Study On Resonance Scattering Of Bessel Beams By Submerged Elastic Spheres

The study about the acoustic scattering characteristics of underwater targets is animportant problem in the field of underwater sound. Its main purpose is to analyze theresponse of underwater targets under the action of the incident field. It is the basis for thedetection, location and recognition of underwater targets and has broad practical meaning.At present, most of the investigations focused on the case of plane wave incidence in thisrespect. As a kind of approximate non-diffracting beam, Bessel beam has an excellentdirecting property and a long transmission distance. Therefore, extensive applicationprospects could be anticipated in the field of underwater sound. When the incident wave isin the form of a Bessel beam, the related parameters of the beam would affect the responseof the target. The problem of the scattering of Bessel beams by submerged simple elasticbodies is a new research area. This thesis has committed to the development of therelevant theory in this area.In this thesis, the scattering of a Bessel beam by a submerged elastic sphere and anelastic spherical shell has been compared with the plane wave in two aspects, thebackground and the elastic resonances based on the Resonance Scattering Theory. Thespecial features of the scattering have been noted. Referring to the partial wave seriesrepresentation of a plane wave scattering, a feasible method to evaluate the form functionof a Bessel beam has been presented. For the Bessel beam scattering by a rigid sphere, thepeak to peak intervals in backscattering form function caused by the interference of thespecular wave and the Franz wave have been analyzed in geometry. The influence of thecharacteristic parameter β of a Bessel beam on the peak to peak intervals has beenindicated. For the scattering by an elastic sphere, the elastic scattering of each partial wavehas been separated according to the Resonance Scattering Theory. The influence of β onthe pure elastic resonance has been studied further. The results show that selecting specificβ can reduce the contribution of a certain partial wave. Therefore the resonance at thecorresponding frequency and the nearby region in the backscattering is remarkablysuppressed. In the discussion of the scattering by an elastic spherical shell, the resonance scattering mode ofa0ands0has been analyzed separately. It is found that there is notonly the suppression of a certain partial wave by a specific β, but also a generalsuppression of the phenomenon of midfrequency enhancement which exists in the planewave scattering. Furthermore, the scattering of high order Bessel beam has been brieflydiscussed, taking the example of first order Bessel beam. The enhancement of resonancein particular scattering angle has been emphasized compared to the case of plane wave andzero order Bessel beam. The work of this thesis could be helpful to the applications ofBessel beams on the acoustic detection of submerged objects.