The Research Of Nonlinear Acoustic Interactions In Water Medium

According to the acoustical nonlinear propagation theory, the amplitude solution ofnonlinear sounds mutual impact is proposed in the paper. The phenomena of energytransmitting with the nonlinear sounds mutual impact are also modeled, and someexperiments are carried out to verify the conclusions. According to the physical characteristicsof ocean medium and the feature of sound propagation, preliminarily develop a sounds mutualimpact dynamical model; then, model and analyze the energy dynamic evolution after soundpassing the model by using the chaos judgment method, which is from nonlinear dynamicstheory. finally, based on the simulation results, propose a technical solution to decreaselow-frequency parts energy of sounds by using the nonlinear mutual impact of the sounds.The verification results support the technical proposal.This paper mainly conduct the following researchs:1. This paper describes the development of nonlinear acoustics, including the researcheson chaotic phenomenon of the water medium, the cross areas of nonlinear acoustics anddynamics. A lot of achievements are obtained on this area since1950s. From the researches onthe nonlinear effect of acoustic waves, the research on nonlinear acoustics is a field need tobe investigated more manpower. The nonlinear acoustics brought many improvements onacoustical techniques directly or indirectly, and formed a good basis for future research.2. This paper analysised kinetics characteristics of the output in the linear system andnonlinear system, explained the the importance of the nonlinear system. Solved the analyticalsolution of the second order variable parametethe differential equation by using Laplacetransform. Taking single freedom Duffing system for example. By using nonlinear dynamicstheory of chaos judge method, this paper analysised the effect of the system parameters orexternal incentive system to the kinetics characteristics of the system output, confirmed theparameters threshold of the system in different motion state Then draw out mathematicalmechanism, which the interaction of the nonlinear variable parameter acoustic wave in thewater medium.3. Based on the theory of wave propagation in nonlinear acoustics, amplituderepresentation of the single frequency large-amplitude sound wave after nonlinearpropagation is solved through the nonlinear wave equation. The energy transfer between thefundamental frequency wave and higher order harmonic waves generated in propagation issimulated. From the Burgers equation, the spectral decomposition method is applied to analyze the nonlinear interactions between the low-frequency weak signal wave and pumpwave (large amplitude wave). The effects of various initial phase differences, frequency ratiosand source levels between the two waves on change of the low-frequency signal energy aresimulated and the amplitude solution of the low-frequency weak signal is given. The energytransfer between the two waves in interaction is studied and result of energy changes of thelow-frequency wave in experiment conform to that in the simulation. The amplituderepresentations of three resonant waves after interaction are derived. The variation curves ofwave energy with and without dissipation are given respectively through numericalcalculation under the conditions of various propagating distances, medium nonlinearparameters and amplitude and frequency of pump waves. The effects of energy reduction andamplification of the low-frequency weak wave after interaction are studied by experiments,which show that values of the reduction and amplification obtained in experiments areconsistent with those of the simulation when source level of the pump wave is within certainlimits.4. In this paper, the empirical model of sound wave nonlinear interaction in marinemedium is built, which is used to analyze the influence of nonlinear module, amplificationmodule, delayed module and various parameters of the system on the output sound energy.Automatic control theory is applied to study the features of open-loop control and closed-loopcontrol. The system output dynamic characters are analyzed by chaos dynamic theory with thegraphs of time-domain, power spectrum, phase diagram and maximum Lyapunov exponentunder the open-loop control and closed-loop control. The research show that open-loopcontrol system can change the sound spectrum, but unable to turn the output wave spectruminto chaos status. The low-frequency wave of the feedback system under the externalhigh-frequency excitation is studied with the graphs mentioned above and the results indicatethat the feedback control system can change the frequency spectrum of the output signalunder the excitation. System output signal energy will present the chaos status and frequencyband of the low-frequency wave will appear widening when the control parameters arewell-chosen. Finally, various parameters of external excitation are chosen to act on thenonlinear control system to interfere the low-frequency wave, which demonstrate that smallextent of external high-frequency disturbance is conductive to lead the low-frequency wave toachieve the chaos status and show energy decrease through nonlinear control system. Theenergy transfer model of sound wave nonlinear interactions in marine medium is establishedpreliminarily.5. Each aspect of research above has been done in the innovative guidance of tutor. The related experiments in the tank and the small flume support the results of theoretical studies inthe thesis. Some new special phenomenon has been found in the experiments, which providereference for subsequent research.