Scot F. Morse

High-frequency backscattering enhancements by thick finite cylindrical shells in water at oblique incidence: Experiments, interpretation, and calculations

Impulse response backscattering measurements are presented and interpreted for the scattering of obliquely incident plane waves by air-filled finite cylindrical shells immersed in water. The measurements were carried out to determine the conditions for significant enhancements of the backscattering by thick shells at large tilt angles. The shells investigated are made of stainless steel and are slender and have thickness to radius ratios of 7.6% and 16.3%. A broadband PVDF (polyvinylidene fluoride) sheet source is used to obtain the backscattering spectral magnitude as a function of the tilt angle (measured from broadside incidence) of the cylinder. Results are plotted as a function of frequency and angle. These plots reveal large backscattering enhancements associated with elastic excitations at high tilt angles, which extend to end-on incidence in the coincidence frequency region. Similar features are present in approximate calculations for finite cylindrical shells based on full elasticity theory and t...

CAPILLARY BRIDGE MODES DRIVEN WITH MODULATED ULTRASONIC RADIATION PRESSURE

The method of modulated ultrasonic radiation pressure, previously used to drive the capillary modes of drops and bubbles, is used to excite the capillary modes of a cylindrical oil bridge in a Plateau tank. Specific modes may be selected by adjusting the modulation frequency and the location or orientation of the bridge in the ultrasonic field. Mode frequencies were measured as a function of the slenderness for the lowest two axisymmetric modes and two nonaxisymmetric modes. The frequencies of the lowest modes agree with an approximate theory which neglects viscous corrections where the interfacial tension is a fitted parameter.

Backscattering of transients by tilted truncated cylindrical shells: Time-frequency identification of ray contributions from measurements

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 degrees for the frequencies examined. Also observed are periodic features corresponding to end-reflected helical waves of the a0-. These scattering features are significantly different from those reported for thin-walled finite cylinders at low frequencies. The present results may be useful for target identification and localization and as a comparison tool for high-frequency computational scattering models.

MERIDIONAL RAY CONTRIBUTIONS TO SCATTERING BY TILTED CYLINDRICAL SHELLS ABOVE THE COINCIDENCE FREQUENCY : RAY THEORY AND COMPUTATIONS

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...

Degradation of meridional ray backscattering enhancements for tilted cylinders by mode conversion: wide-band observations using a chirped PVDF sheet source

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 backscattering enhancements for empty truncated tilted cylindrical shells: measurements, ray model, and effects of a mode threshold.

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 shells 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.

Time-frequency analysis of transient high-frequency dispersive guided waves on tilted cylindrical shells: Review

Measurements of the back scattering by bluntly truncated tilted cylindrical shells in water reveal a dependence on the aspect angle, which can be interpreted using geometrically described coupling mechanisms [Morse et al., J. Acoust. Soc. Am. 103, 785–794 (1998)]. By exciting a shell with a suitable acoustic pulse localized in time, the recorded backscattering response reveals a significant evolution of the spectrum as a function of time and tilt angle. This evolution was interpreted using the dispersion relations of the relevant high-frequency shell guided waves [Morse and Marston, J. Acoust. Soc. Am. 111, 1289–1294 (2002)]. The coupling conditions are affected by the mode properties. This interpretation was facilitated by also computing the scattering properties of an infinitely long titled shell [Morse and Marston, J. Acoust. Soc. Am. 106, 2597–2600 (1999)] and by measuring and modeling the contributions of helical and meridional rays [Morse and Marston 112, 1318–1326 (2002); Blonigon and Marston, 112,...

Angle dependence of the meridional leaky‐ray backscattering enhancement from the end of a tilted finite cylinder. Convolution analysis and a numerical test for shells

Reflection of a meridional leaky ray from the far end of a tilted cylinder produces a backscattering enhancement when the tilt angle is close to the leaky wave coupling angle. The ray lies in the meridional plane defined by the incident wave vector and the cylinder’s axis. The peak magnitude of this enhancement was related to the end‐reflection coefficient by a convolution surface integral [P. L. Marston, J. Acoust. Soc. Am. 102, 358–369 (1997)]. The more difficult integrals descriptive of the dependence on tilt angle are numerically evaluated in the present study. Though the exact analytical solution of the problem considered is unknown, for comparison an approximate partial‐wave series description is available where the boundary conditions at the ends of the cylinder are such that the end‐reflection coefficients become unimodular. For antisymmetric leaky Lamb waves on long thick and thin shells, the enhancement width, magnitude, and location each agree with the result from the integral when a unimodular...

Wave field and evanescent waves produced by a sound beam incident on a simulated sediment

When a sound beam in water is incident on a sediment at a sufficiently small grazing angle, the resulting wave field in the sediment is complicated, even for the case of flat, fluidlike sediments. The wave field in the sediment for a sound beam from a simple, unshaded, finite transducer has an evanescent component and diffractive components. These components can interfere to produce a series of nulls outside the spatial region dominated by the evanescent wave field. This situation has been experimentally simulated by using a combination of previously described immiscible liquids [Osterhoudt et al., J. Acoust. Soc. Am. 117, 2483 (2005)]. The spacing between the observed nulls is similar to that seen in a wave‐number‐integration‐based synthesis (using OASES) for a related problem. An analysis of a dephasing distance for evanescent and algebraically decaying components [T .J. Matula and P. L. Marston, J. Acoust. Soc. Am. 97, 1389–1398 (1995)] explains the spacing of the nulls. [Work supported by ONR.]

Scattering by truncated targets with and without boundary interactions

Ray methods have been applied to the scattering of various truncated targets having wavenumber‐radius products as small as 10 [F. J. Blonigen and P. L. Marston, J. Acoust. Soc. Am. 107, 689–698 (2000); S. F. Morse and P. L. Marston, ibid. 112, 1318–1326 (2002); B. T. Hefner and P. L. Marston, ARLO 2, 55–60 (2001)]. Recent work emphasizes the exploration of scattering enhancements for other situations including plastic cylinders having curved ends, truncated plastic cones, partially exposed cylinders, and objects in simulated conditions for burial in a seabed. Enhanced scattering is often associated with a locally flat outgoing wavefront. For plastic targets it has been helpful to examine the time dependence of the backscattered envelope as a function of target tilt for targets illuminated by short tone bursts. For partially exposed objects it is helpful to examine the backscattering as a function of the target exposure. For simulated buried targets, it has been helpful to excite target resonances. [Work s...