A Quasi-Standing-Wave Phenomenon Due to Oscillating Internal Flow

An acoustic or electromagnetic wave propagating in a material medium experiences fluctuations in amplitude and phase that can be attributed in part to scattering from inhomogeneities due to variations in the refractive index of the transmission medium. Since such “patches of inhomogeneity” are often regarded as lenticular in shape, it is the purpose of this paper to extend, for both plane and spherical sound waves, contemporary wave-fluctuation theory (based on a Gaussian correlation function for refractive index) to include the circumstance of spheroidal inhomogeneities. Mean-square amplitude and phase fluctuations are considered for acoustic waves propagating in a statistically isotropic medium in which random deviations of the refractive index are small. Large-scale inhomogeneities of arbitrary spheroidal shape are treated, and the influence of transmitter-to-receiver range, acoustic wavelength, and “patch” dimensions on variations in wave amplitude and phase is derived for this general case. Particular attention is given the high-frequency ray limit (Bergmann region) and the low-frequency (Fraunhofer diffraction) region. The significant effects on wave fluctuations of both spherical and nonspherical inhomogeneities are compared for plane and spherical acoustic waves.

Helmholtz resonators are commonly used to damp thermoacoustic oscillations in aeroengine and gas turbine combustors. In practice, the Helmholtz resonator is often maintained at a cooler temperature than the combustor, but this tends to be neglected when modeling. In this work, we derive an analytical model for both the acoustic damping and the effect on the combustor thermoacoustics of Helmholtz resonators for which the volume is at a different temperature to the combustor. The energy conservation equation based on stagnation enthalpy, together with the mass and momentum conservation equations, is incorporated into a linear Helmholtz resonator sound-absorption model. These are coupled with linear wave-based models for the plane acoustic waves in the system to which the Helmholtz resonator is attached: here, a combustor. The temperature difference is found to generate an entropy wave that advects downstream of the resonator (and may generate further acoustic waves if accelerated) and to change the acoustic damping near the resonant frequency. This model for the Helmholtz resonator acoustics is then incorporated into a low-order network model for combustor thermoacoustics. The importance of capturing the effect of the Helmholtz resonator temperature difference on the overall thermoacoustic modes is clearly demonstrated.

In an array of microbarographs propagation characteristics of pressure fluctuations have been studied qualitatively. For one station the pressure data was digitized. It was found that acoustic and gravity waves can easily be distinguished on the recordings. The diurnal variation of the amplitude of acoustic waves was found to be mainly due to windinduced turbulence. There are highly significant correlations between the amplitude of both acoustic and gravity waves and classical meteorological parameters which can be summarized as follows: “the better the weather, the smaller the amplitude.” Under Foehn conditions the amplitude of gravity waves is increased while acoustic waves are highly damped.

Helmholtz resonators are a classic means to absorb low frequency acoustic waves with great effectiveness and straightforward implementation. Yet, traditional Helmholtz resonators are tuned to absorb a specific frequency of wave energy and are unable to adaptively tailor damping capability. On the other hand, recent research on elastomeric metamaterials offers concepts for large and tunable damping properties using internal constituents that buckle to trap large elastic energy when subjected to geometric or load constraints. The research reported here draws from the principles of Helmholtz resonance and constrained metamaterials to device a resonant metamaterial with tunable acoustic energy dissipation. Using the fluid-structure interaction of an internal beam-like member with the resonator chamber, the metamaterial absorbs acoustic waves at a target frequency and tailors energy dissipation by changing external constraints that relatively magnify or suppress the interaction between acoustic pressure and damped beam member. An analytical model is developed to qualitatively characterize the behavior of the metamaterial observed in the laboratory. From the combined experimental and analytical studies, it is found that the metamaterial may significantly reduce the sound pressure level at the targeted frequency range while modulating the broadness of the absorption effect by way of external load control.

Oil pipeline leakage will not only cause economic losses, but also pollute the environment, so the leakage detection of pipelines is very important. The acoustic wave method is widely used in pipeline leak detection, and the leak acoustic signal collected by the acoustic wave sensor often contains a lot of noise, which makes it impossible to accurately determine the inflection point of the signal curve and reduces the accuracy of pipeline leak detection. This paper proposes a denoising algorithm based on mutual information optimization complete ensemble empirical mode decomposition with adaptive noise combined with cross-spectral analysis. Compared with other methods, this method can accurately select the effective intrinsic modal function for signal reconstruction, the denoising effect is more obvious, and the original information is preserved to a greater extent. Acoustic waves are attenuated during the propagation process, and will be affected by factors such as pipe connection ports and elbows, making it impossible to accurately determine the amplitude of acoustic waves around the pipeline. According to the propagation characteristics of acoustic waves and various factors that affect the propagation of acoustic waves, this paper establishes a model for calculating the amplitude of acoustic waves, which can accurately determine the amplitude of acoustic waves everywhere in the pipeline. Finally, according to the model, the relationship between pipeline characteristics and detectable leakage rate is analysed. Field experiments show that the proposed model is accurate and the denoising algorithm is efficient. The minimum detectable leakage rate of the oil pipeline can reach 0.43% when the acoustic wave method is used for leak detection.

A vertical combustion wind tunnel was built in which confined turbulent diffusion flames were studied. Mea- surements were made of the mean soot volume fraction in turbulent ethylene flames using laser extinction. Velo- cities were measured with laser Doppler velocity, and radiation was detected with a wide-view-angle radiometer. The flames were burned under three conditions of axial pressure gradient: 0, - 32, and - 92 Pa/m. The acceler- ating flow was produced by a converging wind-tunnel working section. It was found that the latter pressure gra- dient caused the soot loading to fall by one-half, with a commensurate fall in the radiation emitted by the flame. The flame was shorter when it was accelerated. Turbulence intensities were also greater in the convergent duct. The results indicate the potential impact of pressure gradients on the relatively slow chemistry of soot formation. SOTHERMAL free shear layers such as turbulent jets have been studied for many years. More recently, the chemically reacting turbulent shear layer has been the object of consider- able attention. In almost all of the studies of both types of flows there were no mean pressure gradients. An axial pres- sure gradient may be imposed on a free shear layer if it is con- fined in a sufficiently large duct through which an external or outer flow is maintained. The duct may be either convergent or divergent. In the former case a favorable gradient is estab- lished and the outer flow accelerates; in the latter case the un- favorable pressure gradient slows the outer flow. Normally one would expect that a favorable pressure gradi- ent would tend to suppress turbulence in an isothermal flow. This is the concept behind the use of contractions on wind tun- nels. However, if combustion is occurring in the flow and there are significant density variations, then the response of the shear layer to the imposed mean pressure gradient may be different. In one of the few studies to look into this matter, Starner and Bilger1 performed measurements of the velocity field in a hydrogen air diffusion flame that burned in a wind tunnel. They made the interesting observation that the flame became shorter when either a favorable or unfavorable pres- sure gradient was imposed on the flow. Measurements of the velocities indicated that the favorable pressure gradient caused the turbulence intensities to be enhanced. The effect was as- cribed to a pressure-velocity correlation in the equation for turbulent kinetic energy. The experiment suffered from some flaws. The wind tunnel was horizontal, and, as a result, buoyancy forces tended to cause a circulation in the flow. In addition, the working sec- tion walls were adjusted in only one direction so that symme- try was not maintained. Consequently, the flames assumed an elliptical shape that gave rise to some difficulty in interpreting the experimental data. Nevertheless, the results from this ex- periment indicated that the confinement of a turbulent hydro- carbon flame in a convergent duct may have some impact on the slower chemistry in these systems. Among the chemical reactions in a hydrocarbon-air flame, those that lead to soot particles are probably the slowest. This has been demonstrated by Kennedy2 in a series of experiments

Lean premixed prevaporised (LPP) combustion can reduce NOx emissions from gas turbines, but often leads to combustion instability. Acoustic waves produce fluctuations in heat release, for instance by perturbing the fuel-air ratio or flame shape. These heat fluctuations will in turn generate more acoustic waves and in some situations self-sustained oscillations can result. A linear model for thermoacoustic oscillations in LPP combustors is described. A thin annular geometry is assumed and so circumferential modes are included but radial dependence is ignored. The formulation is in terms of a network of modules such as straight ducts and area changes. At certain operating conditions, the flow is predicted to be unstable, with linear waves growing in amplitude. Helmholtz resonators can be used to absorb acoustic energy and, when carefully designed and installed at appropriate locations, can stabilise the flow. Helmholtz resonators are included in the model. Connecting a Helmholtz resonator to an annular duct destroys the axisymmetry of the geometry. This results in coupling of the circumferential modes which must be calculated. The model is used to investigate the best arrangement of resonators around the circumference of an annular duct to achieve maximum damping of a circumferential oscillation.

Acoustic wave scattering and fluid perturbations produced by interactions between incident acoustic wave and confined orifice flow were investigated by a combined numerical solution containing nonlinear flow simulation and linearized acoustic simulation. In flow simulations, the mainstream Reynolds number was fixed at 10,000, which relates to the cooling pipe system of lithography. Turbulent flow fields corresponding to different orifice geometries were solved by an opensource finite volume solver OpenFOAM with Reynolds-averaged Navier–Stokes turbulence model. The turbulence database could efficiently improve the accuracy of subsequent linearized acoustic simulations as the viscosity dissipations were considered. In acoustic simulations, the linearized Navier-Stokes equations were solved by a finite element solver with transformation into frequency domain. The incident acoustic waves with varying frequencies from 500 Hz to 4000 Hz were arranged first at the inlet and then the outlet surfaces, enabling a two-port analysis on the transmission and reflection coefficients of acoustic waves. The numerical setup and the two-port model were well validated by results in literature. Generally, acoustic waves tend to gradually dissipate as their frequencies increase or the opening ratio of the ducted orifice decreases. However, the nonlinear variation in the transmission and reflection coefficients against the frequency variation of the incident acoustic waves could be investigated by increasing the thickness ratio. The acoustically induced fluid perturbations that were characterized by the Q-criterion could form a ring shape vortex structure in the vicinity of the orifice edge and then develop into disk-like packets. When the circumferential shape of the orifice was changed to a square, the attenuation of the incident acoustic waves corresponded to the intensity of the three-dimensionality of the acoustic-induced vortex structures, which indicated a greater energy transfer from the acoustic waves to the fluid perturbations.

The aeroacoustic response of a Helmholtz resonator, when attached to a wind tunnel via junctions with different neck geometries, has been examined. The wind tunnel consists of a 2-inch ID glass tube with variable mean flow at atmospheric pressure connected to a cross-junction with two coaxial 5-liter Helmholtz resonators. During the experiment, the acoustic pressure and corresponding frequency is measured in one of the Helmholtz resonators at different wind tunnel flow velocities. By knowing these quantities, the acoustic velocity of the air at the neck of the resonator can be determined. The ratio of the acoustic velocity to the flow velocity as a function of the Strouhal number can be studied for differing neck geometries. Helmholtz resonator neck geometries studied include 90-deg bends and straight sections in various combinations. [This work was supported by the Arkansas Space Grant Consortium and the University of Central Arkansas University Research Council.]

To meet the requirement for the low disturbance of wind tunnel flow in supersonic boundary layer transition simulation tests, a low-disturbance supersonic research wind tunnel with a nozzle size of 0.3 × 0.15 m2 and test Mach number of 3.0 was built. It was then used to study key design technologies for increasing the size and Reynolds number of low-disturbance supersonic wind tunnel. The wind tunnel adopted a “blowdown + ejection” layout to achieve a wide pressure range of 50–450 kPa. A “sintered wire mesh + honeycomb + damping mesh” structure was adopted in the settling chamber with the flow straightening method to achieve velocity pulsations of less than 0.3% and pressure pulsations of less than 0.1% at the exit. A comprehensive design technique for the supersonic laminar nozzle was adopted, which included nozzle boundary layer suction and highly polished surface nozzle processing. At a total pressure of 450 kPa, a low-disturbance supersonic test area with an axial length of about 9 cm was realized. The airflow pressure pulsations in this area were less than 0.15%, and the Reynolds number based on the characteristic length reached 1.3 × 106.

Examples are presented which show that the velocity, amplitude, attenuation and apparent frequency of several acoustic waves can be recorded in the borehole. Examination of such recordings, termed "character" logs, indicates that the wave types observed include a refracted compressional wave and a wave which travels with formation shear velocity.Laboratory data are used to show that compressional and shear wave velocities are dependent on porosity, effective stress and lithology; but that the change in reciprocal velocity per unit change in porosity is larger for shear waves than for compressional waves. We, therefore, conclude that the accuracy of porosity determinations can sometimes be improved by use of shear wave velocities, provided that the shear wave amplitudes are large enough to delineate the shear arrival from the preceding compressional arrival on the character log. Borehole data are presented which show that the difference between shear wave and compressional wave reciprocal velocities can be used to predict porosities. This is a refinement which may allow the prediction of porosities from single-receiver acoustic logs without introduction of errors from borehole fluid travel-times. Laboratory and field data are presented to show that the relationship between compressional and shear wave velocities can be used to indicate lithology.An example is presented to show that fractures usually cause a greater reduction in borehole shear wave amplitudes than in compressional wave amplitudes, an effect which may offer a more reliable means of detecting fractures.The complexity of the borehole acoustic wave train can make presently available cement bond logs highly sensitive to the gate and bias settings used. The character log offers a means to circumvent possible misinterpretations by recording all amplitudes, from which the interpreter can select the appropriate data for evaluating the cement bond.Character logs may also be used as a quality control for open-hole transit-time logs when existence of small compressional wave amplitudes interferes with the proper functioning of bias-controlled timing devices.Evaluation of the potential uses of character log data is not complete; but a character log presented in a form convenient for routine use would be a desirable addition to currently available logs. To summarize, possible applications for such a log in formation evaluation include the followingquality control of transit-time logs,refinement of porosity predictions,determination of lithology,improvement of fracture detection andimprovement of cement bond evaluation. Suggestions are made regarding the requirements for a sufficient but practical character log for routine use. Introduction Acoustic logs have become a widely used porosity tool in formation evaluation. In addition, there is a growing application of acoustic logs in cement bond evaluation and fracture detection. These applications have mainly involved the use of logs of first-arrival transit times and amplitudes and have not included detailed studies of the complete signal.The purpose of this paper is to show that significant benefits in formation evaluation can be gained by a more complete use of the acoustic wave train generated in the borehole by an acoustic logging tool. We hope that this discussion will also stimulate further development of logs suitable for routine use so that these benefits may be realized.Examples of acoustic wave train logs, termed "character" logs, are presented to show that several identifiable acoustic waves are present in the borehole. The measurable characteristics of these acoustic waves and some of their relations to formation properties of interest are also discussed. The more obvious potential uses of character logs are listed, and some suggestions are made regarding the requirements for a sufficient but practical character log for routine use. CHARACTER LOGS Some 10 years ago, Vogel and Summers and Broding noted that the signals received uphole from an acoustic logging tool located in a borehole had a number of interesting characteristics. The logging tool consisted of two or more pressure transducers spaced on an acoustically insulated body (Fig. 1a). One of the pressure transducers was used as a transmitter to generate pressure waves in the borehole fluid. The other transducer served as a receiver to detect any pressure waves reaching it in the borehole. The receiver then converted these pressure waves to electrical signals which were transmitted to the surface and displayed on an oscilloscope as a record of time vs receiver-signal amplitude. Fig. 1b is a schematic representation of a typical record. JPT P. 639^

Combustion instabilities are caused by a coupling between acoustic waves and unsteady heat release. Helmholtz resonators are widely used as acoustic dampers to stabilize unstable combustion systems. Such dampers are typically subjected to a low Mach number grazing flow and are normally only effective over a narrow frequency range, close to the resonant frequency. To increase the effective frequency range, a Helmholtz resonator with an oscillating volume, implemented via an electromagnetic shaker and vibrating backplate, was designed and experimentally tested at the University of Loughborough. It was found that volume oscillation can either increase or decrease the acoustic power being absorbed by the resonator, depending on the phase with which it is driven. A nonlinear numerical model of a Helmholtz resonator with an oscillating volume was then developed to simulate the experiments. Excellent agreement between the numerical and experimental results is found. Furthermore, insight into how to obtain maximum power absorption was provided by the numerical model and validated by the experiments. Finally, to optimize the phase in real time (by minimizing the amplitude of the pressure oscillations), active control of the backplate vibration was experimentally investigated. For the low Mach number grazing-flow regime investigated, this was found to give increased damping and to increase the effective frequency range of the resonator.

This study investigated the effects of using bionics in reducing the low-frequency fluctuation phenomenon in three-fourths open-jet automobile wind tunnels. Low-frequency fluctuation in these wind tunnels can cause pulsations of velocity and pressure in the test section, leading to measurement accuracy reduction. The unique shape of the seal vibrissa had been demonstrated to effectively suppress vortex-induced vibrations. Here the wind tunnel nozzle was optimized with seal-vibrissa-shaped protrusions. The effects and corresponding mechanism of seal-vibrissa-shaped nozzle toward jet shear layer vortex structures and low-frequency fluctuation phenomenon were analyzed. A 1:15 scaled three-fourths open-jet wind tunnel was used in the study. The results indicated that after the implementation of the seal-vibrissa-shaped nozzle, the turbulence intensity was decreased at the bottom of the test section and the pressure and velocity fluctuations were effectively suppressed. The three-dimensional vortex structures separating from the surface of vibrissa destroyed the original two-dimensional vortex structures produced by the original nozzle. Many small vortex structures with dispersed frequencies and energies were formed, which decreased the low-frequency fluctuations. The study showed that the seal-vibrissa-shaped nozzle had the potential of inhibiting the large-scale vortex separation in the jet shear layer, which could effectively reduce the low-frequency fluctuations.

We study the impact of electrical contact barriers in hybrid WSe2-LiNbO3-based acousto-electric and acousto-photoelectric devices using a combination of scanning photocurrent and acousto-electric current spectroscopy. Static scanning photocurrent measurements provide a qualitative measure of the band bending and spatial distribution of the Schottky barrier between semiconducting WSe2 and gold electrodes, whereas the surface acoustic wave-induced acousto-electric current reveals the height of the tunneling barrier created by the van der Waals gap between the multilayered WSe2 flake and the gold electrodes in addition to the Schottky barrier. The combination of both techniques shows a 10-fold increase in the photocurrent by the acoustic wave. Moreover, the observed spatial redistribution of the current is attributed to the interplay between the contact properties at the source and drain electrodes and the charge carrier dynamics induced by the surface acoustic wave. The ratio of the acoustic wavelength to the electrical channel length is found to impact the SAW-induced charge carrier transport. For a channel length shorter than one acoustic wavelength, carriers undergo a seesaw-like motion, which changes to charge conveyance for channel lengths comparable to or exceeding the acoustic wavelength.

In this paper, a direct method and a simple inverse method, which can be used to determine the velocity profile of a shear layer, are presented. Specifically, an infinite acoustic medium, with constant density and sound speed, containing a free shear layer of infinite extent, is considered. The free shear layer is probed with a two-dimensional plane wave, incident on the layer from outside, and it is assumed that the maximum Mach numbers for the flows in the shear layers are small, e.g., M=10−3 to 10−4 are typical values for oceanic shear layers. Then, the method of matched asymptotic expansions is employed to obtain an asymptotic expansion of the solution of the direct scattering problem as M→0 that is uniformly valid in the dimensionless wave number k and the angle α of the incident plane wave. It is found that the reflection coefficient of the scattered wave is proportional to the Fourier transform of the velocity profile of the free shear layer. This transform is inverted to obtain an asymptotic approximation of the solution of the inverse scattering problem. That is, given an incident plane wave and measurements of the reflected wave for a sequence of frequencies, approximations of the thickness and flow velocity distribution of the layer are determined. These results may be useful for studies that use acoustic exploration methods to determine oceanographic properties.