Theoretical And Experimental Study Of The Underwater Plasma Acoustic Source

The underwater plasma acoustic source (UPAS) has many advantages such as widefrequency band, high repetition rate and better safety, and it is being applied in theunderwater security area in recent years. In this dissertation, a series of theoretical studiesare carried out on the three working phases of the UPAS: breakdown-detonation-propagation and focusing. For these three working phases, physical models aredeveloped, and corresponding numerical simulations and experimental studies arecarried out.This dissertation mainly consists of the following aspects:1. The electrothermal breakdown, which usually happens under the common workingcondition of the UPAS (E~10kV/cm and conductive water), is theoretically analyzed.The generation process of an underwater arc in the electrothermal breakdown isproposed to be divided into three succeeding phases: ignition/streamer connection�'pre-breakdown-heating�'spark breakdown. Two necessary conditions to realize anelectrothermal breakdown,‘achieving the border boiling’ and ‘enough remainingvoltage’, are concluded and studied. The ‘border boiling’ can be explained to theprocess that, only when the weak electric field (E~10kV/cm) deposits enough energyinto the border water around the initial arc in advance, then can the‘evaporation-ionization’ cycle be possible. The ‘enough remaining voltage’ can beexplained to the condition that, after the pre-breakdown-heating phase, the field strengthin the underwater discharge gap must be enough to maintain the temperature and theionization ratio of the initial arc, and then, to provide enough initial electrons for thesucceeding spark breakdown phase.2. A physical model for the pre-breakdown-heating phase is developed, in which twoheating mechanisms in the pre-breakdown-heating phase, the holistic Joule heating fromthe ionic current and the local radiation heating from the initial arc, are firstlyconsidered equally. Basing on such model, the time-evolutions of the key variables inthe pre-breakdown-heating phase are simulated, of which the results disclose theimportant influence of the initial arc to the electrothermal breakdown. Correspondingexperiment is carried out towards this pre-breakdown-heating model. The experimentresults not only verify the basic assumption that the ‘border boiling’ temperature is about500oC(773K) which is proposed by the model, but also indicate that the minimumfield strength to realize the spark breakdown is about8kV/cm.3. The generation process of the dense discharge plasma (of which the density is in10×1020/cm3order) in the spark breakdown phase is theoretically analyzed. It is proposedthat, under a weak field condition (E~10kV/cm), the dense discharge plasma isgenerated in a ‘delayed avalanchine ionization’ process, which can be described as: onlywhen the temperature of the molecules is raised to be close to the temperature of theelectrons and the energy transfer caused by the elastic collision between electrons andmolecules is low enough, can the avalanchine ionization process be realized. Thetime-evolutions of the key variables in the generation process of the dense dischargeplasma are numerically simulated. On one hand, from the simulation results, theapproximate linear relationship between the field strength, in which the ‘delayedavalanchine ionization’ can be realized, and the corresponding critical density of themolecules is obtained. On the other hand, the simulation result also show that thequantity of the electrons in the initial arc is very important for the generation of thedense discharge plasma.4. A ‘cylinder detonation’ model, which finally aims at the calculation of themechanical output of the underwater arc, is developed. The equation of states in theisothermal process for ideal gas (‘p=nkT’) is used to calculate the pressure of thedischarge plasma. The cylinder expansion is used to describe the kinetic evolution of theunderwater arc. The ‘guess-iteration’ algorithm is used to solve the temperature of thedischarge plasma. Based on the ‘cylinder detonation’ model, the time-evolutions of thekey variables in the arc detonation process are simulated. In addition, by using theexperiment data, the possible initial radius for the arc generated in fresh water and saltywater are speculated.5. By using a numerical simulation software LS-DYNA, the time-evolution of thewave front of the discharge shockwave, which is generated by a cylindrical source withhigh length-radius ratio (100:1), is simulated. The simulation results shows that, afterthe propagation distance (l) exceeds10times of the arc length (d), the cylindricaldischarge shockwave will evolve to a quasi-sphere shock wave. Therefor,“l>10d” canbe regarded as an empirical criterion to set the minimum dimension of the focusingmantle. By using the LS-DYNA, the acoustic focusing of the discharge shockwave on the ellipse mantle with specified parameter (a=500mm, b=250mm) is simulated.The difference between the theoretical focusing gain (17.96) which is estimated by thenumerical simulation and the practical focusing gain (12.77) which is obtained by theexperiment shows that the dissipation of energy which occurs on the reflecting face ofthe ellipse mantle can not be ignored.