Sound Speed Field Research Articles

Coupling heat transfer and large eddy simulation for combustion instability prediction in a swirl burner

Large eddy simulations (LES) of combustion instabilities are often performed with simplified thermal wall boundary conditions, typically adiabatic walls. However, wall temperatures directly affect the gas temperatures and therefore the sound speed field. They also control the flame itself, its stabilization characteristics and its response to acoustic waves, changing the flame transfer functions (FTFs) of many combustion chambers. This paper presents an example of LES of turbulent flames fully coupled to a heat conduction solver providing the temperature in the combustor walls. LES results obtained with the fully coupled approach are compared to experimental data and to LES performed with adiabatic walls for a swirled turbulent methane/air burner installed at Engler-Bunte-Institute, Karlsruhe Institute of Technology and German Aerospace Center (DLR) in Stuttgart. Results show that the fully coupled approach provides reasonable wall temperature estimations and that heat conduction in the combustor walls strongly affects both the mean state and the unstable modes of the combustor. The unstable thermoacoustic mode observed experimentally at 750 Hz is captured accurately by the coupled simulation but not by the adiabatic one, suggesting that coupling LES with heat conduction solvers within combustor walls may be necessary in other configurations in order to capture flame dynamics.

Implications of the evolving sound speed field on detection and classification of targets in shallow water

Acoustic refraction caused by gradients in the sound speed field within a region covered by an active sonar system affects its performance and quality of information. From the perspective of target strength, refraction can focus sound and enhance target returns, or it can redirect sound and create shadow zones where targets will drop out. These effects depend on the evolving sound speed field, as well as the placement of the transmitters and receivers. In addition, focused sound at the water-sediment interface can produce a small region of enhanced backscatter, which can rise above the nominal reverb level by 6 dB or more. Continuous evolution of the time-dependent sound speed field can cause these relatively compact returns to appear as moving targets, increasing false-alarm detections. Advances in fine scale oceanographic models facilitate studying these propagation effects, both the enhanced/degraded target returns and the target-like bottom backscattered features. A combined acoustic and oceanographic model is presented for discussion on what conditions cause these effects, the possibility of identifying their occurrence within data, and what the consequences are on a practical sonar system operating in shallow water.

Evaluating direction of arrival uncertainty in undersea canyons with internal tides

The slopes of undersea canyon regions impart complexity to underwater acoustic fields for two reasons: intricate patterns of reflection from the seabed, and sound-speed anomalies from internal tides generated at the slopes. Both may spread the horizontal directional spectrum of sound from “line of sight” to a source. The directional spectrum is tied to the covariance matrix of the field, a fundamental quantity that can be measured or modeled. Here, sound-field horizontal-lag spatial covariance matrices and other derived quantities are generated from time-stepped 3D parabolic equation acoustic simulations made using sound-speed fields from ocean models. The covariance matrices are then inserted into the direction-of-arrival (DOA) estimation problem. Analysis of DOA estimates and error bounds is done for a conventional beamformer and for a Gauss-Markov inverse-based beamformer for a variety of signal-to-noise ratios. The method treats non-line of sight acoustic energy as a form of noise. At low signal-to-noise ratio, the Gauss-Markov estimator can perform better that the other. This analysis of field variability allows performance degradation caused by evolving ocean structures to be directly compared to other detrimental influences such as excess noise and array deformation. Detection, localization, and tracking are affected by the processes examined here.

Four dimensional sound speed environments in ocean acoustic simulations

Current ocean acoustic simulation environments often rely on a single depth-varying sound speed profile. This work introduces a robust and configurable Octave/C + + implementation of a four-dimensional sound speed environment (4D SSP) for use with Bellhop in the MIT/LAMSS software package for acoustic simulations. This tool shows considerable advances in that the 4D SSP environment can be created from MIT GCM model output, CTD data, or manually varied from existing historical profiles. By considering the fluctuations in sound speed in longitude, latitude, depth, and time, output pressure data better approximates experimental data. Two experiments in the Arctic and the Santa Barbara Channel (SBC) are simulated in 1-D and 4-D to compare to experimental data taken. A framework for acoustic data assimilation using this new simulated data environment in tandem with experimental data is shown to derive the true sound speed field. [Work supported by ONR under the Information in Ambient Noise MURI.]

Ray travel times in the northern Philippine Sea circulation from state estimates

The North Pacific Acoustic Laboratory (NPAL) Philippine Sea experiment deployed a variety of instruments in the northern Philippine Sea during April 2010 through April 2011, including six acoustic transceivers for reciprocal measurements of travel times between instruments. Five transceivers were moored in a pentagon approximately 660 km in diameter with the sixth transceiver moored in the center. Observed travel-time time series were compared with travel times computed from ocean state estimates made using an eddy-active regional implementation of the MITgcm that were constrained by satellite sea surface height and sea surface temperature observations and by temperature and salinity profiles from Argo, CTDs, and XBTs but not by the acoustic data. The similarities cross-validate the state estimates, while the differences provide a simple estimate of the novel information present in the travel times. Smoothness in the modeled sound speed field had a large effect on the ability to find eigenrays. The ocean state estimates were then re-computed to fit the acoustic travel times. The state estimate was able to match the travel times within their error bars and did not significantly increase the misfits with the other observations.

Using broadband acoustic signals and modal behavior to assess scales of ocean variability in shallow water waveguides

The sound speed fields of the continental shelf water column are anisotropic and have “fronts” related to propagating internal wave packets. The angle between a broadband acoustic signal’s source–receiver propagation track and the propagating “fronts” affects its modal composition. If the temporal variability of the modal behavior of the acoustic signals are monitored in time and space, one might identify the fluid process that causes the changes to the sound speed field. In this paper, we first present a heuristic view of how the modal properties of broadband signals can be affected by the sound speed field variability caused by propagating internal wave packets. We then present three days of acoustic field data acquired on vertical and horizontal arrays placed in ~100 m of water about 20 km from the location of acoustic signal sources. Temporal changes in the received acoustic signal's modal composition will be compared to numerical simulations. The changes in modal composition will be correlated to internal wave induced variability in the horizontal refraction of the signals.

Wind turbine radiated acoustic signals—Propagation in a temporally and spatial variable marine meteorological boundary layer

The temporal and spatial properties of an acoustic field radiated from a wind turbine operating in a marine meteorological boundary layer and a spatial temporal variable impedance boundary will be quantified using paraxial equation based numerical simulations. A sound speed field will be constructed using wind field measurements made with a 915 MHZ radar wind speed sensor and local temperature and humidity measurements obtained from a meteorological tower. Temporally variable impedance boundary conditions for a tidal marine marsh will be included in the simulations.

海洋観測フロートによって観測された大規模海底地震の震源近傍における音速プロファイルの変化

The Tohoku region Pacific Ocean earthquake that occurred on March 11, 2011, resulted in extensive damage in a vast area ranging from the Tohoku to Kanto. However, at the time of the earthquake, no research vessel was confirmed to be investigating marine life in the seas adjacent to epicenter area, so physical data like water temperature and salinity, as well as ocean sound speed data required for the echo sounder, could not be acquired. Therefore, in this study, we compute the sound speed profile from the Argo system data(water temperature, salinity, pressure) that was being deployed near the area of the epicenter both before and after the earthquake, and attempt to detect the change in the sound speed field. In addition, the Tohoku region Pacific Ocean earthquake and the Sumatra earthquake in December 2004, which were accompanied by a giant tsunami, were also investigated. Results showed that in the main temperature front between the central thermocline and deep-sea isothermal layer, water temperature did not change very much. However, we confirmed the tendency for the underwater sound speed to suddenly increase dramatically after an earthquake.

Modeling three dimensional environment and broadband acoustic propagation in Arctic shelf-basin region

Rapid climate change over the last decade has created a renewed interest in the nature of underwater sound propagation in the Arctic Ocean. Changes in the oceanography and surface boundary conditions are expected to cause measurable changes in the propagation and scattering of low frequency sound. Recent measurements of a high-resolution three-dimensional (3-D) sound speed structure in a 50 x 50 km2 region in an open-water shelf-basin region of the Beaufort Sea offer a unique and rare opportunity to study the effects of a complex oceanography on the acoustic field as it propagates from the deep basin onto the continental shelf. The raw oceanography data were analyzed and processed to create a 3-D sound speed field for the water column in the basin-slope-shelf area. Recent advances in both 2-D and 3-D acoustic modeling capability allow one to study the effects of the range- and azimuth-dependent water column layers on the frequency-dependent acoustic modal structure. Of particular interest is the nature of the 3-D and mode-coupling effects on the frequency response induced by the oceanography. The results will likely be useful in designing acoustic experiments with serious logistical constraints in the rapidly changing Arctic Ocean.

Echo statistics of individual and aggregations of scatterers in the water column of a random, oceanic waveguide.

The relative contributions of various physical factors to producing non-Rayleigh distributions of echo magnitudes in a waveguide are examined. Factors that are considered include (1) a stochastic, range-dependent sound-speed profile, (2) a directional acoustic source, (3) a variable scattering response, and (4) an extended scattering volume. A two-way parabolic equation model, coupled with a stochastic internal wave model, produces realistic simulations of acoustic propagation through a complex oceanic sound speed field. Simulations are conducted for a single frequency (3 kHz), monostatic sonar with a narrow beam (5° -3 dB beam width). The randomization of the waveguide, range of propagation, directionality of the sonar, and spatial extent of the scatterers each contribute to the degree to which the echo statistics are non-Rayleigh. Of critical importance are the deterministic and stochastic processes that induce multipath and drive the one-way acoustic pressure field to saturation (i.e., complex-Gaussian statistics). In this limit predictable statistics of echo envelopes are obtained at all ranges. A computationally low-budget phasor summation can successfully predict the probability density functions when the beam pattern and number of scatterers ensonified are known quantities.

Inversion of Internal Wave-Perturbed Sound-Speed Field Via Acoustic Data Assimilation

Model-based signal processing methods, such as matched-field processing, are concerned with exploiting the combination of the sound propagation physics in an ocean waveguide and the observation data. As such, they require accurate knowledge of the ocean acoustic environment. This paper presents an approach of environmental inversion via acoustic data assimilation (ADA), intended specifically to reconstruct the sound-speed field perturbed by linear internal waves. It exploits a variety of information sources, including direct local sound-speed measurements, an internal wave model, a full field acoustic propagation model, and acoustic pressure measurements. Compared to the previous work on ADA, four important improvements have been made: 1) a linear internal wave model is included; 2) a more realistic shallow-water environment from the Shelf Break PRIMER 1996 experiment is considered in the numerical simulations; and 3) effect of the sediment on acoustic propagation is considered in the inversion algorithm. The results further demonstrate the validity of the ADA approach.

The effects of environmental variability and spatial sampling on the three-dimensional inversion problem.

The overall goal of this work is to quantify the effects of environmental variability and spatial sampling on the accuracy and uncertainty of estimates of the three-dimensional ocean sound-speed field. In this work, ocean sound speed estimates are obtained with acoustic data measured by a sparse autonomous observing system using a perturbative inversion scheme [Rajan, Lynch, and Frisk, J. Acoust. Soc. Am. 82, 998-1017 (1987)]. The vertical and horizontal resolution of the solution depends on the bandwidth of acoustic data and on the quantity of sources and receivers, respectively. Thus, for a simple, range-independent ocean sound speed profile, a single source-receiver pair is sufficient to estimate the water-column sound-speed field. On the other hand, an environment with significant variability may not be fully characterized by a large number of sources and receivers, resulting in uncertainty in the solution. This work explores the interrelated effects of environmental variability and spatial sampling on the accuracy and uncertainty of the inversion solution though a set of case studies. Synthetic data representative of the ocean variability on the New Jersey shelf are used.

The effect of large- and small-scale sound velocity field structure on shallow water sound propagation in the East China Sea

During the Transverse Acoustic Variability Experiment (TAVEX) in 2008, towed CTD measurements were used to record the sound speed field over an area of the East China Sea surrounding 34- and 20-km acoustic paths. The data revealed several propagating internal wave packets along with sound speed variability due to density-compensated thermohaline variations (spice). Concurrent acoustic data were also recorded from 300- and 500-Hz sources. Parabolic equation modeling is used to examine the effects of the measured internal waves and spice on acoustic propagation and to make comparisons with the recorded acoustic data.

Future shallow water low frequency (100–1000 Hz) acoustic signal propagation physics studies

The measurement and prediction of the phase and amplitude properties of acoustic signals (1 to 1000 Hz) propagating in the dynamic shallow water (30 to 500 m water depths) environment has been a focus of the underwater acoustic research community for nearly 20 years. The majority of the ocean studies have occurred during late spring to summertime oceanic conditions when the sound speed variability was influenced by nonlinear mode 1 internal wavefields. Sound speed variability during the summer-to-winter and winter-to-summer water column transition periods will cause variability in the phase coherent properties of acoustic signals. Few experimental studies explicitly focused on quantifying the variability of phase coherent properties of acoustic signals during these periods have been performed. Illustration of the types of fluid processes that will perturb the sound speed structure during the fall-to-winter transition periods will be presented. Included will be mode 1 and mode 2 internal wave perturbation of the sound speed field as well as the locations and spatial scales of sound speed variability caused by interleaving water masses of varying temperature and salinity (Spice). Fall-to-winter transition sound speed fields will be contrasted with summer time sound speed conditions. [Work supported by the Office of Naval Research.]

Deep water towed array measurements at close range

During the North Pacific Acoustic Laboratory Philippine Sea 2009 experiment, towed array receptions were made from a towed source as the two ships transited from a separation of several Convergence Zones through a Closest Point of Approach at 3 km. A combination of narrowband tones and broadband pulses were transmitted covering the frequency band 79-535 Hz. The received energy arrives from two general paths-direct path and bottom bounce. Bearing-time records of the narrowband arrivals at times show a 35° spread in the angle of arrival of the bottom bounce energy. Doppler processing of the tones shows significant frequency spread of the bottom bounce energy. Two-dimensional modeling using measured bathymetry, a geoacoustic parameterization based upon the geological record, and measured sound-speed field was performed. Inclusion of the effects of seafloor roughness and surface waves shows that in-plane scattering from rough interfaces can explain much of the observed spread in the arrivals. Evidence of out-of-plane scattering does exist, however, at short ranges. The amount of out-of-plane scattering is best observed in the broadband impulse-beam response analysis, which in-plane surface roughness modeling cannot explain.

Environmental acoustic parameters of the Sea of Japan shelf (Peter the Great Gulf)

The paper describes general geoacoustic model of the Peter the Great Gulf (the largest gulf of the Sea of Japan) and detailed model of its part of 400 square km in size. The general geoacoustic model is formed on the basis of geological, seismic, and bathymetric data. It includes distribution of P-, S-wave, density, and attenuation of friable sediments at the bottom surface and averaged characteristics for different types of rocks in the gulf. Bathymetric data processing was carried out by means of two-dimensional singular spectrum analysis, based on the ratio between the size of relief structures and the energy expended under the development of tectonic processes. Detailed geoacoustic model is based on processing of hydroacoustic and seismic authors’ studies (3.5 kHz sonar, 50–100 kHz sonar and air-gun). Three sedimentary layers and the location of upper edge of the granite stratum are marked in the observable shelf area. The model demonstrates significant changes in P-, S-wave in the sediments of the lateral (1500–1750 m/s—P-wave and 120–600 m/s—S-wave) and vertical (1500–5400 m/s—P-wave and 120–3300 m/s—S-wave). Some of the most important oceanographic mechanisms determining both large- and small-scale spatio-temporal variations of sound speed field in the same area are presented. These data were obtained by the authors within last several years. Qualitative analysis of the propagation of low-frequency signals is presented as an example of application of the model.

The effect of surface and linear internal waves on higher order acoustic moments in shallow water

Acoustic fields in shallow water have a statistical nature due to complex, time-evolving sound speed fields, and scattering from rough boundaries. Previously, coupled-mode transport theory [Raghukumar and Colosi (2012)] was applied to high frequency acoustic fluctuations in an environment typical of the Shallow Water 2006 (SW06) experiment on the New Jersey continental shelf. As a consequence of the strong adiabatic component in SW06 propagation, a hybrid approach was used to calculate mode coherences where mode energies from the Dozier-Tappert approach were combined with adiabatic phase terms. Mode energies, coherences and acoustic intensities were examined, and it was found that internal and surface waves preferentially couple low and high modes respectively. Here, we extend that study to include higher moments such as scintillation index and shift focus to modes that are coupled by both internal and surface waves. Oceanographic and sea surface measurements are used to constrain the internal wave and sea surface models. The relative importance of linear internal waves and surface scattering effects are studied using transport theory and Monte Carlo simulations.

Acoustic interface treatment with an adjoint operator for linear range-dependent ocean index of refraction inversions

Hurskey et al. [J. Acoust. Soc. Am. 115(2), 607–619 (2004)] introduced the adjoint method and incorporated local sound-speed measurements into range-dependent ocean sound-speed inversion. They left two practical issues for the implementation of this method unresolved in this paper: the collection of appropriate environmental measurements and the implementation of bottom boundary conditions. The first of these issues was considered in a simulation study that used an oceanographic glider to collect range-dependent sound-speed measurements [Richards, CMRE memorandum (2012)]. A covariance matrix was constructed from the changes observed in the range-dependent sound-speed field. The adjoint inversion was performed in a reduced element subset of the empirical orthogonal functions (EOF) base of the covariance matrix. The second issue is the focus of this paper, which describes a bottom boundary condition with a defined adjoint operator. A horizontal fluid-bottom interface is implemented using the implicit finite difference form of the parabolic equation introduced by McDaniel and Lee [J. Acoust. Soc. Am. 71(4), 855–858 (1982)]. Combined with local range-dependent sound-speed statistics gathered with gliders, this development may provide a method of near real-time acoustic measurement of ocean sound-speed variations between an acoustic source and vertical hydrophone array.

A time-domain finite volume approach for prediction of muffler transmission loss including thermal effects

A time-domain finite volume approach is presented for predicting the transmission loss of muffler including thermal effects with non-uniform sound speed field and density field, in which the acoustic wave equation in heterogeneous media is solved by using unstructured finite volume method with the temperature field specified or solved by some commercial code. An improved time-domain impulse method based on the absorbing boundary condition is applied to predict the acoustic attenuation characteristics of mufflers. The approach is validated by numerical simulations of a simple expansion chamber muffler and a complex muffler with five chambers. The predicted results agree well with the corresponding experimental ones and numerical ones obtained by finite element method with commercial code SYSNOISE. The results of both mufflers under different thermal conditions indicate that the temperature distribution has a significant influence on transmission loss. According to the analysis of a complex muffler with ideal medium, it is shown that the variation of working conditions can obviously affect density and sound speed distributions but have little influence on transmission loss. On the other hand, the obtained transmission loss with the solved temperature field deviates much from the one with specified uniform temperature field.

Mode-2 internal wave generation and propagation-impact on acoustic signal properties

Mode 2 internal wave spatial distributions, generation, propagation and dissipation in the vicinity of bathymetry variably on the continental shelf and near the shelf break of New Jersey, USA will be overviewed for early fall oceanographic conditions. The 3-D spatial and temporal evolution of the mode 2 waves, as simulated with the Naval Research Laboratory nonhydrostatic Model for Coastal Oceans, will be compared to tow-yo conductivity, temperature and depth (CTD) and high frequency acoustic flow visualization observations of mode -2 internal waves. Sound speed field perturbation and acoustic field spatial and temporal distributions in the presence of mode-2 internal waves will be summarized.