Acoustic Energy Dissipation Research Articles

Modeling of ultrasonic enhancement on membrane distillation

Ultrasonic technology is successfully applied to enhance the permeate flux of membrane distillation. In this study, two major mechanisms of ultrasonic enhancement on membrane distillation, ultrasonic cavitation and acoustic streaming, are formulated in an analytical modeling. Ultrasonic induced membrane vibration, acoustic energy dissipation in viscous fluid, and membrane absorption and transmission of acoustic energy are also included in the modeling approach. Based on this model, effects of ultrasonic intensity, ultrasonic frequency, solution temperature and excited membrane area on permeate flux of membrane distillation are theoretically analyzed. It is found that the enhancement ratio can be improved by increasing ultrasonic intensity, decreasing ultrasonic frequency or decreasing the solution temperature. The current study also demonstrates that an enhancement up to 200% can be reached with an ultrasonic intensity from 0 to 5 W/cm 2.

Extended reaction acoustic liner for jet engines and the like

An extended reaction acoustic liner for use in jet engine noise mitigation has a substantially non-porous outer layer, a honeycomb core disposed in laminar juxtaposition to the outer layer, and a porous inner layer in laminar juxtaposition to the honeycomb core such that the honeycomb core is sandwiched between the outer layer and the inner layer. The honeycomb core comprises a plurality of cell walls defining a plurality of cells. Some of the cells are in fluid communication with one another via the openings formed in the cell walls. The openings in the cell walls cause viscous acoustic losses, resulting in acoustic energy dissipation and enhanced noise attenuation.

Basal Heating in Main‐Sequence Stars and Giants: Results from Monochromatic Acoustic Wave Models

We calculate time-dependent models of acoustically heated chromospheres for main-sequence stars between spectral type F0 V and M0 V and for two giants of spectral type K0 III and K5 III assuming monochromatic waves. The hydrodynamic equations are solved together with the radiative transfer and statistical equilibrium equations to investigate the propagation of acoustic waves into the chromospheric regions. The emergent radiation in Mg II h + k and Ca II H + K is calculated and compared with observations. We find good agreement, over nearly 2 orders of magnitude, between the time-averaged emission in these lines and the observed flux emission, which had been suspected to be due to nonmagnetic (i.e., acoustic) operating in all late-type stars. The height dependence of the acoustic energy flux can be explained by the limiting strength property of the acoustic shocks and is consistent with that found in models of quiet solar regions. We also confirm the validity of the Ayres scaling law, which has originally been derived for semiempirical chromosphere models and is thus independent of assumptions about the chromospheric mechanism. Our results strongly support the idea that the basal heating of chromospheres of late-type stars as revealed by the frequency-integrated Mg II and Ca II line emission is fully attributable to the dissipation of acoustic wave energy.

High damping composite joint for mechanical vibration and acoustic energy dissipation

A design for a high damping composite joint which dissipates vibrations through the use of air gaps and viscoelastic material minimizing the transfer of vibrations to the metallic coupling. Viscoelastic material is used with adhesive to provide for increased energy dissipation by the joint. As the load increases on the joint, the load transfers from adhesive to the viscoelastic. The viscoelastic begins to take the load of the joint at the point where the adhesive becomes plastic. Acoustic vibrations are then dissipated in the viscoelastic and are prevented from being transferred to the metallic coupling by air gaps provided in the joint. The amount of viscoelastic and adhesive used depends on the anticipated load. Finite element analysis is used to calculate optimal amounts of viscoelastic and adhesive.

PHASE COMPENSATION FOR FEEDBACK CONTROL OF ENCLOSED SOUND FIELDS

A method of improving the active control of reverberant sound fields, using phase compensation within a local feedback control loop, is presented in this study. A coupled, feedback model of the reverberant sound field, including in bandwidth transducers, is developed to capture accurately the phase characteristics of the system response, particularly at low-frequencies which dominate closed-loop system performance and stability. Phase compensation is introduced to alleviate problems resulting from in-bandwidth transducers by constraining the open loop system response to emulate positive-real system behaviour over the bandwidth to be controlled. An increase in the dissipation of acoustic energy is observed from the approach presented herein, as opposed to direct output feedback control originally proposed by Olson and May [21].

Measurement of structural intensity

The modeling of structural behavior leads to simplified models of very often complex structures. To get a better understanding of the energy flow inside and out of the structures, the development of measurement methods is very important. Dynamic movements are associated with vibratory and acoustic energy dissipation. If the surface area is large compared to an acoustic wavelength, acoustic intensity is a good indicator of dynamic activity. If the area is small compared to a wavelength at the frequencies in question, surface measurements using the vibratory motion on the structure surface may be a good indicator. By transfer of energy between structural parts, point power measurements should be applied. Measurements may be carried out using acoustical techniques for determination of structure borne energy. The acoustical transducers used must be able to measure correctly in very reactive environments. A proper calibrated intensity probe will offer a noncontact measurement of an area, which may be small or large depending on wavelength for measurement of vectors caused by dispersed waves. The use of two accelerometers or an accelerometer and a force transducer may also be useful for detection of energy flow in a structure. Acoustic calibration of intensity probes has been improved to meet standard requirements. Calibration of accelerometers and force transducers is carried out by substitution measurements against a third transducer. Measurements on structures has found many practical applications.

Sound attenuation system for jet aircraft engines

The present invention relates to a novel sound attenuating system. More particularly, the invention relates to a simpler, lighter, and more effective noise attenuating laminate which is made up of five specific layers of material. The laminate includes a duct liner, a moisture barrier, a fire barrier, acoustic attenuating material, and a solid backing sheet. The laminate is readily inserted into various sections of a jet engine compartment in order to attenuate the sound produced by the jet engine. Hollow rivets are used to conduct acoustical energy to the noise attenuating laminate. This novel means for conducting the acoustical energy to the laminate of the jet engine compartment allows for an improved anti-icing system, for more efficient jet engine operation, and for more efficient dissipation of acoustic energy.

On the dipole acoustic source level of breaking waves

Wave breaking is believed to be the primary physical mechanism for generation of ambient noise. Yet previous measurements of ambient noise have been based on averaged results of noise sources randomly distributed in space and related primarily to wind speed. Recent progress in direct measurement of breaking waves has made it possible to examine the sound radiated from individual sources and relate it to breaking wave parameters. Here we present field observations of dipole acoustic source levels of breaking waves obtained during the surface wave processes program. The acoustic source level is related to measured breaking wave speed, which is believed to be an important parameter relevant to wave energy dissipation. It is found that the source level has a much better correlation with the breaking wave speed than with the wind speed. The data also indicate that the acoustic source level can be modeled as an exponential distribution. These results are explained using laboratory observations of acoustic radiation and wave energy dissipation due to breaking.

Perturbation of the electrified interface and the response of the thickness-shear mode acoustic wave sensor under conductive liquid loading

The behavior of the thickness-shear mode (TSM) acoustic wave sensor under conductive liquid loading has been examined by acoustic network analysis. The response of the TSM sensor arises from the perturbation of the electrified interface, including the acoustoelectric coupling of the surface potential with ionic species and the formation of the electrical double layer. In dilute solutions, dissipation of acoustic energy resulting from acoustoelectric coupling dominates the complete energy-transfer process, which leads to a rapid decrease of the series resonant frequency and in increase in the motional resistance

Toward linear macrosonics.

Large resonant acoustic pressure amplitudes were mechanically generated inside resonators filled with refrigerant HFC-134a. The nonlinear losses normally associated with resonant macrosonic waves were dramatically attenuated via resonators whose higher resonant modes were noninteger multiples of the fundamental. Such resonators reduced Q amplification of the fundamental’s harmonics, minimized viscous and thermal wall losses and minimized peak acoustic velocity. These noninteger resonators provided acoustic pressure amplitudes of up to 60 psi peak-to-peak for ambient pressures of 70 psi. At peak acoustic pressure amplitudes of 25% ambient, measured acoustic energy dissipation was within 34% agreement with strictly linear predictions. A high-amplitude resonator was used to construct a laboratory prototype of an oil-free resonant acoustic compressor. Compression ratios were provided via high-speed valves that converted acoustic pressure oscillations into an external discharge pressure. A 300-Hz discharge valve was tested that converted 95% of a resonator’s peak acoustic pressure amplitude into an external discharge pressure. Efficiency calculations indicate that resonant acoustic compressors could provide improved efficiency for home refrigerators. [Work conducted at Los Alamos National Laboratory in collaboration with Greg Swift and supported by DOE.]

Optimal design of sound-proof multilayer structures with allowance for acoustic-energy dissipation

Optimal design of sound-proof multilayer structures with allowance for acoustic-energy dissipation

Measurement and Description of Tensile Fracture in Granite

A study of the process zone in Charcoal and Rockville granites, average grain sizes of 1 and 10 mm, is conducted using the double‐cantilever‐beam and double‐edge‐notched geometries. The fracture process for mode I (tensile) opening is inferred from ultrasonic probing, where the extent of the inelastic zone is interpreted by the energy loss in surface wave transmission, and from acoustic emission, where the locations of microseismic events outline the region of interest. For Charcoal granite, a zone of major acoustic activity is observed to form a single, multiconnected region within the fracture. Crack growth in Rockville granite is more discontinuous. In general, the tensile fracture process seems to be a gradual separation of surfaces along a preferential path set up by microcracking. This development of crack propagation accounts for the dissipation of acoustic energy from unbroken and interlocked crystals within a localized process zone, the length of which may be a substantial portion of the effectiv...

Non-linear effects in finite amplitude wave propagation through ducts and nozzles

In this paper an extensive study of non-linear effects in finite amplitude wave propagation through ducts and nozzles is summarized. Some results from earlier studies are included to illustrate the non-linear effects on the transmission characteristics of duct and nozzle terminations. Investigaiations, both experimental and analytical, were carried out to determine the magnitudes of the effects for high intensity pulse propagation. The results derived from these investigations are presented in this paper. They include the effect of the sound intensity on the acoustic characteristics of duct and nozzle terminations, the extent of the non-linearities in the propagation of high intensity impulsive sound inside the duct and out into free field, the acoustic energy dissipation mechanism at a termination as shown by flow visualizations, and quantitative evaluations by experimental and analytical means of the influence of the intensity of a sound pulse on the dissipation of its acoustic power.

Damping of bubble oscillations induced by transport of surfactants between the adsorbed film and the bulk solution

At frequencies well below the resonance, the forced oscillations of a microbubble suspended in a surfactant solution are affected by the dilational elasticity of an adsorbed film and by the adsorbate exchange occurring due to bubble’s surface area oscillations. The model introduced for computation of the exchange rate uses the relaxation time of the adsorption–desorption process, on which the experimental data are available in the literature, as the governing parameter. An important manifestation of the surfactant exchange, as found by solving a linearized problem of the bubble motion coupled with the adsorbate flux, is that it induces a considerable dissipation of acoustic energy. Computations are presented for the driving frequencies (ω) ranging from 102 to 105 s−1 and bubble diameters 10–200 μm. The total damping of a clean bubble is also computed, for comparison. The predicted damping constant for a 20-μm bubble covered with a condensed film of a fatty substance can exceed that for a clean bubble by several orders of magnitude at ω<104 s−1.

On the Absorption of Sound by Turbulence and Other Hydrodynamic Flows

The dissipation of acoustic energy which occurs when sound generates vorticity at the boundaries of a fluid or propagates across a field of turbulence is examined. Vorticity generation occurs typically during scattering or diffraction by surfaces with corners or edges, and an understanding of the consequent attenuation is important in the design of mufflers and other devices used for suppressing acoustic and mechanical vibrations. The rate of dissipation is generally a non-linear function of the acoustic intensity, but becomes linear and significantly greater in the presence of a mean flow. When sound propagates through turbulence it is partly scattered and partly absorbed. Lighthill's (1953) theory of scattering is extended to include the possibility of acoustic absorption by a direct transfer of energy to the turbulent motions. A similar but greatly enhanced level of absorption takes place when sound propagates in turbulent pipe flow. The energy transfer occurs principally in the acoustic momentum and thermal boundary layers at the walls, whose properties are subject to the modifying influences of turbulence convection. This is discussed in relation to experimental data of Ronneberger & Ahrens (1977).

High amplitude acoustic transmission through duct terminations: Theory

Recent experimental measurements have demonstrated that net acoustic energy dissipation can occur when sound waves interact with free shear layers, which are produced either by boundary layer separation in mean fluid flow at sharp edges, or by separation of the boundary layer in the acoustic flow at an edge in the absence of mean flow. This paper presents theoretical results which are offered in an attempt to explain these observations quantitatively. Comparison is made between the predicted and measured net energy loss which occurs upon transmission of high amplitude impulsive acoustic waves through various duct terminations, and also between calculated and measured reflection coefficients in the duct. The agreement is generally at least qualitatively good, and would appear to justify the physical assumptions on which the theoretical arguments are based.

The influence of grazing flow on the acoustic impedance of a cylindrical wall cavity

The acoustic impedance at low frequencies of a circular cylindrical cavity in the wall of a duct in the presence of a low Mach number mean flow is examined. A linearized theoretical model is proposed which involves the unsteady shedding of vorticity from the upstream edge of the cavity aperture. The shed vorticity causes a “potential difference” to be established across the aperture which modifies the reciprocating volume flux and results in the dissipation of acoustic energy. Comparison of theoretical predictions with preliminary experimental data obtained by Parrott (1978, private communication) at the NASA Langley Research Center provides tentative support for the present analysis. Certain difficulties associated with the linearized treatment of cavity oscillations are also discussed.

Acoustic streaming

This lecture attempts to correct widespread misunderstanding of the nature of acoustic streaming, and to provide an adequate theoretical treatment of the vigorous, and usually turbulent, jet-like winds that are generated by powerful ultrasonic sources in air. It is appropriate to recognize (section 2) that there are two different kinds of mean motion (Lagrangian and Eulerian), differing by a multiple of the acoustic intensity (equation (16)); but much more important to recognize that both kinds vastly exceed this difference between them in typical cases of acoustic streaming. Acoustic streaming is forces by the action of a Reynolds stress, defined as the mean value of the acoustic momentum flux; but it is exclusively the dissipation of acoustic energy flux that permits gradients in momentum flux to force acoustic streaming motions (section 3). On the other hand, these do not tend to zero as a dissipation coefficient such as the viscosity μ tends to zero, because resistance to the motions so forced itself depends on μ; note also that several other mechanisms of acoustic energy dissipation need to be taken into account (section 4). The classical treatment [1, Nyborg 1953; 2, Westervelt 1953], of acoustic streaming due to the action of Reynolds stress forcing resisted by viscosity, ignored the effect of the fluid's inertia on the streaming motions. That theory is further developed in section 5, but shown to be valid only for acoustic sources of very low power. For high ultrasonic frequencies in fact (section 6), that theory is applicable only for powers up to 10 −6; by contrast, the acoustic streaming motion takes the form of an inertially dominated jet at powers above 3 × 10 −5W, and this becomes a turbulent jet above 4 × 10 −4W. Section 7 gives a comprehensive treatment of turbulent jets produced as attenuation of the energy flow in a general acoustic beam makes momentum flow available to force streming motions. Schlichting's relationship between eddy viscosity and momentum flow rate is used to obtain descriptions of the motion which are satisfactorily compared with observations of the powerful streaming generated by a 2W source at 20 kHz. The lecture's main purpose, then, is to explain how turbulent jets are generated by sound (the opposite of the more familiar process!). The lecture concludes, however, with a review of modern developments in the other main type of acoustic streaming, associated with the interactions between sound waves and solid boundaries (section 8). These are still dominated by the important equation (94) for the streaming motion's effective “slip” at the boundary, given originally in the second edition of Rayleigh's Theory of Sound [3].

Aeroacoustic fluid dynamic equations and their acoustic energy conservation corollary with O 2 and N 2 vibrational relaxation effects included

A formulation of fluid dynamic equations for air is given based on concepts of irreversible thermodynamics. An explicit expression for the instantaneous entropy function is derived from statistical physical principles and is approximated for the case when only O2 and N2 molecular vibrations are not in internal equilibrium. The fluid model includes an entropy balance equation based on this instantaneous entropy function. The model is complete; numerical values and formulas are included for all coefficients. In the latter part of the paper, an acoustic energy conservation theorem is derived which includes a non-negative expression for acoustic energy dissipation per unit time and volume. The latter is shown to be a lowest order approximation of temperature times the irreversible entropy production term in the entropy balance equation, even though this term does not appear in the linear acoustic equations. An independent derivation of the absorption coefficient of plane waves is given in which the energy theorem is utilized.

Selective damping of resonant acoustic waves in tubes

A study of acoustic energy dissipation in a resonant tube fitted at one end with a hollow rodis presented. The inside diameter of the rod is a small fraction of the tube's diameter, so that the nature of the acoustic field in the tube is, in general, not significantly affected by the opening. It is shown, however, that under certain conditions the fields in the rod and in the tube can be resonantly coupled, and that in those conditions the damping in the tube is substantially increased owing to energy transfer from the tube to the hollow rod, where viscosity and heat conduction act more effectively. The analytical results are compared to experimental data also reported here and found to be in close agreement.