Acoustic Signal Fluctuations Research Articles

Detection of Wavy Sea Surface Oil-Derivative Contamination with Forward Specular High-Frequency Scattering

Abstract A spectrum of low-frequency (20–30 Hz) amplitude fluctuations of the ultrasonic (10 MHz) signal specularly scattered from water surfaces covered with monomolecular and thicker crude oil origin films of well-defined, oceanographically relevant viscoelastic properties was examined in laboratory and at-sea conditions. The relationship between the surface water wave (30 Hz) damping coefficient and the oil layer thickness was established, and compared to the one predicted by the classical Stokes theory. The depression of the spectral energy density of wind-driven waves by surface films was inferred from the ratio of acoustic signal fluctuations spectra with/without films, and compared to that resulting from the Marangoni damping theory applicable to monolayers of particular surface viscoelasticity. The agreement between the theory and experimental data was satisfactory. As shown in at-sea experiments performed with a free-floating, buoy-like acoustic system, and an artificial oil slick spread over the Baltic Sea surface, the film’s rheological surface properties can be recovered from acoustic surface probing, as well as oil spill edge detection. Simultaneous statistical analyses of the scattered signal amplitude distribution parameters turned out to be unequivocally related to the oil substance fraction weight, oil layer thickness, and the form of oil contamination.

Numerical modeling for characterization of CO2 bubble formation through submerged orifice in ionic liquids

Industrial applications of Ionic liquids (ILs) in CO2 capture are still limited due to lack of research on CO2-ILs bubble dynamics. A CFD model was developed to understand CO2 bubble formation in the pool of ILs by coupling improved VOF (Volume of Fluid) method with FW–H (Ffowcs Williams and Hawkings) acoustic formulation. Conventional VOF method was improved by adding suitable drag force for CO2-ILs interaction and ILs viscosity was considered as a variable with dissolved CO2. The predicted results are in good agreement with experimental measurements. Based on current CFD model, distinct acoustics pressure signals were examined to characterize CO2 bubble formation stages (growth, necking, detachment). It was investigated that bubble diameter and detachment time were increased with increasing of orifice diameter. Moreover, effects of gas and ILs inlet velocities were explained by fluctuations in Reynolds number and acoustic pressure signals respectively. Higher dissolved CO2 quantity and earlier bubble detachment time were obtained in aqueous ILs than pure ILs. The results have potential to give some useful guidelines to design industrial gas distributor (multiple-holes) for CO2-ILs system.

Acoustic and heat release signatures for swirl assisted distributed combustion

The acoustic signal and heat release fluctuations are examined from a swirl combustor using methane as the fuel. The focus was on flame stability and noise emission that have direct relevance in further developing distributed combustion for gas turbine applications and oxy-fuel combustion. Three regimes are examined in this paper, the first being a swirl mode at equivalence ratios between 0.9 and 0.55. The second one being distributed combustion, achieved through N2/CO2 dilution to reach oxygen concentration below 15%, fostering distributed reaction. The third was oxy-fuel flame using increased amounts of CO2 dilution to reach distributed reaction. For the first case, lowering the equivalence ratio led to a reduction in the peak sound pressure level around 500Hz and a decrease in heat release fluctuations. For all the equivalence ratios, close coupling between acoustic signature and heat release fluctuations existed around 200Hz. Distributed combustion, achieved at oxygen concentration below 15%, showed a much lower peak sound pressure levels at the 500Hz range with no coupling between heat release fluctuations and acoustic signal, outlining the flame stability at this regime. Also, the noise emission levels were significantly reduced under this mode. For the third regime, increase in CO2 dilution resulted in high heat release fluctuations and an unstable flame which oscillated between two different flame modes, a feature that did not exist in the first two regimes. Further increase in CO2 led to achieving distributed reaction and a much more stable flame as compared to its oscillatory behavior at lower CO2 amounts, along with reduced noise emission levels. This outlines the possibility of achieving distributed combustion in a stable manner via CO2 dilution in oxy-fuel flames.

Laboratory measurements of sound propagation in a continuously stratified ocean containing internal waves

A tank experiment is being conducted to explore sound propagation in a continuously stratified ocean containing internal gravity waves that produce fluctuations in acoustic signals. The tank is filled with water of increasing salinity with increasing depth in the fluid; the variation in salinity results in a 150 m/s variation of the sound speed over the 50 cm-depth fluid. Sound arrival times determined in three-dimensional hydrophone scans map out the sound diffraction pattern in the stratified fluid. Good agreement is found between the measurements and results obtained from wavefront modeling by ray acoustics. Sound fluctuations produced by internal waves are measured for several acoustic track locations and at different phases of the internal waves that radiate from a mechanical wave generator. Internal wave fields measured by particle image velocimetry provide input for a parabolic equation model, which is used to predict the sound fluctuations for comparison with measurements. This research is designed to improve the understanding and modeling of sound propagation and scattering by internal waves in the realistic oceans. [Research support: The 2013-14 ASA F. V. Hunt Postdoctoral Research Fellowship (L. Zhang) and ONR MURI Grant N000141110701 (WHOI).]

Modification of receiver operating characteristic curves for ocean variability

In this study, acoustic signal fluctuation in a continental shelf break is analyzed to examine the effects of detection probability and false alarm probability due to internal waves frequently observed on the east coast of Korea. Internal waves induce ocean temperature changes with time and space, and thus cause acoustic signal fluctuation. Internal waves are analyzed by measuring vertical ocean temperature changes with time using three thermister chains placed at three different positions. An acoustic signal is also measured during internal wave events. The observed internal waves have the following characteristics: a typical period of 25 min, a wavelength of 1,200 m, and an average amplitute of 5 m. The acoustic fluctuation parameters are estimated to be 3.7, 3.9, and 4.8 at frequencies of 300, 500, and 700 Hz, respectively. As a result, using a modified receiver operating characteristic curve, it is concluded that the detection probability decreases from 95 to 64% at a false alarm probability of 1% and increases from 23 to 44% at a false alarm probability of 10−4% at a frequency of 300 Hz.

Modeling mid-to-high frequency acoustic signal fluctuations induced by fetch limited sea surface roughness in shallow water

Surface waves are among several environmental parameters that significantly influence mid-to-high frequency (1–18 kHz) acoustic wave propagation. In coastal regions, surface waves are fetch limited with reduced spectral level at lower frequencies. In order to assess the detail of an acoustic signal interaction with the sea surface in such regions, a combined approach based on experimental observation and modeling of both surface waves and acoustic waves has been adopted. Data from two broadband shallow water acoustic experiments are presented. These data include simultaneous wind speed and acoustic propagation measurements. The experimental design allowed an examination of received signals corresponding to single surface bounced ray paths. Measured data analysis shows a high correlation between time–angle–intensity fluctuations of received signals and varying sea surface conditions. An empirical fetch-limited ocean wave spectrum has been combined with an acoustic ray-based model to study acoustic wave propagation. Rough sea surface realizations are generated and used as sea surface boundaries with the acoustic model. This combined sea surface/acoustic model predicts the variability of acoustic signal fluctuations as a function of varying sea surface. Modeled time–angle–intensity signal fluctuations compare excellently with field data at lower wind speeds.

The effects of the ocean environmental variability on sound propagation and reverberation from ASIAEX (ECS)

During the duration of the Asian Seas International Acoustics East China Sea Experiment, the ocean environment (e.g., wind speed, sea state, internal wave) varied significantly. Propagation and reverberation data from explosive charges recorded by a vertical line array and horizontal line array at different times during the experiment show that the acoustic data have a strong fluctuation. In this paper, the recorded data are analyzed to estimate the relationship between the acoustic signal fluctuation and the environmental variability. A PE code is used to simulate the effect of the recorded ocean environmental variability on the sound signal fluctuation. The experimental data and numerical results indicate the strong dependence between the acoustic fluctuation and ocean environmental variability. A coupled normal-mode-based model is also used to explain some of the acoustic fluctuation. [Work supported by the National Natural Science Foundation of China and ONR.]

Influence of fetch limited surface roughness on mid-to-high frequency acoustic propagation in shallow water

Surface waves are among several environmental parameters that can influence broadband mid-to-high frequency (1–18 kHz) acoustic wave propagation. Understanding the interaction of sound waves at a rough surface requires a detailed description of the ocean wave spectrum. In shallow water regions, due to proximity to land, surface waves are usually fetch and duration limited with reduced spectral level and higher frequency components. A model that approximates the wave spectra for a fetch limited sea has been combined with acoustic ray-based method for analysis of forward scattered acoustic signals in such a coastal environment. Numerical modeling is employed to investigate time-frequency-angle characteristics of ray paths reflected from a rough sea surface. Temporal variability of acoustic signal fluctuations has been examined as a function of varying sea surface. To validate this model, results are compared against a unique set of experimental data collected in a fetch limited region. The experimental design allowed an examination of time evolution of a single surface bounced ray path. Simultaneous wind speed and acoustic propagation measurements allowed correlation between the individual ray paths and the sea surface at varying sea state conditions. For low wind speed conditions, model results predict the temporal fluctuations of the measured acoustic signal propagation.

Acoustic wave propagation in atmospheric boundary layer above a rough terrain

In this paper we will present the latest development in the numerical simulation of acoustic wave propagation in the lowest atmospheric boundary layer above a rough terrain with a finite difference time domain (FDTD) algorithm. It is of great interest to investigate the effect of dynamic turbulent boundary layer meteorology coupled with surface terrain on acoustic wave propagation in frequency bands relevant to battlefield sensor systems. Turbulent mixing in the vicinity of the earth–air boundary layer is the key factor contributing to acoustic signal fluctuations such as upward refraction, scattering, high attenuation, and coherence degradations. Using first principles acoustic wave equation expressions for a moving fluid will allow us to couple acoustic models with time varying, 3-D large eddy simulation (LES) results. The specific meteorological parameters are vector wind velocity, temperature, pressure, and density as a function of space and time. Our acoustic simulation uses a coupled system of first-order temporal and spatial differential equations relating wind velocity, pressure, and density variations. This system of equations was evaluated with an explicitly time-stepping finite difference numerical approach. The coupling of LES and acoustic FDTD simulation will allow us to directly identify spectral portions of turbulent boundary layer meteorology contributing to short- and long-term acoustic signal variations.

Normal-mode analysis for signal fluctuations in the Yellow Sea

This paper presents a normal mode approach for analyzing signal fluctuation in shallow water with a thermocline which fluctuates due to tides and internal waves. In the Yellow Sea 1996 experiment, variations of thermocline structure and internal waves were observed by CTD and thermistor chains. The cw and FM acoustic signals were measured over a 12-h period with two 16-hydrophone vertical arrays deployed 3.11 and 10.32 km from the source. The cw pulses were used to observe fluctuation at four specified frequencies. Amplitude fluctuations of more than 20 dB were observed. The FM signals were used to isolate individual normal modes by pulse compression and mode filtering. The influence of internal waves on the normal-mode structure and signal fluctuation was examined. It was found that the variation of the thermocline was significant in determining acoustic signal fluctuation. The normal-mode structure and dispersion relations in the channel depend strongly on the thermocline. Variation of the thermocline thus causes phase and group velocity to become variable. This leads to signal fluctuation, since the amplitude of the received signal is determined by interference between arriving modes. The study also showed that the amplitude fluctuation is weaker at shorter range.

Large-array acoustic signal processing in random deep-water channels: Effects of coherence

Approximate analytical solution of the matrix transport equation for the mutual coherence function (MCF) of an acoustic field in terms of normal modes was derived. The proposed approach is capable of including stochastically rough surface as well as volume fluctuations in the index of refraction, and allowed one to reduce the problem of the MCF calculation in a refractive channel to the analogous problem in a free space. These results have been then employed to simulate the statistical effects of long-range sound propagation in realistic deep-water environments from the North-West Pacific on the horizontal array detection performance for several beamforming techniques. The most attention was paid to the gain degradation for the sound frequency of 250 Hz over megameter ranges under the basic assumption that internal waves or surface wind waves are, respectively, the main source of acoustic signal fluctuations. Rough surface scattering has been shown to cause the most significant effects. In particular, for the array length of order 100 acoustic wavelengths and wind speed of 15 m/s the coherence loss was of about 6 dB even for optimal quadratic processor and, in comparison, of about 12 dB for conventional plane-wave beamformer. [Work supported by Russian Basic Research Foundation, Grant No. 94-02-04544a.]

Acoustic propagation through anisotropic, surface-layer turbulence

The effects of atmospheric wind and temperature fluctuations on acoustic signal variability is discussed, with emphasis on the effects of large-scale turbulence (motions having size larger than or comparable to the integral length scale). Such large-scale turbulence is anisotropic, is generated by both shear and buoyancy instabilities, and has structure that depends strongly on the meteorological conditions as well as the distance from the ground. Previous research in the atmospheric sciences literature regarding length scales and anisotropy is reviewed and incorporated into an acoustic propagation model. The model is based on a multiply scaled, six-termed, sound-speed correlation function. A second, simpler model, based on fluctuating curvature of the vertical wind profile, is also proposed. Both models are compared with experimental measurements of amplitude and travel-time fluctuations obtained during the Rock Springs Tomography Experiment, which involved concurrent monitoring of acoustic fluctuations and atmospheric turbulence statistics. An important conclusion is that large-scale turbulence is a significant cause of acoustic signal fluctuations, particularly in the signal phase.

Acoustic Tomographic Monitoring of the Atmospheric Surface Layer

Abstract Acoustic tomography is proposed as a method for monitoring near-surface atmospheric temperature and wind velocity fields. Basic issues relating to the feasibility and implementation of atmospheric tomography are discussed. Among these issues are the causes of fluctuations in acoustic signals propagated through the atmosphere, appropriate spatial dimensions of an array, signal detection and processing techniques, mathematical inverse techniques and their numerical implementation, and whether or not tomography m provide measures of dynamical variables of interest to atmospheric scientists. Surface-layer, horizontal-slice tomography was implemented experimentally, with an array of three sources and seven receivers distributed over a region approximately 200 m square. Travel-time fluctuations at the receivers were used to reconstruct the temperature and wind fields with about 50-m resolution in the horizontal plane.

Acoustic tomographic monitoring of the atmospheric boundary layer

Abstract Acoustic tomography is proposed as a method for monitoring near-surface atmospheric temperature and wind velocity fields. Basic issues relating to the feasibility and implementation of atmospheric tomography are discussed. Among these issues are the causes of fluctuations in acoustic signals propagated through the atmosphere, appropriate spatial dimensions of an array, signal detection and processing techniques, mathematical inverse techniques and their numerical implementation, and whether or not tomography m provide measures of dynamical variables of interest to atmospheric scientists. Surface-layer, horizontal-slice tomography was implemented experimentally, with an array of three sources and seven receivers distributed over a region approximately 200 m square. Travel-time fluctuations at the receivers were used to reconstruct the temperature and wind fields with about 50-m resolution in the horizontal plane.

A method for predicting acoustic signal fluctuations using a distribution of Gaussian correlation functions

The autocorrelation function of the refractive index inhomogeneities in a turbulent medium was modeled as a continuous distribution of Gaussian functions. The advantage of this approach lies in the simplicity of the solutions of the wave scattering integral for a weakly inhomogeneous medium with a Gaussian spatial correlation function. The density function of the distribution for the case of a turbulent medium was constructed. Convolution of the solution of the wave scattering integral for a unit Gaussian function with the density function yields the solution for the turbulent medium. The results are identical with those obtained by Tatarskii [Wave Propagation in a Turbulent Medium (McGraw-Hill, New York, 1961 and Dover, New York, 1967); and The Effects of the Turbulent Atmosphere on Wave Propagation, translated by the Israel Program of Scientific Translations Ltd., available as TT-68-50464 from the U.S. Dept. of Commerce] by the more complicated approach of using the wavenumber spectrum and the structure function.

Generating random sound speed fields in water using heating or cooling

Both heating and cooling techniques have been developed for generating random sound speed fields along a ten meter acoustic propagation path in a 14 m tank having a 3.7 × 3.7 m cross section. A 1.5-MHz cw signal was propagated along this path for the purpose of studying high-frequency acoustic signal fluctuations in a random medium. Previous investigators of acoustic fluctuations have produced random sound speed distributions by locating heating elements beneath the propagation path. Localized heating of water generates bubbles due to degassification, and it has been suggested that observed fluctuations may be due to resonant scattering by these rising microbubbles [J. A. Neubert and J. L. Lumley, J. Acoust. Soc. Am. 64, 1148–1158 (1978)]. Preliminary qualitative results indicate that statistically similar random sound speed fields generated by heating from below or by cooling from above produce similar acoustic fluctuations.

Turbulence and its effect upon the transmission of sound in water

When sound is transmitted in the sea the amplitude of the received signal is found to fluctuate. Various causes of signal fluctuations are discussed and the possibility of turbulence in the water causing some acoustic signal fluctuation is put forward. The statistical properties of turbulence are discussed and a theoretical prediction for the amount of signal fluctuation likely to be expected for a given degree of turbulence is made. Practical measurements, both of signal amplitude fluctuations and of the energy distribution of turbulent eddies agree very closely with the theoretical predictions.

Acoustic Signal Fluctuations in a Simulated Random Medium

A statistical study has been conducted under controlled laboratory conditions of sound-wave fluctuations inherent in underwater acoustic propagation through a random (in space) medium. Large (with respect to acoustic wavelength) inhomogeneities of equal volume were formulated, using spherical, liquid-filled neoprene balloons, and the refractive indices (relative to water) of the fluid spheres were arranged on the order of unity with deviations up to 1%. (Such weak, large-scale refractive variations are representative, in part, of circumstances in the ocean.) A large number of these balloon inhomogeneities were suspended underwater at random in an experimental acoustic chamber. Signal fluctuations were measured at four different acoustic frequencies and transmitter-to-receiver ranges within the simulated random medium. For each range frequency combination, numerous ensembles were obtained by rotating the fixed transmitter-to-receiver line of sight about its midpoint. The experimental arrangement and procedures involved in the study are discussed, and a comparison is made between laboratory results and extant theoretical treatments. [Work supported by the Lockheed Independent Research Program.]