Three-dimensional Sono-elasticity Of Ships

For many years, theoretical and experimental researches on the fluid-structure coupledvibrations and sound radiations of a ship have been carried out at home and abroad. A varietyof published research works in this field were about the vibrations and sound radiations oftypical simplified structures such as a flat plate, a stiffened plate and a stiffened cylindricalshell etc. in unbounded uniform fluid field. The corresponding experimental studies weremostly conducted for the mechanism investigations based on the tests of small scale models.Although great progress has been made in the past decades in this research field, it still hasbeen a focusing point about how to rationally represent the coupled fluid-structureinteractions in describing the distribution characteristics and clarifying the differences ofmachinery excited radiation sound fields of a ship in different water depth, differentsubmerging depth, different observation distance and direction, as well as in identifying thetransmission and variation behaviors of near field self-noise and far field radiation noise. Thisrequires the further development of a three-dimensional sono-elastic analysis method that isexpected to be suitable for ships with arbitrary shapes and complicated internal and externalstructures, capable to include the influence of free surface and sea bed, the effect of forwardspeed of the ship in the vibration and noise predictions, and also having acceptable numericalaccuracy and adequate computational demand in engineering applications to a full scale ship.It is against to this engineering background, as a creative extension and furtherdevelopment of the existing three-dimensional hydroelasticity theory of Wu (1984), athree-dimensional sono-elasticity theory of floating bodies is presented together with thecorresponding numerical methods in this thesis. The major achievements contained in thisthesis are as follows.1. The first, a three-dimensional sono-elasticity theory of a ship advancing in uniformacoustic medium with free surface is established by introducing the Green’s function for theideal compressible fluid and employing the Price-Wu generalized fluid-structure interfaceboundary conditions. This theory is applicable to either the sound radiation problem or thescattering problem. As the result of inclusion of the forward speed effect of the ship, thesono-elastic theory of ships seems theoretically more complete. The order analysis of theformulas and the numerical examples show that the ship speed will influence the coupledvibration and sound radiation. However the influence is only limited to the low frequencyrange and near field. 2. The second, in view of the actual geographical conditions of shallow China Sea,paying attention to the effects of sea bed and free surface, rather than the influence of waterdensity and sound velocity variations and stratum in depth, a three-dimensionalsono-elasticity theory of a ship in ocean acoustic environment of finite depth and thenumerical methods are established. To achieve this, the three-dimensional sono-elasticitytheory of a ship in uniform acoustic medium is combined with the ocean sound propagationtheory and the Pekeris waveguide model. The methods for solving the coupled dynamicequations of sono-elastic responses of the structure and the waveguide equation for acousticpropagation are presented in detail. An approximate series expansion formula of the Pekeriswaveguide Green’s function is described to greatly reduce the complexity in acoustic fieldcomputations.3. The third, three specific techniques are proposed for increasing the computationalefficiency and fulfilling the requirements of practical applications of the three-dimensionalsono-elasticity theories of ships and the corresponding numerical methods:(1) A ship sono-elastic sub-structure separation and integration method (SSSI) and amixed analytical-numerical sub-structure method (MANS)By separating the ship outer hull and the internal sub-structures (such as the bulkheads,decks, machinery foundations etc.), employing the super-element modal synthesis method toform the dynamic stiffness matrices of condensed input and output degrees of freedom,solving the coupled fluid and outer-hull interaction equations, and finally integrating the mainouter-hull and the internal sub-structures with their conjunction boundary conditions, the SSSImethod is proposed. This method has an intrinsic ability to enhance the calculation efficiencyand is particularly applicable to predict and to optimize the vibration and noise propagationamong ship hull and sub-structures. Further more, for a submerged vessel, a submarine or anAUV for example, the outer main hull can be modeled as a single-hull stiffened cylindricalshell simply supported at both ends, its fluid-structure interaction problem can be analyticallysolved, while the sub-structure inside the vessel can be numerically modeled. The MANSmethod may then be established to predict the sono-elastic responses of the vessel. Thismethod increases the computational efficiency as well as the frequency band of the responsesolution.(2) The sono-elastic analysis method for a structure covered with acoustic layersAcoustic layers, anechoic tiles so to call, are commonly used to cover on the hull surfaceof a submarine for depressing the deflected acoustic signal and the self radiated noise. Toallow for prediction of the sono-elastic responses of a underwater structure covered with acoustic layer, the three-dimensional sono-elasticity theory and numerical method are furtherextended by introducing a four-terminal parameter method to describe the acoustic wavetransmission behavior between the inside and outside surfaces of the acoustic layer. Thecoupled sono-elastic responses of the “hull structure-acoustic layer-water medium” systemmay then be solved.(3) The sono-elastic analysis method for a coupled fluid-structure system withdouble-fluid regionsIn a double-hull underwater vessel, water is filled in broadside space between the outerand inner hulls. The three-dimensional sono-elasicity theory is further modified toaccommodate the coupled response analysis of the system with double-fluid regions.4. The fourth, a “Closed Virtual Impedance Surface (CVIS) Method” for depressing the“irregular frequencies” encountered in the numerical solutions based on the sourcedistribution method is proposed. In the past decades the Helmholze integral method werewidely employed in solving the sono-elastic problems. Certain methods were simultaneouslydeveloped to successfully eliminate the problem of non-unique solutions appeared as the“irregular frequencies”. When applying the simple source distribution method instead of theHelmholze method, the problem of irregular frequencies also exists that remains as a difficulttask to deal with. In this thesis it is proposed to introduce a virtual closed surface withprescribed impedance inside the imaginary fluid region occupied by the floating body (theship). This surface absorbs the acoustic energy and suppresses the cavity resonances in theimaginary fluid region, and hence efficiently eliminates the irregular frequencies.The above results have formed a set of theoretical framework, numerical methods andapplication tools for sono-elastic analysis of the coupled vibration and acoustic radiation of acomplicated ship structure in the low and medium frequency range. The computer programdeveloped in this work has been integrated into the software entitled “Three-dimensionalHydroelastic Analysis of Floating Traveling Structures”(THAFTS) as an individual“Acoustic Module”.5. The fifth, for vilification and validation purposes, a number of numerical examples areused to compare the predictions obtained by the present methods and programs with theavailable analytical solutions of typical simple structure, the published data, and theexperimental results. The experimental data used in the comparisons include the structuralvibrations and acoustic pressures measured in the tests of a small scale stiffened cylindricalhull in open water with finite depth, and a two-cabin segment of an full scale old ship in alarge water basin, both were excited by operating machineries. The comparisons show that the present theory, numerical method and the corresponding program can be efficiently applied topredict the sono-elastic responses of a ship.6. The sixth, as a part of application examples, the method and program presented in thisthesis are particularly used to investigate and examine the following problem that is of greatinterests in measuring and assessing the noise source level of a submerged ship: thequantitative influences of the sampling duration, the observation distance and direction of thepressure measurement, the water depth and submerging depth of the ship on the assessment ofthe ship’s radiation noise source level. Based on the investigations, the reasonable samplingduration of the pressure measurement and processing method are proposed to providerelatively more stable (consistent) description of radiation noise spectrum and source levelassessment; the distribution characteristics of the near and far field radiation noise in differentwater depth and submerging depth are briefly exhibited; a method is proposed anddemonstrated to obtain an adjustment diagram for converting the resultant radiation noiselevel within a frequency range of a ship from one environment of water depth and submergingdepth to another based on the calculation results of the same ship in different water depth andsubmerging depth. Finally, the applications of the sono-elastic response analysis method andprogram developed in this thesis are briefly exhibited in two examples of machinery excitedvibrations and sound radiations of full scale ships. The comparisons of the predictions and themeasured results again confirm to some extent the applicability of the major achievementssummarized above.