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  • 1
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    Acoustic mode radiation from the termination of a truncated nonlinear internal gravity wave duct in a shallow ocean area (2010)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2009. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 126 (2009): 1752-1765, doi:10.1121/1.3203268.
    Description: Horizontal ducting of sound between short-wavelength nonlinear internal gravity waves in coastal environments has been reported in many theoretical and experimental studies. Important consequences arising at the open end of an internal wave duct (the termination) are examined in this paper with three-dimensional normal mode theory and parabolic approximation modeling. For an acoustic source located in such a duct and sufficiently far from the termination, some of the propagating sound may exit the duct by penetrating the waves at high grazing angles, but a fair amount of the sound energy is still trapped in the duct and propagates toward the termination. Analysis here shows that the across-duct sound energy distribution at the termination is unique for each acoustic vertical mode, and as a result the sound radiating from the termination of the duct forms horizontal beams that are different for each mode. In addition to narrowband analysis, a broadband simulation is made for water depths of order 80 m and propagation distances of 24 km. Situations occur with one or more modes absent in the radiated field and with mode multipath in the impulse response. These are both consistent with field observations.
    Description: This work was supported under ONR Grant No. N00014-05-1-0482 and the ONR postdoctoral fellowship award, Grant No. N00014-08-1-0204.
    Keywords: Acoustic field ; Acoustic intensity ; Approximation theory ; Parabolic equations ; Underwater acoustic propagation
    Repository Name: Woods Hole Open Access Server
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  • 2
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    Observationally constrained modeling of sound in curved ocean internal waves: Examination of deep ducting and surface ducting at short range (2011)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2011. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 130 (2011): 1173-1187, doi:10.1121/1.3605565.
    Description: A study of 400 Hz sound focusing and ducting effects in a packet of curved nonlinear internal waves in shallow water is presented. Sound propagation roughly along the crests of the waves is simulated with a three-dimensional parabolic equation computational code, and the results are compared to measured propagation along fixed 3 and 6 km source/receiver paths. The measurements were made on the shelf of the South China Sea northeast of Tung-Sha Island. Construction of the time-varying three-dimensional sound-speed fields used in the modeling simulations was guided by environmental data collected concurrently with the acoustic data. Computed three-dimensional propagation results compare well with field observations. The simulations allow identification of time-dependent sound forward scattering and ducting processes within the curved internal gravity waves. Strong acoustic intensity enhancement was observed during passage of high-amplitude nonlinear waves over the source/receiver paths, and is replicated in the model. The waves were typical of the region (35 m vertical displacement). Two types of ducting are found in the model, which occur asynchronously. One type is three-dimensional modal trapping in deep ducts within the wave crests (shallow thermocline zones). The second type is surface ducting within the wave troughs (deep thermocline zones).
    Description: Grants from the Office of Naval Research funded this work. Use of the vessels Ocean Researcher I and Ocean Researcher II in this experiment was funded by the Taiwan National Science Council.
    Keywords: Acoustic field ; Acoustic focusing ; Acoustic intensity ; Acoustic wave scattering ; Acoustic wave velocity ; Ocean waves ; Oceanographic regions ; Underwater acoustic propagation
    Repository Name: Woods Hole Open Access Server
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  • 3
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    A higher-order split-step Fourier parabolic-equation sound propagation solution scheme (2012)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2012. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 132 (2012): EL61-EL67, doi:10.1121/1.4730328.
    Description: A three-dimensional Cartesian parabolic-equation model with a higher-order approximation to the square-root Helmholtz operator is presented for simulating underwater sound propagation in ocean waveguides. The higher-order approximation includes cross terms with the free-space square-root Helmholtz operator and the medium phase speed anomaly. It can be implemented with a split-step Fourier algorithm to solve for sound pressure in the model. Two idealized ocean waveguide examples are presented to demonstrate the performance of this numerical technique.
    Description: This work was sponsored by the Office of Naval Research under Grants No. N00014-10- 1-0040 and No.N00014-11-1-0701.
    Keywords: Acoustic waveguides ; Helmholtz equations ; Parabolic equations ; Underwater acoustic propagation
    Repository Name: Woods Hole Open Access Server
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  • 4
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    Horizontal coherence of low-frequency fixed-path sound in a continental shelf region with internal-wave activity (2012)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2012. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 131 (2012): 1782-1797, doi:10.1121/1.3666003.
    Description: Sound at 85 to 450 Hz propagating in approximately 80-m depth water from fixed sources to a joint horizontal/vertical line array (HLA/VLA) is analyzed. The data are from a continental shelf area east of Delaware Bay (USA) populated with tidally generated long- and short-wavelength internal waves. Sound paths are 19 km in the along-shore (along internal-wave crest) direction and 30 km in the cross-shore direction. Spatial statistics of HLA arrivals are computed as functions of beam steering angle and time. These include array gain, horizontally lagged spatial correlation function, and coherent beam power. These quantities vary widely in magnitude, and vary over a broad range of time scales. For example, correlation scale can change rapidly from forty to five wavelengths, and correlation-scale behavior is anisotropic. In addition, the vertical array can be used to predict correlation expected for adiabatic propagation with cylindrical symmetry, forming a benchmark. Observed variations are in concert with internal-wave activity. Temporal variations of three coherence measures, horizontal correlation length, array gain, and ratio of actual correlation length to predicted adiabatic-mode correlation length, are very strong, varying by almost a factor of ten as internal waves pass.
    Description: This work was supported by Office of Naval Research (ONR) Grants Nos. N00014-05-1-0482 and N00014-11-1- 0194 to T.F.D., ONR Grant No. N00014-04-1-0146 to J.F.L., and an ONR Ocean Acoustics Postdoctoral Fellowship awarded to J.M.C. under Professor William Carey at Boston University.
    Keywords: Acoustic arrays ; Acoustic correlation ; Statistics ; Underwater acoustic propagation
    Repository Name: Woods Hole Open Access Server
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  • 5
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    A three-dimensional parabolic equation model of sound propagation using higher-order operator splitting and Padé approximants (2012)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2012. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 132 (2012): EL364-EL370, doi:10.1121/1.4754421.
    Description: An alternating direction implicit (ADI) three-dimensional fluid parabolic equation solution method with enhanced accuracy is presented. The method uses a square-root Helmholtz operator splitting algorithm that retains cross-multiplied operator terms that have been previously neglected. With these higher-order cross terms, the valid angular range of the parabolic equation solution is improved. The method is tested for accuracy against an image solution in an idealized wedge problem. Computational efficiency improvements resulting from the ADI discretization are also discussed.
    Description: This work was sponsored by the Office of Naval Research under Grant Nos. N00014-10-1-0040 and N00014-11-1-0701.
    Keywords: Helmholtz equations ; Mathematical operators ; Parabolic equations ; Underwater acoustic propagation
    Repository Name: Woods Hole Open Access Server
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  • 6
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    Horizontal refraction of propagating sound due to seafloor scours over a range-dependent layered bottom on the New Jersey shelf (2012)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © Acoustical Society of America, 2012. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 131 (2012): 2587-2598, doi:10.1121/1.3687446.
    Description: Three-dimensional propagation effects of low frequency sound from 100 to 400 Hz caused by seafloor topography and range-dependent bottom structure over a 20 km range along the New Jersey shelf are investigated using a hybrid modeling approach. Normal modes are used in the vertical dimension, and a parabolic-equation approximate model is applied to solve the horizontal refraction equation. Examination of modal amplitudes demonstrates the effect of environmental range dependence on modes trapped in the water column, modes interacting with the bottom, and modes trapped in the bottom. Using normal mode ray tracing, topographic features responsible for three-dimensional effects of horizontal refraction and focusing are identified. These effects are observed in the measurements from the Shallow Water 2006 experiment. Specifically, signals from a pair of fixed sources recorded on a horizontal line array sitting on the seafloor show an intensification caused by horizontal focusing due to the seabed topography of 4 dB along the array.
    Description: Work sponsored by the Office of Naval Research under Grants No. N00014-04-1-0146, No. N00014-10-1-0040, and No. N00014-10-1-0649.
    Keywords: Acoustic focusing ; Acoustic signal processing ; Acoustic wave refraction ; Modal analysis ; Ray tracing ; Underwater acoustic propagation
    Repository Name: Woods Hole Open Access Server
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