Anton Nagl

Refraction effects in the generation of helical surface waves on a cylindrical obstacle

Abstract We investigate the scattering of compressional waves from an infinite, circular-cylindrical obstacle, and the excitation during the scattering process of surface waves that propagate along helical paths over the cylinder surface. For the case of a rigid or soft obstacle, the surface waves are external , and are obtained via the use of a Watson transformation. For the case of a penetrable cylinder, additional internal , resonant surface waves are generated for which the phase and group velocity dispersion curves can be obtained from the Resonance Scattering Theory. We perform a detailed study of certain refraction effects which take place upon the generation of the surface waves by the incident plane wave.

Adiabatic mode theory of underwater sound propagation in a range‐dependent environment

For an ocean sound channel whose environmental parameters depend not only on depth, but in a gradual fashion also on range, the wave equation may be separated by the adiabatic range variation method of Pierce [J. Acoust. Soc. Am. 37, 19 (1965)]. This method is used here to calculate underwater sound propagation in a channel with arbitrary (but gradual) range dependence, and also with arbitrary depth dependence of the sound velocity profile, by employing Airy function solutions of segmentwise linearized problems. Our results are illustrated for a realistic deep‐water propagation case with profile data collected in the western North Atlantic, as well as a shallow‐water example from the Norwegian Sea, and compared against the experimental transmission loss data, and the results of calculations using other methods for the same cases.

Isolation of the resonant component in acoustic scattering from fluid‐loaded elastic spherical shells

The theory of resonance scattering [L. Flax, L. R. Dragonette, and H. Uberall, J. Acoust. Soc. Am. 63, 723 (1978)], recently developed for acoustic wave scattering from elastic objects and elastic‐wave scattering from cavities, is applied to the problem of sound scattering from fluid‐filled elastic spherical shells in a fluid, in particular air‐filled aluminum shells in water. The eigenvibrations of the shell appear as resonances in the scattering amplitude of each normal mode; these are superimposed on a smooth background which if the shell is made thinner, converts from the amplitude of a rigid to that of a soft sphere. We have isolated the resonant component from the non‐resonant background by using an “intermediate background” formalism in order to exhibit the pure resonances. The sequence of resonance peaks in successive partial waves may be interpreted as due to circumferential waves (Regge poles) which resonate when n + 12 wavelengths span the sperical‐shell circumference; they undergo a phase jump...

Complex acoustic and electromagnetic resonance frequencies of prolate spheroids and related elongated objects and their physical interpretation

A numerical calculation of the complex eigenfrequencies of prolate spheroids and ellipsoids, and of finite‐length circular cylinders undergoing acoustic or electromagnetic eigenvibrations is reported. While mainly longitudinal eigenvibrations have been studied previously, here we obtained eigenfrequencies of vibrations which contain azimuthal components. These give rise (e.g., for the case of a cylinder) to helical surface waves, and we were able to interpret the corresponding eigenfrequencies in terms of resonances caused by the phase matching of such surface waves as they repeatedly engulf, and propagate around, the vibrating object. Phase and group velocities and absorption coefficients of the surface waves are obtained numerically from the set of complex eigenfrequencies.

Electromagnetic and Acoustic Resonance Scattering Theory

Abstract The excitation of the eigenfrequencies of finite radar or sonar targets, of inhomogeneities in elastic materials, of geological strata or of the entire earth by the impact of propagating waves (of electromagnetic or acoustic nature, or of ultrasonic, elastic, or seismic character, respectively) manifests itself in the appearance of poles in the resulting wave amplitudes, as described by the Resonance Scattering Theory (RST). In the complex frequency plane, these poles relate to the ringing of the scattering resonances. In the complex mode number plane, corresponding poles are connected with circumferential or creeping waves. An analytic relation between these two descriptions is indicated here, and a number of examples from the above-mentioned fields will be discussed. We introduce the concepts of ‘ Acoustic Spectroscopy ’ and of ‘ Radar Spectroscopy ’, respectively, by exhibiting the targets resonance frequency spectrum in a form familiar from atomic spectroscopy, in order to study the shifting and splitting of resonances ‘levels’ under changes of target shape, and to provide us with possible solutions for the ‘inverse problem’ (i.e., determination of target properties from echo properties - here, resonant echoes).

Calculation of electron channeling radiation with a realistic potential

Abstract The emission of radiation by electrons moving through a crystal channel has been quantitatively predicted by Kumakhov, and was experimentally observed by Swent et al . The present study of this effect employs a realistic crystal potential, which was developed by Pantell and Swent in order to furnish the location of the emission peaks. A detailed calculation of the peak intensities, including the transition matrix elements and the populations of the channeling states, is carried out here. The results agree satisfactorily with the planar-channeling data of Swent et al . We further present a numerical calculation of the spectra, angular distributions and polarization of the radiation from 56 MeV planar-channeled electrons.

Resonance spectra of elongated elastic objects

The eigenfrequencies at which smooth convex objects resonate under the incidence of an acoustic wave correspond to the real parts of those complex frequency values at which circumferential waves generated by the incident signal phase‐match after repeated circumnavigations around the object [H. Uberall, L. R. Dragonette, and L. Flax, J. Acoust. Soc. Am. 61, 711 (1977)]. A resonance condition based on this principle is formulated, and applied to the case of elastic prolate spheroids and cylinders with hemispherical endcaps. Using then the known phase velocities of surface waves on elastic spheres, with a radius equal to the local radius of curvature along the surface path, the elastic resonance frequencies of these objects can be predicted. This was done for the Rayleigh wave on a prolate spheroid, where comparison with resonances in the scattering amplitude as obtained by a T‐matrix calculation led to good agreement.

Resonances in Acoustic Bottom Reflection And Their Relation to the Ocean Bottom Properties

An analysis of the resonances in the acoustic reflection coefficient of a layered ocean bottom is carried out, ilustrated by a representative example with constant sound velocities in each layer. Resonances are evident in the reflection coefficient both as a function of frequency and of angle of incidence. They are shown to depend on the properties of the layered ocean bottom in such a way that the bottom parameters may be extracted from a measurement of certain characteristic features of the resonances, thus providing a solution of the inverse scattering problem.

Bistatic scattering and identification of the resonances of elastic spheroids

Resonances appear in the backscattering cross section of elastic objects (interfering with the contribution of specular echoes) when observed as a function of frequency. Only the resonance frequencies and resonance widths can be determined from such an observation, but not the resonance order. For targets of separable geometry, it has been shown both experimentally [G. Maze and J. Ripoche, J. Acoust. Soc. Am. 73, 41 (1983)] and theoretically [M. F. Werby and H. Uberall, J. Acoust. Soc. Am. 82, 265 (1987)] that bistatic observations can determine the resonance order after subtraction of the specular background. The method is applied here to bistatic scattering from elastic spheroids, as analyzed by a T‐matrix formalism and computer code. Mode mixing takes place now, but it is shown that for moderate aspect ratios (up to 3:1) a dominant mode can still be used to characterize the resonances.

Charged pion photoproduction from 12C with excitation of analog states

Abstract Differential cross sections of π + ( π − ) photoproduction near threshold on a 12 C target with excitation of the 12 B( 12 N) analog levels are calculated. Charged pion photoproduction excites predominantly the spin flip states, so that such an experiment may determine the degree of spin flip strength of the ΔT =1 levels.