results from a full-order coupled finite element model. Predicted beam displacements from a CROM agree well with full model simulations.

Acoustic Radiation From a Laminated Composite Plate Excited by Longitudinal and Transverse Mechanical Drives

Abstract: Analytical expressions are derived for the vibration and far-field acoustic radiation from a fluid-loaded laminated composite plate, which is excited by three types of mechanical drives: point, line, and uniformly distributed forces in longitudinal and transverse directions. Through numerical results, it is shown that the lamination schemes of the laminated composite plate play an important role in its far-field acoustic radiation, especially when the plies of the plate are asymmetric with its middle plane. This paper would help better understand the main mechanism of acoustic radiation from laminated composite plates, which has not been adequately addressed in its companion paper (Yin, et al., 2007, "Acoustic Radiation From a Laminated Composite Plate Reinforced by Doubly Periodic Parallel Stiffeners, " J. Sound Vibrat., 306, pp. 877―889).

Inversion of spinning sound fields.

Abstract: A method is presented for the reconstruction of rotating monopole source distributions using acoustic pressures measured on a sideline parallel to the source axis. The method requires no a priori assumptions about the source other than that its strength at the frequency of interest varies sinusoidally in azimuth on the source disk so that the radiated acoustic field is composed of a single circumferential mode. When multiple azimuthal modes are present, the acoustic field can be decomposed into azimuthal modes and the method applied to each mode in sequence. The method proceeds in two stages, first finding an intermediate line source derived from the source distribution and then inverting this line source to find the radial variation in source strength. A far-field form of the radiation integrals is derived, showing that the far-field pressure is a band-limited Fourier transform of the line source, establishing a limit on the quality of source reconstruction, which can be achieved using far-field measurements. The method is applied to simulated data representing wind-tunnel testing of a ducted rotor system (tip Mach number of 0.74) and to control of noise from an automotive cooling fan (tip Mach number of 0.14), studies which have appeared in the literature of source identification.

Prediction of acoustic radiation from axisymmetric surfaces with arbitrary boundary conditions using the boundary element method on a distributed computing system

Abstract: This paper presents a computational technique using the boundary element method for prediction of radiated acoustic waves from axisymmetric surfaces with nonaxisymmetric boundary conditions. The aim is to predict the far-field behavior of underwater acoustic transducers based on their measured behavior in the near-field. The technique is valid for all wavenumbers and uses a volume integral method to calculate the singular integrals required by the boundary element formulation. The technique has been implemented on a distributed computing system to take advantage of its parallel nature, which has led to significant reductions in the time required to generate results. Measurement data generated by a pair of free-flooding underwater acoustic transducers encapsulated in a polyurethane polymer have been used to validate the technique against experiment. The dimensions of the outer surface of the transducers (including the polymer coating) were an outer diameter of 98mm with an 18mm wall thickness and a length ...

Choosing of Optimal Voltage Amplitude of Four Pairs Square Piezoelectric Elements for Minimization οf Acoustic Radiation of Vibrating Plate

Abstract: The study is a next part of earlier works by the authors, and explores how does the sequence of activated actuators and the level of applied voltage affect the radiated acoustic energy. The analysis uses the finite element method for structural vibrations and combination of the finite element and the intensity hybrid method to assess the level of sound radiation. The vibrating element is a steel plate with glued on actuators, supported on one edge and excited by a harmonically variable, concentrated load with a constant amplitude value.

Stirring and mixing of liquids using acoustic radiation force.

Abstract: The possibility of using acoustic radiation force in standing waves for stirring and mixing small volumes of liquids is theoretically analyzed. The principle of stirring considered in this paper is based on moving the microparticles suspended in a standing acoustic wave by changing the frequency so that one standing wave mode is replaced by the other, with differently positioned minima of potential energy. The period-average transient dynamics of solid microparticles and gas microbubbles is considered, and simple analytical solutions are obtained for the case of standing waves of variable amplitude. It is shown that bubbles can be moved from one equilibrium position to another two to three orders of magnitude faster than solid particles. For example, radiation force in a standing acoustic wave field may induce movement of microbubbles with a speed of the order of a few m/s at a frequency of 1 MHz and ultrasound pressure amplitude of 100 kPa, whereas the speed of rigid particles does not exceed 1 cms under the same conditions. The stirring effect can be additionally enhanced due to the fact that the bubbles that are larger and smaller than the resonant bubbles move in opposite directions. Possible applications of the analyzed stirring mechanism, such as in microarrays, are discussed.

Vibro-magnetometry: Theoretical aspects and simulations

Abstract: In this article, we introduce the theory for vibro-magnetometry (VM) with computational simulations for an idealized experimental setup. A method based on acoustic radiation force and magnetic measurement has been proposed for interrogating the mechanical vibrations of a target immersed in fluid medium. In this method, ultrasound radiation is used to exert a low-frequency (in kHz range) force on a rigid magnetized target immersed in a viscoelastic medium. In response, the target vibrates sinusoidally in a pattern determined by viscoelastic properties of the medium. The magnetic field resulting from target vibration is related to both the ultrasonic and low-frequency (kHz range) mechanical characteristics of the medium. We report the relationship between the magnetic field signal and the incident ultrasonic pressure field in terms of the mechanical parameters of the medium. Simulations were conducted to demonstrate a simple approach based on using amplitude-modulated ultrasound to generate a dynamic acoustic radiation force on a magnetic target. The magnetic field generated by vibration of this target is then obtained and used to estimate the radiation-force-induced displacement as a function of time. It was observed that the intensity of the dynamic component of the magnetic field caused by the acoustic excitation is high enough to be registered by a conventional magnetic sensor. When a low stress is applied on a reactive magnetic target embedded in the medium, the subsequent oscillating magnetic field is measured by a dedicated magnetic sensor, yielding the applicable mechanical information of the host medium. The proposed methodology presents a powerful tool for evaluation of acoustic radiation force as well as the mechanical properties of soft materials.

The inverse problem of acoustothermography with correlation reception of thermal acoustic radiation

Abstract: For the one-dimensional inverse problem of acoustothermography with correlation reception of thermal acoustic radiation, an integral equation is presented and experimentally verified. A method of solving the inverse problem is proposed. The method is based on combining the correlation functions of thermal acoustic radiation that were obtained for different distances between the receivers.

Passive acoustic radiation control for a vibrating panel with piezoelectric shunt damping circuit using particle swarm optimization algorithm

Abstract: This paper presents a new acoustic radiation optimization method for a vibrating panel-like structure with a passive piezoelectric shunt damping system in order to minimize well-radiating modes generated from the panel. The optimization method is based on an idea of using the p-version finite element method(p-version FEM), the boundary element method(BEM), and the particle swarm optimization algorithm(PSOA). Optimum embossment design for the vibrating panel using the PSOA is first investigated in order to minimize noise radiation over a frequency range of interest. The optimum embossment design works as a kind of stiffener so that well-radiating natural modes are shifted up with some degrees. The optimized panel, however, may still require additional damping for attenuating the peak acoustic amplitudes. A passive shunt damping system is thus employed to additionally damp the well-radiating modes from the optimized panel. To numerically evaluate the acoustic multiple-mode damping capability by a shunt damping system, the integrated p-version FEM/BEM for the panel with the shunt damping system is modeled and developed by MATLAB. Using the PSOA, the optimization technique for the optimal multiple-mode shunt damper is investigated in order to achieve the optimum damping performance for the well-radiating modes simultaneously. Also, the acoustic damping performance of the shunt damping circuit in the acoustic environment is demonstrated numerically and experimentally with respect to the realistically sized panel. The simulated result shows a good agreement with that of the experimental result.

Acoustic Radiation from Two Concentric Cylindrical Shells with Periodic Supports and a Viscoelastic Perforated Outer Coating

Abstract: A solution is developed for the acoustic radiation from two concentric cylindrical shells which are connected by a set of periodic annular plates, and the outer of which is coated with a viscoelastic perforated layer. All the perforations in the coating have an identical cavity configuration with variable (thickness-dependent) cross-sections. The classical theory of Kennard type and the linear acoustic field equation are employed to simulate the two shells and their entrained fluid, respectively. It is assumed that only membrane motions exist in the annular plates, based on which a two dimensional membrane theory is employed to derive their reactive forces. The viscoelastic perforated coating is divided into many sub-layers each has an equivalent complex wave number by using a thick walled cylinder model. Transfer matrix method is then used to develop the relation between the quantities (velocity and pressure) at the inner and outer surfaces of the coating. The solution is found by using Fourier wave number transforms, and the stationary phase approximation is used to find an expression for the far field pressure. Comparison of numerical results for the two concentric infinite cylindrical shells without the outer coating to measured data from a cabin model is in good agreement. Far field acoustic pressure results are presented for two cylindrical shells with and without the viscoelastic perforated coating, which indicate that the viscoelastic perforated coating can effectively reduce the radiated sound pressure at middle and high frequency ranges. The influences of the coating parameters on far field acoustic pressure are also discussed.

Transition scattering in stochastically inhomogeneous media

Abstract: When a physical object ("a source") without its own eigenfrequency moves through an acoustically homogeneous medium, the only possible form of acoustic radiation is the emission of Mach shock waves, which appear when the source velocity surpasses sonic speed. In nonhomogeneous media, in nonstationary media, or in the neighborhood of such media, the source motion is accompanied by the so-called "transition" radiation (diffraction or scattering), which has place even when the source moves with subsonic velocity. Key features pertaining to the formation of the acoustical transition scattering in media with fluctuating acoustical parameters are established. To analytically study the effect, the Green's function method formulated in terms of functional derivatives is used. The relationship between the wave number and frequency, k=k(omega), for acoustic waves is found. The results serve to determine the phasing conditions necessary for opening the transition scattering and Cherenkov radiation channel and to establish the physical explanation for the phenomenon-scattering (transformation) on inhomogeneities of the accompanied source field; i.e., formation of radiation appears when the attached field readjusts back to the equilibrium state after being deformed while passing through the fluctuations of the medium.

Sound Radiation Modes of MDOF Waveguide Suppressor in Outer Fluid

Abstract: For a multiple degree-of-freedom (MDOF) waveguide suppressor attached to a large baffleplate, we achieved the analysis solution to this problem by replacing a MDOF waveguide suppressor as an equivalent Timoshenko beam. Based on the theory of acoustic radiation mode, we discussed the properties of acoustic energy radiated from MDOF waveguide suppressor and acoustic energy transfer in outer fluid, which indicate the patterns of acoustic energy radiation and transfer in fluid. The surface sound intensity and fluid sound intensity may be classified into two parts1 one part can propagate towards the far field, while the other part may exchange between suppressor and fluid within the near field, which means only when we curb the acoustic energy radiation patterns of waveguide suppressor propagating to the far field effectively, can we depress the sound radiation level in outer fluid.

Research on vibration and acoustic radiation of the submerged complicated shell structure with multiple excitations

Abstract: In order to evaluate the structural vibration and the acoustic radiation of underwater complicated shell structure with multiple excitations,a complicated multiple compartment shell structure,similar to real ships,is designed and the underwater vibration and acoustic radiation experiments with single and multiple excitations are implemented. The experimental results show that sound pressure caused by multiple excitations can be approximated as the incoherent superposition of those caused by each excitation separately.The transfer relations between the vibro-acoustical characteristics of the structures and the excitations are deduced analytically.The rationality and the error of the incoherent superposition are analyzed as well.It is indicated the incoherent superposition method can be used to calculate the vibration and sound radiation of the complicated shell structure with multiple excitations,and it has important significance to predict the real vibration response and acoustic field.

Effect of electrical circuits on duration of an acoustic pulse radiated by a piezoplate.

Abstract: By means of numerical computation, pulse mode of a piezoelectric radiator in the form of a plate, loaded with a liquid and connected to R-L circuits in various combinations, is investigated. The computations were carried out having taken into account the internal resistance of the electric generator. Numerical results, based on the theory, are provided for examples of parallel and series circuits connected to a piezoelectric plate. The optimal values of parameters are determined, providing minimal duration of radiated acoustic pulses.

Estimation of vibration amplitudes for resonating sensors immersed in liquids

Abstract: For a lot of vibrating sensors operating in liquids, it is difficult to accurately model the displacement amplitude exerted at the interface to the liquid. The presented approach provides an estimate by relating the power in the radiation resistance of the transducer's equivalent circuit to the acoustic radiation into the liquid. The only prerequisite for the application of the method is that a dominantly shear or normally polarized mode is present and that an effective area characterizing interaction between transducer and liquid can be estimated.