Numerical Analysis Study On Shock Wave Characteristics Of Pulsed Streamer Charges In Water

Transmitting information in a complex marine environment is a key part of developing marine resources.Sound wave is the only way to transmit information in the ocean,so developing an optimum underwater sound source has become an important sub-subject under this major issue.A plasma underwater sound source based on the principle of underwater pulsed streamer charges has the advantages of high sound source level,small sound source volume,wide frequency response range,high electro-acoustic conversion efficiency and programmable sound source in common,at the same time,its controllability and repeatability are superior to the traditional plasma sound source manufactured by the principle of arc discharge,which has a great application foreground in ocean communication,ocean sound channel measurement,ocean exploration,etc.Consequently,it is necessary to study the acoustic characteristics of pulsed streamer charges in water,such as acoustic radiation,frequency spectrum and bunching sound field distribution,which provides a reference for the engineering application.In this paper,coupled numerical simulations are performed to study the propagation of shock wave due to pulsed streamer charges in water by using ANSYS and LS-DYNA.And based on some typical ways of shock wave bunching,the characteristics of the bunching are discussed,from which some conclusions are summarized.The main work done in this thesis is as follows:(1)The physical mechanism of the shock wave induced by underwater pulse streamer charges is introduced,and compared in detail with the general acoustic wave.Numerical methods mainly used in this chapter are introduced and discussed,including required governing equation,state equation and difference scheme.The feasibility of calculating the transient sound field of shock wave by using arbitrary Euler method and Mie-Gruneisen water state equation is proved.(2)A numerical model for calculating the single shock wave is established.Then some parameters required for the calculation are analyzed parametrically and further determined based on experiments.Furthermore,with the spatiotemporal parameters of the streamers,a numerical model for calculating propagation properties of shock wave is established,then the time-frequency characteristic of shock wave due to single and double streamer is studied numerically.It is found that sound pressure level of the single shock wave can reach up to 280 dB,and it is significantly attenuated within 0-1mm from the discharge needle tip.The spectrum of single shock wave generated by pulsed streamer charges can reach up to MHz,and its sound pressure is mainly distributed in the low frequency band.The shock wave generated by a single streamer propagates more in the direction perpendicular to the streamer.The shock wave generated by the two streamers will eventually form a converging wave in the direction of the normal line inside and outside the streamer,but the difference is that the larger the angle of the streamer is,the sharpened the directivity of shock wave is.(3)The bunching principle and application background of underwater plasma sound source with some typical bunching methods such as ellipsoidal reflector,parabolic reflector and array bunching,are introduced.Based on Euler equation and Mie-Grüeisen equation of state,a bunching sound field distribution model of underwater shock wave is established,which meets the requirement of minimum bunching size of the source.The bunching process of the shock wave with three bunching methods is calculated numerically.Simultaneously,for ellipsoidal reflector and parabolic reflector,the comparison is made from the bunching nephogram,bunching gain and bunching delay.It is found that the ellipsoidal reflector can form a concave shock wave surface,and the one of the parabolic reflector is smooth;the shock wave surface of the line array is concave.And the bunching bias effect obtained by adjusting the timing is remarkable,but the bunching gain is smaller than the normal timing;the line array bunching gain is much smaller than the reflector bunching;the bunching gain of the ellipsoidal reflector is always greater than the parabolic reflector under the same conditions.As the direct wave pressure increases,the time delay of two types of reflectors is reduced.When the direct wave pressure reaches 0.6 MPa,the time delay of the shock wave does not change substantially,but the time delay of the parabolic reflector is always smaller than the ellipsoidal reflector.The ellipsoidal bunching gradient decreases with the increasing direct wave pressure,which means the ellipsoidal bunching shock wave front is excessively flattened from the bunching area to the non-bundling area,and the gradient nonlinearity becomes obvious.