| Moonpool structure are widely applied in the equipments of offshore petroleum resources drilling and marine energy exploitation.and can be found to have important application in special ships and naval vessels.Because of the special structure form of the moon pool in which a free surface is existed inside and an open area is existed at the bottom,violent resonance of free surface can be occured when the moonpool is subjected to external flow and wave excitation.In order to provide a safe working environment for sonar equipment.the sonar trunk is set up in the middle of the new generation of mine countermeasures ship.The sonar trunk is a typical kind of moonpool structure.To achieve the rise and fall of sonar at any angle,the opening at the bottom of the moonpool in the mine countermeasures ship has been enlarged on the original level.But after the opening enlarged,the instrument behind the moonpool was affected by the bubbles produced in the moonpool.specifically,the results from bathymeter in high speed range show poor accuracy and the results from vialog with respect to water is inaccurate.Therefore,a thorough analysis of the mechanism and characteristics of the fluid oscillation in the moonpool and a comprehensive study of the formation causes,evolution and development of bubbles in the moonpool as well as bubble distribution on the bottom of the ship are of great academic value and engineering guiding significance.In this thesis,the research progress of hydrodynamic characteristics of moonpool is reviewed from three aspects firstly,that is.experiment approach,analytical method and numerical approach.Meanwhile,the research status of self-sustaining oscillation of shear layer and gas entrainment by bow wave and breaking wave is summarized.By summed up the research work of scholars at home and abroad,it can be found that most of the analytical and semi-analytical methods are focused on the two-dimensional moonpool.and the research on the high order modes of the three-dimensional moonpool needs further development.The equipment in the moonpool is often neglected in the research,the hydrodynamic characteristics of the moonpool considering the influence of the large-scale equipment are lack of relevant research.Most of the studies on the self-sustaining oscillation of shear layer are carried out in the case of planer jet and cavity flow under higher velocity.The characteristics of shear layer self-sustaining oscillation and its development on the bottom of the moonpool need to be further analyzed,when the velocity of flow is much less than the sound velocity.Moreover,the study of bubbles in the moonpool has not been found in the existing studies,the mechanism of the bubble formation,the change regulation of bubble evolution and dev elopment in the moonpool,the distribution characteristics of bubbles on the bottom of the ship and the measures to effectively suppress bubbles in the moonpool are the urgent problems to be solved.In view of the shortcomings of the existing study,research work of this thesis are carried out in the respects of semi-analytical and experiment approaches.Under the assumption of linear potential flow theory,the gonverning equations of two-dimensional and three-dimensional rectangular moonpool are established in this thesis.And the modified Fourier series method is introduced to solve the equations.The velocity potential function is assumed to be the sum of standard Fourier series and two auxiliary functions.By establishing the equations of unknown coefficients in Fourier series,the solution of partial defferential equation in the original problems is simplified as a standard eigenvalue problem.The validity of the proposed method is verified by comparing with the results in the literature.The influence of the moonpool dimension on oscillation characteristics is systematically analyzed by solving the natural frequencies and mode shapes of moonpool with different widths and depths.The moonpool dimension parameters which are more sensitive to frequency variation are pointed out.Considering the limitations of numerical and analytical methods in the study of bubbles in the moonpool.a hydrodynamic and bubbles characteristics experiment system is developed in this thesis.The whole experiment system is consist of two types of test,which are non-ventilation ship model test in towing tank and ventilation PIV test in towing tank respectively.With considering the influence of large equipment,the fluid oscillation and bubble characteristics in the moonpool are studied comprehensively,in which a variety of measurement means are used,such as high speed camera,dynamic pressure sensor and towing submerged PIV measurement system.In the non-ventilation ship model test,the main work is to analyze the mechanism of bubble formation in the moon pool.According to the comparison of bubbles existence in the moonpool recorded by high speed camera under different current velocity and different oscillation modes.the causes of bubble emergence can be determined.Meanwhile,the characteristics of fluid oscillation with complex boundary conditions under the uniform flow and interaction waves and currents are studied.The empirical mode decomposition method is adopted to analyze the data of dynamic pressure sensor.The intrinsic mode function representing the self-sustaining oscillation of shear layer at the bottom of the moonpool are separated from the experiment results.By summarizing the frequency characteristics and feedback model of the shear layer oscillation of the moonpool and observing the phenomena of free surface oscillation captured by high speed camera,the oscillation mechanism under different excitation modes is analyzed.Combined the pressure data obtained from the bottom of the ship,development of shear layer toward stern is explored,and the phenomenon of fluctuation in the bottom of the ship is pointed out.The ventilation PIV test in towing tank is mainly to study the distribution of bubbles on the bottom of the ship.Aiming at the problems of short navigating time and less bubbles at high current velocity in ship model test,an artificial ventilation device is designed to simulate the bubble environment of full-scale ship.Through the integral superposition of the single-frame images taken by the underwater camera during the towing navigation,the statistical characteristics of bubble distribution in four typical sections are analyzed.The effects of the opening shapes,the ship hull form,current velocity and quantity of ventilation are discussed.The vital information of bubble distribution,such as the width and thickness of the bubble layer and the location the bubble concentration area,are obtained.And the validity of experimental data from the ventilation PIV test is verified by compared with the results of ship trial.On the basis of fully understanding the mechanism of the fluid oscillation and bubble emergence in the moonpool,various fluid and bubble control measures are designed,which are bottom baffle,internal damping plate,bottom closure plate,diversion trough,collection plate and splitter board,specifically.The principle of these control measures can be summarized as the following four aspects,that is,controlling the flow separation at bottom of the moonpool to reduce the excitation of waves and currents,increasing damping of fluid system in the moonpool to consume the energy of fluid motion,reducing the generation and leakage of bubbles in the moonpool and improving the bubble distribution on the bottom of ship.The effect of these measures is validated and evaluated in model test.It is found that the combination of bottom baffle and internal damping plate can effectively reduces the free surface oscillation in the moonpool,and remarkably reduces the bubble production rate and quantity of bubble in the moonpool.The combination of diversion trough and collection plate greatly optimizes the the bubble distribution on the bottom of ship.These results show a strong guiding significance and provide a substantial technical support for solving practical engineering problems. |
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