Yu. V. Petukhov

Formation, Propagation, and Focusing of Acoustic Beams in Oceanic Waveguides. The Quasi-Optics Approximation

Using the WKB approximation and assuming that the spatial spectrum of modes excited by a vertical array is sufficiently narrow, a quasi-optical theory is developed to describe the fundamental regularities manifesting themselves in the formation, propagation, and focusing of multimode acoustic beams in oceanic waveguides. Functional dependences on the parameters of both the transmitting array and the oceanic waveguide are obtained for horizontal distances at which ordinary beams form refraction focusing zones. Conditions ensuring the formation of a beam with the minimum wave front divergence are formulated for the distribution of the source excitation factor over the array aperture.

Caustics and weakly divergent beams in oceanic waveguides

It is found that the weakly divergent beam formed around the reference ray with minimal cycle length does not stand out in intensity against the background of the total field. It is shown that, under the same conditions, another beam stands out in intensity: it is the caustic beam composed of the system of caustic lines, which are the envelopes of the family of rays forming the weakly divergent beam.

Conditions for forming weakly diverging acoustic bundles in ocean waveguides

Necessary and sufficient conditions for the dependence of the cycle length of the Brilloin waves or rays of geometrical acoustics on the ray parameter that is inversely proportional to their phase velocity are formulated. The formulated conditions determine the existence of weakly diverging acoustic bundles in vertically stratified oceanic waveguides, even with a point sound source.

Weakly divergent ray and diffraction beams in oceanic waveguides

Conditions that should be satisfied by the sound velocity profile of an oceanic waveguide for the dependence of the ray cycle length on the ray phase velocity to contain smooth extrema are formulated. The extrema correspond to weakly divergent ray beams forming “caustic” beams. It is found that diffraction effects cause a considerable smoothing of the sharp extrema that occur in the dependence of the interference period of neighboring modes on their phase velocity. As a result, in addition to the weakly divergent ray beams, weakly divergent diffraction beams and the corresponding “diffraction” caustics can be formed.

Formation, propagation, and focusing of acoustic beams in oceanic waveguides: Simulation of processes

A quasi-optic theory developed in [1] for describing the propagation and focusing of multimode acoustic beams in oceanic waveguides smoothly inhomogeneous along the track is generalized with the use of the adiabatic approximation. Analytical calculation and numerical simulation are carried out for the corresponding processes using the simplest model of a surface channel that is homogeneous along the track with a linear dependence of the squared refraction index on depth. The results confirm the validity of the laws established in [1].

Formation of caustic beams in a refractive oceanic waveguide

We consider the patterns in the formation of the spatial interference structure of an acoustic field excited in the near-surface channel of a vertical array consisting of point sources emitting a tonal signal inphase. It is established that when the array aperture is increased to a certain optimal size, only one caustic beam forms in the channel. As the array aperture further increases, the sequential formation of a certain number of beams after the first is observed.

Formation of a weakly diverging caustic beam in an underwater sound channel open to the bottom

The paper studies the patterns manifesting themselves in the formation and propagation of caustic and weakly diverging beams using the geometric acoustic approximation and mode theory applied to the simplest model of an underwater sound channel open to the bottom in the form of the bilinear dependence of the square of the refractive index on depth. It is established that when a vertical array emitting a tonal sound signal is located at a certain critical depth, the multimode caustic beam which forms near the reference ray horizontally departing from its center and which predominates in intensity is simultaneously the most weakly diverging beam. It is shown that the reference ray of such a weakly diverging caustic beam corresponds to the smoothest minimum for the dependence of the length of the ray cycle on their angle of departure from the center of the array and to the depth level of revolution at the critical depth characteristic of the given oceanic waveguide.

Manifestation of weakly diverging beams of rays in the spatiotemporal structure of the acoustic signal field in oceanic waveguides

It is established that the existence of weakly divergent beams in an oceanic waveguides leads, in a certain time interval and depth range, to the formation of a characteristic triplet structure of wavefronts. It is explained that a weakly diverging beam of rays represents an Airy wave in the spatiotemporal structure of the acoustic signal field in an oceanic waveguide.

Parametric generation of acoustic-gravity waves in the solar atmosphere

Based on a plane isothermal solar-atmosphere model, we investigate the parametric generation of acoustic-gravity waves (AGWs) in the approximation of a fixed field for vertically propagating disturbances. Both nonpropagating and propagating AGWs are shown to be generated at the difference frequency via the nonlinear interaction of primary waves in the frequency range “forbidden” for the propagation of AGWs during their linear generation. An acoustic wind has been found to be formed in the solar atmosphere at zero difference frequency; its velocity increases with height in inverse proportion to the decreasing ambient density. We study the nonlinear generation of AGWs at the second harmonic during the interaction of disturbances from the forbidden frequency range.

Operation of linear and ring arrays in shallow water

With the aim of unambiguously determining the direction to a tonal emission source, comparative analysis of the directed properties of sufficiently extended linear and ring horizontal arrays in shallow water was performed. An isovelocity waveguide with a homogeneous fluid bottom was used as the shallow water model, in which a multimode regime of acoustic signal propagation was achieved.