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Philip L. Marston - One of the best experts on this subject based on the ideXlab platform.

  • Quantitative Ray methods for scattering by tilted cylinders
    The Journal of the Acoustical Society of America, 2018
    Co-Authors: Philip L. Marston
    Abstract:

    After a review of Ray methods and phenomena associated with high frequency scattering by spheres and cylinders in water viewed broadside, generalizations to tilted cylinders relevant to spectral and imaging domains will be summarized. These extensions were found to be useful for Meridional as well as helical Ray backscattering enhancements associated with leaky (or supersonic) waves on shells [F. J. Blonigen and P. L. Marston, J. Acoust. Soc. Am. 112, 528–536 (2002)]. For such enhancements, Fermat’s principle is useful for identifying Ray paths of interest. In the case of helical waves (and in the broadside special case) the scattering amplitude can be expressed in terms of a Fresnel patch area where the guided wave is excited on the shell. Fresnel patches also give insight into the relatively large magnitude of Meridional Ray contributions. The coupling coefficient is proportional to the radiation damping of the leaky wave. For some shells it is necessary to take into account the anisotropy of the phase ...

  • Quantitative Ray methods for scattering by tilted cylindrical shells
    The Journal of the Acoustical Society of America, 2014
    Co-Authors: Philip L. Marston, Scot F. Morse
    Abstract:

    Starting with a review of Ray methods and phenomena associated with high frequency scattering by spheres and cylindrical shells in water viewed broadside, generalizations to tilted shells will be summarized. These extensions were found to be useful for Meridional as well as helical Ray backscattering enhancements associated with leaky (or supersonic) waves on shells [Morse and Marston, J. Acoust. Soc. Am. 112, 1318–1326 (2002); Blonigen and Marston, J. Acoust. Soc. Am. 112, 528–536 (2002)]. For such enhancements Fermat’s principle is useful for identifying Ray paths of interest. In the case of helical waves (and in the broadside special case), the scattering amplitude can be expressed in terms of a Fresnel patch area where the guided wave is excited on the shell. Fresnel patches also give insight into the relatively large magnitude of Meridional Ray contributions. The coupling coefficient is proportional to the radiation damping of the leaky wave on the shell and in some cases it is necessary to take into...

  • Meridional Ray backscattering enhancements for empty truncated tilted cylindrical shells: measurements, Ray model, and effects of a mode threshold.
    The Journal of the Acoustical Society of America, 2002
    Co-Authors: Scot F. Morse, Philip L. Marston
    Abstract:

    Narrow-band backscattering experiments are used to characterize a Meridional Ray enhancement on a tilted, finite empty cylindrical shell having a blunt truncation. The Meridional Ray of the lowest order flexural leaky Lamb wave is examined, which has previously been shown to lead to large backscattering enhancements for excitation frequencies near and above the shell's coincidence frequency. The measurements are used to validate a convolution formulation Ray theory describing the far-field backscattered amplitudes. Comparisons are also made with an approximate partial wave series solution for the finite cylindrical shell. The amplitude of the Meridional Ray enhancement is dependent on the nature of the reflection of the leaky wave from the shell truncation. While the peak measured amplitude agrees with predictions at low frequencies, experiments indicate the enhancement is degraded at high frequencies and exhibits an abrupt drop near the frequency of the mode threshold (cutoff) for the next-highest flexural mode. The nature of the leaky wave end reflection is examined using an approximate calculation of the energy reflection coefficient for leaky waves on a semi-infinite free plate. Results suggest the observed degradation is the result of mode conversion effects.

  • Backscattering of transients by tilted truncated cylindrical shells: Time-frequency identification of Ray contributions from measurements
    The Journal of the Acoustical Society of America, 2002
    Co-Authors: Scot F. Morse, Philip L. Marston
    Abstract:

    Impulse backscattering measurements by a thick-walled finite cylindrical shell are examined in the time-frequency domain to identify and characterize individual Ray contributions from generalized Lamb waves excited on the shell. Previous experiments and analysis in the frequency-aspect angle domain [S. F. Morse et al., J. Acoust. Soc. Am. 103, 785–794 (1998)] indicate that large backscattering enhancements occur in the midfrequency region for the shell tilted at large angles. Presently this experimental data is examined in the time-frequency domain for selected angles of incidence. Individual Ray contributions are evident and their evolution over aspect angle is discussed. The most prominent contribution is due to the Meridional Ray of the a0 leaky Lamb wave. This feature distinctly highlights the truncation of the shell and is found over a range of aspect angles spanning 200° for the frequencies examined. Also observed are periodic features corresponding to end-reflected helical waves of the a0−. These s...

  • Leaky helical flexural wave backscattering contributions from tilted cylindrical shells in water: Observations and modeling
    The Journal of the Acoustical Society of America, 2002
    Co-Authors: Florian J. Blonigen, Philip L. Marston
    Abstract:

    For tilt angles smaller than the Meridional Ray coupling condition previously investigated [S. F. Morse et al., J. Acoust. Soc. Am. 103, 785–794 (1998)], flexural helical waves on cylindrical shells can significantly enhance the backscattering. These contributions are compared and modeled here for an empty cylinder. Experiments using tone bursts were performed on a tilted stainless steel shell to investigate the contributions caused by flexural leaky Lamb waves above the coincidence frequency of the shell. In some of the measurements the tone bursts were of sufficient duration to superpose helical wave contributions of successive circumnavigations, along with the Meridional contribution near the critical tilt, to arrive at a quasi-steady-state backscattering amplitude for the cylinder. These measurements are compared with an approximate numerical partial-wave series solution and a Ray theory as a function of the tilt angle. The data for ka=20 follow the basic shape of the Ray theory and the relevant features of the partial-wave model. They illustrate the importance of the interference of successive helical wave contributions. Measurements (also as a function of the tilt angle) using tone bursts that were sufficiently short to separate the earliest helical wave contribution from later contributions also support the Ray theory.

Scot F. Morse - One of the best experts on this subject based on the ideXlab platform.

  • Quantitative Ray methods for scattering by tilted cylindrical shells
    The Journal of the Acoustical Society of America, 2014
    Co-Authors: Philip L. Marston, Scot F. Morse
    Abstract:

    Starting with a review of Ray methods and phenomena associated with high frequency scattering by spheres and cylindrical shells in water viewed broadside, generalizations to tilted shells will be summarized. These extensions were found to be useful for Meridional as well as helical Ray backscattering enhancements associated with leaky (or supersonic) waves on shells [Morse and Marston, J. Acoust. Soc. Am. 112, 1318–1326 (2002); Blonigen and Marston, J. Acoust. Soc. Am. 112, 528–536 (2002)]. For such enhancements Fermat’s principle is useful for identifying Ray paths of interest. In the case of helical waves (and in the broadside special case), the scattering amplitude can be expressed in terms of a Fresnel patch area where the guided wave is excited on the shell. Fresnel patches also give insight into the relatively large magnitude of Meridional Ray contributions. The coupling coefficient is proportional to the radiation damping of the leaky wave on the shell and in some cases it is necessary to take into...

  • Meridional Ray backscattering enhancements for empty truncated tilted cylindrical shells: measurements, Ray model, and effects of a mode threshold.
    The Journal of the Acoustical Society of America, 2002
    Co-Authors: Scot F. Morse, Philip L. Marston
    Abstract:

    Narrow-band backscattering experiments are used to characterize a Meridional Ray enhancement on a tilted, finite empty cylindrical shell having a blunt truncation. The Meridional Ray of the lowest order flexural leaky Lamb wave is examined, which has previously been shown to lead to large backscattering enhancements for excitation frequencies near and above the shell's coincidence frequency. The measurements are used to validate a convolution formulation Ray theory describing the far-field backscattered amplitudes. Comparisons are also made with an approximate partial wave series solution for the finite cylindrical shell. The amplitude of the Meridional Ray enhancement is dependent on the nature of the reflection of the leaky wave from the shell truncation. While the peak measured amplitude agrees with predictions at low frequencies, experiments indicate the enhancement is degraded at high frequencies and exhibits an abrupt drop near the frequency of the mode threshold (cutoff) for the next-highest flexural mode. The nature of the leaky wave end reflection is examined using an approximate calculation of the energy reflection coefficient for leaky waves on a semi-infinite free plate. Results suggest the observed degradation is the result of mode conversion effects.

  • Backscattering of transients by tilted truncated cylindrical shells: Time-frequency identification of Ray contributions from measurements
    The Journal of the Acoustical Society of America, 2002
    Co-Authors: Scot F. Morse, Philip L. Marston
    Abstract:

    Impulse backscattering measurements by a thick-walled finite cylindrical shell are examined in the time-frequency domain to identify and characterize individual Ray contributions from generalized Lamb waves excited on the shell. Previous experiments and analysis in the frequency-aspect angle domain [S. F. Morse et al., J. Acoust. Soc. Am. 103, 785–794 (1998)] indicate that large backscattering enhancements occur in the midfrequency region for the shell tilted at large angles. Presently this experimental data is examined in the time-frequency domain for selected angles of incidence. Individual Ray contributions are evident and their evolution over aspect angle is discussed. The most prominent contribution is due to the Meridional Ray of the a0 leaky Lamb wave. This feature distinctly highlights the truncation of the shell and is found over a range of aspect angles spanning 200° for the frequencies examined. Also observed are periodic features corresponding to end-reflected helical waves of the a0−. These s...

  • Degradation of Meridional Ray backscattering enhancements for tilted cylinders by mode conversion: wide-band observations using a chirped PVDF sheet source
    IEEE Journal of Oceanic Engineering, 2001
    Co-Authors: Scot F. Morse, Philip L. Marston
    Abstract:

    High-frequency backscattering measurements are discussed for a thick-walled hollow finite cylindrical shell. The frequency range examined is above the coincidence frequency where energy coupled to supersonic Lamb-type waves dominates the backscattering for off-beam aspect. One particular mechanism, involving a Meridional leaky Ray, leads to large enhancements along well-defined arcs in frequency-aspect angle space. It is shown that the backscattered levels for the flexural mode Meridional Ray drop dramatically above a certain frequency. This degradation of the energy in the return signal is found to be correlated with mode conversion at the truncation of the shell of the lowest order flexural mode into the next higher flexural mode. An approximate calculation utilizing simply supported boundary conditions does not display the observed degradation since it fails to allow for mode conversion.

  • Meridional Ray contributions to scattering by tilted cylindrical shells above the coincidence frequency: Ray theory and computations
    The Journal of the Acoustical Society of America, 1999
    Co-Authors: Scot F. Morse, Philip L. Marston
    Abstract:

    High-frequency scattering enhancements from a submerged infinite cylindrical shell are studied by means of partial wave series (PWS) calculations and a Ray approximation. For certain oblique angles of incidence it is possible to launch a generalized leaky Lamb wave which propagates strictly in the axial direction on the shell. This Meridional leaky wave radiates with a locally flat wavefront curvature and produces large far-field amplitudes over a range of tilt angles. When combined with a reflection from a discontinuity in the shell, this process can give rise to large backscattering enhancements at large tilt angles for thick finite cylindrical shells [S. F. Morse et al., J. Acoust. Soc. Am. 103, 785–794 (1998)] and is present in sonar images. A convolution formulation Ray theory for the far-field Meridional Ray amplitude is compared with the exact PWS solution in the region where enhancements are observed for both the a0 and s0 Lamb waves. The Ray theory correctly predicts both the peak enhancement amp...

V. I. Smirnov - One of the best experts on this subject based on the ideXlab platform.

Florian J. Blonigen - One of the best experts on this subject based on the ideXlab platform.

  • Leaky helical flexural wave backscattering contributions from tilted cylindrical shells in water: Observations and modeling
    The Journal of the Acoustical Society of America, 2002
    Co-Authors: Florian J. Blonigen, Philip L. Marston
    Abstract:

    For tilt angles smaller than the Meridional Ray coupling condition previously investigated [S. F. Morse et al., J. Acoust. Soc. Am. 103, 785–794 (1998)], flexural helical waves on cylindrical shells can significantly enhance the backscattering. These contributions are compared and modeled here for an empty cylinder. Experiments using tone bursts were performed on a tilted stainless steel shell to investigate the contributions caused by flexural leaky Lamb waves above the coincidence frequency of the shell. In some of the measurements the tone bursts were of sufficient duration to superpose helical wave contributions of successive circumnavigations, along with the Meridional contribution near the critical tilt, to arrive at a quasi-steady-state backscattering amplitude for the cylinder. These measurements are compared with an approximate numerical partial-wave series solution and a Ray theory as a function of the tilt angle. The data for ka=20 follow the basic shape of the Ray theory and the relevant features of the partial-wave model. They illustrate the importance of the interference of successive helical wave contributions. Measurements (also as a function of the tilt angle) using tone bursts that were sufficiently short to separate the earliest helical wave contribution from later contributions also support the Ray theory.

  • Leaky helical flexural wave scattering contributions from tilted cylindrical shells: Ray theory and wave-vector anisotropy.
    The Journal of the Acoustical Society of America, 2001
    Co-Authors: Florian J. Blonigen, Philip L. Marston
    Abstract:

    At sufficiently high frequencies, cylindrical shells support a wave whose properties are analogous to those of the lowest antisymmetric Lamb wave on plates. When the shell is in water and the frequency exceeds the coincidence frequency, the flexural wave is a leaky wave that can be a major contributor to the scattering by tilted shells [G. Kaduchak, C. M. Wassmuth, and C. M. Loeffler, J. Acoust. Soc. Am. 100, 64–71 (1996)]. While the Meridional Ray-scattering contributions for such leaky flexural waves were previously modeled, the helical contribution can also be significant. A Ray theory for those contributions is compared with the exact partial wave series (PWS) solution for infinitely long empty shells. The agreement between the Ray theory and the PWS is only possible when a weak anisotropy of the flexural wave parameters is included in the evaluation of the Ray theory. The anisotropy is determined numerically from the roots of a denominator in the PWS because approximations for the anisotropy from thin-shell mechanics are not applicable significantly above the coincidence frequency.

  • Backscattering from empty and water‐filled tilted cylindrical shells due to leaky helical flexural waves: Comparison and Ray theory
    The Journal of the Acoustical Society of America, 2001
    Co-Authors: Florian J. Blonigen, Philip L. Marston
    Abstract:

    For tilt angles smaller than the Meridional Ray coupling condition previously investigated [S. F. Morse et al., J. Acoust. Soc. Am. 103, 785–794 (1998)], helical Rays on empty and water‐filled steel shells can significantly enhance the backscattering. These contributions are compared and modeled in the present work. Such contributions in the water‐filled case are found to be weaker in amplitude, which is to be expected since additional energy is lost through radiation leaked to the inner fluid. A modified Ray theory taking into account the increased radiation damping was compared with experimental results for the earliest helical wave arrival, yielding satisfactory agreement. However, the energy lost to the interior of the shell may be at least partially recovered, as internal Rays can excite additional leaky waves on the shell. Contributions from Rays with one internal chord inside the cylinder were superposed with those of the first helical wave, and the resulting backscattering amplitude was nearly tha...

  • Leaky wave scattering contributions for tilted thick cylindrical shells in water above the coincidence frequency: Ray theory and computational models
    The Journal of the Acoustical Society of America, 1999
    Co-Authors: Florian J. Blonigen, Philip L. Marston
    Abstract:

    Scattering measurements [Morse, Marston, and Kaduchak, J. Acoust. Soc. Am. 103, 785–795 (1998)] and sonar images [Kaduchak, Wassmuth, and Loeffler, J. Acoust. Soc. Am. 100, 64–71 (1996)] demonstrate that above the coincidence frequency, the responses of tilted cylindrical shells associated with leaky antisymmetric Lamb waves can give major contributions to the scattering of sound. Thin‐shell dynamics is not applicable for the modeling of the shell’s response and an approximate Ray formulation has been developed for situations where exact solutions are unavailable. Relevant contributions include Meridional [P. L. Marston, J. Acoust. Soc. Am. 102, 358–369 (1997)] as well as helical [P. L. Marston, J. Acoust. Soc. Am. 102, 1628–1638 (1997)] leaky Rays. The present research emphasizes computational tests for situations where helical antisymmetric Lamb wave contributions are important. These include situations where contributions from the broad Meridional Ray enhancement interfere with helical Ray contribution...

Toyokazu Sakamoto - One of the best experts on this subject based on the ideXlab platform.

  • Analytic Solutions of the Eikonal Equation for a GRIN-rod Lens 2. Skew Rays
    Journal of Modern Optics, 1995
    Co-Authors: Toyokazu Sakamoto
    Abstract:

    Abstract Analytic solutions of the eikonal equation for a skew Ray in a GRIN-rod lens are derived together with those of the Ray equation by the perturbation method. It is algebraically shown that the derived solutions agree with the exact ones for a Meridional Ray in the sech profile and for a helical Ray in the helical exact profile. The numerical accuracy is also examined by the Runge-Kutta-Gill method for the sech profile.

  • Analytic Solutions of the Eikonal Equation for a GRIN-rod Lens
    Journal of Modern Optics, 1993
    Co-Authors: Toyokazu Sakamoto
    Abstract:

    Abstract A systematic method based on the perturbation method is proposed for analytically solving the eikonal equation for a Meridional Ray in a GRIN-rod lens. Using this method, the perturbation solutions up to third order are derived together with those of the Ray equation. The computational accuracy of the derived perturbation solutions is examined against the exact one for the sech index profile. At the typical numerical aperture 0·45 of a compact disc pickup objective, the accuracy of the second-order perturbation solution is found to be as high as 1/100 waves.