Rheology of a sonofluidized granular packing

We report experimental measurements on the rheology of a dry granular material under a weak level of vibration generated by sound injection. First, we measure the drag force exerted on a wire moving in the bulk. We show that when the driving vibration energy is increased, the effective rheology changes drastically: going from a non-linear dynamical friction behavior --weakly increasing with the velocity-- up to a linear force-velocity regime. We present a simple heuristic model to account for the vanishing of the stress dynamical threshold at a finite vibration intensity and the onset of a linear force-velocity behavior. Second, we measure the drag force on spherical intruders when the dragging velocity, the vibration energy, and the diameters are varied. We evidence a so-called “geometrical hardening" effect for smaller-size intruders and a logarithmic hardening effect for the velocity dependence. We show that this last effect is only weakly dependent on the vibration intensity.     

Amplitude and frequency characteristics of acoustic jets

The prerequisites to formation and the characteristics of a jet that occurs at the open end of a waveguide guiding an intense sound wave are studied. The velocity field is measured by a hot-wire anemometer. The previously developed method of separating the jet velocity V_jet from the amplitude of the oscillating particle velocity V_osc, which is applicable only when V_jet>V_osc, is supplemented with the method of oscillogram processing applicable for V_jet≤V_osc. Thus, a full picture of the jet evolution in space, starting from the waveguide outlet, is obtained. The experimentally determined spatial distribution of the jet velocity is found to agree well with the numerically simulated dependences reported by other authors. For the oscillating velocity amplitude at the open end of the waveguide, a threshold value, beyond which the formation of the acoustic jet takes place, is revealed. The frequency dependence of this threshold value is determined. The dependence of the maximal jet velocity on the oscillating velocity amplitude in the outlet waveguide cross section is found to be close to linear.     

Song of the dunes as a self-synchronized instrument

Since Marco Polo it has been known that some sand dunes have the peculiar ability to emit a loud sound with a well-defined frequency, sometimes for several minutes. The origin of this sustained sound has remained mysterious, partly because of its rarity in nature. It has been recognized that the sound is not due to the air flow around the dunes but to the motion of an avalanche, and not to an acoustic excitation of the grains but to their relative motion. By comparing singing dunes around the world and two controlled experiments, in the laboratory and the field, we prove that the frequency of the sound is the frequency of the relative motion of the sand grains. Sound is produced because moving grains synchronize their motions. The laboratory experiment shows that the dune is not needed for sound emission. A velocity threshold for sound emission is found in both experiments, and an interpretation is proposed.     

Non-linear effect of wall linings on sound attenuation in ducts

The equivalent surface source method is extended to the analysis of high intensity sound propagation in a duct whose wall is partially treated with a sound absorbing material. The propagation of sound in the gas is assumed to be linear, but the acoustic resistance of the sound absorbing material is assumed to be a function of the normal acoustic velocity. The problem is reduced to a non-linear integro-differential equation for the fluid particle displacement at the lined wall surface, which can be solved by a successive approximation method. Numerical examples show that the non-linear effect decreases or increases the peak sound attenuation rate of the lowest mode depending upon the linear component of the resistance. The dependence of the attenuation spectrum on modal phase difference of multi-mode incident waves is heavily affected by the non-linear effect. In the case of incident waves of multi-circumferential modes, different circumferential modes are generated by the non-linear effect.     

Low frequency sound produced by a ventilated supercavitating vehicle

An analytical investigation is made of the low frequency noise produced by gas jet impingement on the gas-water interface of a ventilated supercavity. Enclosure within a supercavity enables an underwater vehicle to attain high-speed forward motion. Whereas high frequency components of the cavity self-noise can interfere with the vehicle guidance system, low frequency sound tends to radiate in the water to large distances from the cavity. A canonical mathematical problem is examined that extends a previous study by Foley et al. (Journal of Sound and Vibration329 (2010) 415-424.) of sound generation by a specially modified, model scale supercavitating vehicle involving gas jet impingement at normal incidence to the interface. Our analysis determines the influence on low frequency sound production of cavity aspect ratio and the manner in which the efficiency of sound production increases with decreasing distance of the jet impact region from the circular ‘cavitator’ at the vehicle nose, where the supercavity is formed.     

Experimental examination of vortex-sound generation in an organ pipe: A proposal of jet vortex-layer formation model

Aero-dynamical models of sound generation in an organ pipe driven by a thin jet are investigated through an experimental examination of the vortex-sound theory. An important measurement requirement (acoustic cross-flow as an irrotational potential flow reciprocating sinusoidally) from the vortex-sound theory is carefully realized when the pipe is driven with low blowing pressures of about 60 Pa (jet velocities of about 10 m/s). Particle image velocimetry (PIV) is applied to measure the jet velocity and the acoustic cross-flow velocity over the mouth area at the same phase by quickly switching the jet drive and the loudspeaker-horn drive. The vorticity of the jet flow field and the associated acoustic generation term are evaluated from the measurement data. It is recognized that the model of the “jet vortex-layer formation” is more relevant to the sound production than the vortex-shedding model. The acoustic power is dominantly generated by the flow–acoustic interaction near the edge, where the acoustic cross-flow velocity takes larger magnitudes. The acoustic generation formula on the vortex sound cannot deny the conventional acoustical volume-flow model because of the in-phase relation satisfied between the acoustic pressure at the mouth and the acoustic volume flow into the pipe. The vortex layers formed along both sides of the jet act as the source of an accelerating force (through the “acceleration unbalance”) with periodically alternating direction to oscillate the jet flow and to reinforce the acoustic cross-flow at the pipe mouth.     

Suppression effect of jet flow on pulsating pressure of cavity using scale-adaptive simulation model

The suppression of the aerodynamic noise in the cavity has a great significance to solve relevant puzzles of weapon bays.Acoustic field of the standard cavity model is simulated by using the computational fluid dynamics technology based on scale-adaptive simulation(SAS)model.The results obtained by the proposed method in this paper show reasonable agreement with experiments.On the basis of this,effect of different jet flow rates on the time-averaged variables,turbulent kinetic energy,root mean square(RMS)of sound pressure,sound sources distribution and the pulsating pressure distribution in the cavity is studied.The analysis shows that the jet flow has great influence on the cavity flow field and the distribution of pulsating pressure RMS by changing the morphology of the shear layer.The most obvious of these measures is spout4 configuration,the influence mainly in the form of reducing the pulsating pressure of the whole cavity and changing the sound pressure level in the far field.The results show that different jet flow rates have different control effects on pulsating pressure in the cavity and sound pressure level in the far field.Furthermore,the jet flow rates and the suppression effect on the pulsating pressure have no linear relation.     

Finite element simulation of non-linear acoustic generation in a horn loudspeaker

The loudspeaker is an electro-acoustic device for sound reproduction which requires the distortion as small as possible. The distortion may arise from the magnetic non-linearity of the york, the uneven magnetic field distribution, the mechanical non-linearity at the diaphragm suspension and the acoustic non-linearity due to the high sound pressure and velocity in the duct-radiation system. A horn is sometimes provided in front of the vibrating diaphragm radiator, which plays an important role to increase the efficiency by matching the acoustic impedance between the radiator and the ambient medium. The horn is in many cases folded twice or three times to shorten the length, which further degrades the reproduction quality. The sound intensity and velocity are apt to attain very high in the small cross-sectional area in the throat and in the folded regions, which may cause the distortion due to the non-linear effect of the medium. The present paper is to investigate the frequency characteristics of the loudspeaker numerically evaluating the generation of the harmonics and sub-harmonics. An axisymmetric folded horn is considered for which the wave equation with the non-linear term retained is solved by the finite element method. The solution is made in time domain in which the sound pressure calculated at the opening end of the horn is Fourier-transformed to the frequency domain to evaluate the distortion, while the wave marching in the horn is visualized.     

受激布里渊散射的非线性增强和饱和

针对典型激光聚变等离子体参数条件,利用弗拉索夫程序研究非均匀流等离子体中受激布里渊散射的非线性行为。在动理学效应占主导的参数区域,观察到受激布里渊散射激发的离子声波由于非线性动理学频移和非均匀流空间失谐相互补偿引起的离子声波自共振增长,这会导致受激布里渊水平量级的增强;提出用光束时间去相干抑制这种绝对增长。在流体非线性占主导的参数区域,观察到由于离子声波谐波导致的孤立波产生、离子加热以及受激布里渊散射饱和现象。     

Direct computation and aeroacoustic modelling of a subsonic axisymmetric jet

A numerical algorithm for acoustic noise predictions based on solving Lilley's third order wave equation in the time-space domain is developed for a subsonic axisymmetric jet. The sound field is simulated simultaneously with the source field calculation, which is based on a direct solution of the compressible Navier-Stokes equations. The computational domain includes both the nearfield and a portion of the acoustic farfield. In the simulation, the detailed sound source structure is provided by the nearfield direct numerical simulation (DNS), while the sound field is obtained from both the DNS and the numerical solution to the non-linear Lilley's equation. The source terms of Lilley's equation are used to identify the apparent sound source locations in the idealized axisymmetric low-Reynolds number jet. The sound field is mainly discussed in terms of instantaneous pressure fluctuations, frequency spectra, acoustic intensity and directivity. A good agreement is found between the predictions from the axisymmetric Lilley's equation and the DNS results for the sound field. Limitations and perspectives of the simulation are also discussed.     

Experimental study of targeted energy transfer from an acoustic system to a nonlinear membrane absorber

This paper deals with the application of the concept of targeted energy transfer to the field of acoustics, providing a new approach to passive sound control in the low frequency domain, where no efficient dissipative mechanism exists. The targeted energy transfer, also called energy pumping, is a phenomenon that we observe by combining a pure nonlinear oscillator with a linear primary system. It corresponds to an almost irreversible transfer of vibration energy from the linear system to the auxiliary nonlinear one, where the energy is finally dissipated. In this study, an experimental set-up has been developed using the air inside a tube as the acoustic linear system, a thin circular visco-elastic membrane as an essentially cubic oscillator and the air inside a box as a weak coupling between those two elements. In this paper, which mainly deals with experimental results, it is shown that several regimes exist under sinusoidal forcing, corresponding to the different nonlinear normal modes of the system. One of these regimes is the quasi-periodic energy pumping regime. The targeted energy transfer phenomenon is also visible on the free oscillations of the system. Indeed, above an initial excitation threshold, the sound extinction in the tube follows a quasi-linear decrease that is much faster than the usual exponential one. During this linear decrease, the energy of the acoustic medium is irreversibly transferred to the membrane and then damped into this element called nonlinear energy sink. We present also the frequency responses of the system which shows a clipping of the original resonance peak of the acoustic medium and we finally demonstrate the ability of the nonlinear absorber to operate in a large frequency band, tuning itself to any linear system.     

ACOUSTIC IDENTIFICATION OF COHERENT STRUCTURES IN A TURBULENT JET

Fluid-dynamic events associated with noise generation in a subsonic jet are educed by conditioning in-flow velocity and pressure signals on farfield sound measurements. The jet is located in an anechoic chamber, and farfield noise measurements are performed simultaneously with in-flow anemometric and acoustic measurements at a number of distances x from the nozzle (0?x/D?20, with D the jet diameter). The experimental data are then analyzed with a conditional averaging procedure using peaks in the acoustic signal as a trigger. An analysis of the method is developed and supported by numerical simulations. The averaging procedure permits the identification of the average time signatures of in-flow velocity and pressure associated with noise-generating coherent structures in the flow, their position at the emitting instants and their temporal statistics. The physical properties of the events associated with the averaged time signatures are then discussed.     

Acoustic-excited vortex shedding and acoustic nonlinearity at a rectangular slit with bias flow

The acoustical response of a slit with a mean bias flow is numerically studied. By means of a potential flow model based on the discrete vortex method and a spanwise-averaged three-dimensional Green?s function, both unsteady vortical flow and slit impedance are obtained in a unified theoretical framework. The numerical simulation focuses on the acoustic-excited vortex structures of the slit flow while neglecting the viscous damping effect. Three representative flow features are demonstrated, which are the destabilized jet flow, the rolling up of vortex sheets and formation of vortex pairs, and the reversal flow with alternating vortex shedding on both sides of the slit. These features are corresponding to low, moderate, and high sound amplitude, respectively. The acoustic behavior of the slit can be divided into linear, transition, and nonlinear regimes. During its evolution through the three regimes, the resistance exhibits a constant value, a slight decrease, and a significant increase with the increasing sound amplitude. Correspondingly, the reactance first remains constant and then shows a modest decrease as the sound amplitude increases. The nonlinear effect also causes the gradual decrease of the mean bias velocity in company with the marked increase of the amplitude of the fluctuating velocity in the slit. The mean bias velocity decreases to about 80 percent of its linear value at the transition point where reversal flow begins to occur, and further decreases to only 10 percent in the highly nonlinear region. The slit impedance is also presented as a function of frequency and for different aspect ratios. And the effects of frequency and slit geometry are discussed.     

The instability of the surface of helium crystal in a superfluid flow

The problem on crystal growth under conditions of normal incidence of fluid on the boundary is investigated for stability. The threshold velocity of the emergence of instability is found; at low temperatures, this velocity proves to be much lower than the sound velocity. The stability is examined of the shape of cylindrical crystal in a fluid flow parallel to the crystal axis. The behavior of the atomically rough surface of crystal helium is experimentally investigated in a jet of fluid in the temperature range from 1 to 1.4 K, where the emergence is observed of an instability of the type previously predicted by Kagan, as well as by Nozieres and Uwaha. Observations reveal that, below the roughening transition, the (0001) basal face is stable in a jet of fluid.     

Linear Analyses of Langmuir and EM Waves in Relativistic Hot Plasmas

The linear Langmuir and electromagnetic (EM) waves in relativistic hot plasmas are discussed, and the dispersion relations are obtained based on the covariant Maxwell's and fluid equations. When kBT/mc²>1, the effective mass of electrons will be increased obviously. As the results, many other influences are induced, such as the decrease of the plasmas frequency and the critical frequency, the reduction of the electron sound velocity and the electrons' oscillation velocity, and so on. Numerical results show that these influences can affect the dispersion relations of Langmuir and EM waves seriously even in linear regime.     

Non-linear acoustic radiation by an underwater vibrating spherical shell

I.IntroductionMostofthepublishedstudiesontheacousticwaveradiationfromelasticbodyconcen-tratedon1ineartheory.However,iftheamplitudeislargeandthefrequencyishigh,theresultingsoundwavepropagatinginthemediumwillgenerateconsiderablenonlineardistor-tion.Sothest.di.s[1]inthisfieldarelimited.Inthispaperwetreatedasphericalshell.Itconsistsoftwoaspects:oneisthestructuralresponseofasphericalshellinacousticfield.Becausethenonlineareffectofacousticwavecumulateswithpropagatingdistance,itisnegligibleonthesph…     

On combustion generated noise from turbulent diffusion gaseous flames

A study is made into the mechanism of noise emission process from an ‘inverse’ open turbulent diffusion gaseous double-concentric jet flame. Correlations are presented between the mean temperatures, mean and fluctuating velocity, and the fluctuating pressure on one hand and the overall noise emission on the other from the natural gas-air flames and the He-air jets. Time mean and temporal component of velocity were measured using a laser doppler anemometry system. The LDA was operated in the fringe mode with a radial rotating diffraction grating. Signal processing was carried out by a counter linked to an on-line mini computer.A comparison is presented between the experimental results and the theoretical predictions based on a rational correlation and shows a good agreement for the peak frequency of the radiated sound power.     

Translational oscillations of a microsphere in superfluid helium

The translational motion of a microsphere (radius 100 μm) in liquid helium is investigated. The sphere is levitating inside a superconducting capacitor and oscillates about its equilibrium position. The velocity amplitude and the resonance frequency are measured as a function of driving force and temperature (0.35 K up to 2.2 K). By increasing the driving force we first find a linear regime (laminar flow) which changes abruptly into a nonlinear one (turbulent flow). For temperatures below 0.7 K the linear drag is given by ballistic roton and phonon scattering whereas for temperatures above 1.1 K the hydrodynamic force on the sphere is described by Stoke's solution. In the turbulent regime, above a temperature independent threshold velocity, we find the drag force to be given by turbulence in the superfluid component plus an essentially laminar drag by the normal component.     

Renormalized theory of magnetic field generation in resonance absorption

DC magnetic field generation in resonance absorption is studied, in a non-linear regime, when it becomes of sufficient order of magnitude to affect wave propagation, as well as electron-ion collisions or thermal dispersion do. It is shown that a simple expression obtained in the linear regime is still valid in the non-linear theory. Scaling laws are set up.     

On the role of glottis-interior sources in the production of voiced sound

The voice source is dominated by aeroacoustic sources downstream of the glottis. In this paper an investigation is made of the contribution to voiced speech of secondary sources within the glottis. The acoustic waveform is ultimately determined by the volume velocity of air at the glottis, which is controlled by vocal fold vibration, pressure forcing from the lungs, and unsteady backreactions from the sound and from the supraglottal air jet. The theory of aerodynamic sound is applied to study the influence on the fine details of the acoustic waveform of "potential flow" added-mass-type glottal sources, glottis friction, and vorticity either in the glottis-wall boundary layer or in the portion of the free jet shear layer within the glottis. These sources govern predominantly the high frequency content of the sound when the glottis is near closure. A detailed analysis performed for a canonical, cylindrical glottis of rectangular cross section indicates that glottis-interior boundary/shear layer vortex sources and the surface frictional source are of comparable importance; the influence of the potential flow source is about an order of magnitude smaller.