Numerical Research On Truncated Model Method Of Ship Structural Borne Noise Prediction

To ensure that a ship has good underwater quiet performance, it is utterly important tocarry out prediction evaluations of ship’s underwater noise characteristics in the stages ofdesigning and constructing to discover problems in the acoustic design of the ship. Therefore,foreign powerful navy countries have invested massive human and material resources in theunderwater noise prediction and evaluation of ship structures, forming a series of methods ofassessment of ship’s underwater noise prediction, such as analytical method, engineeringestimation method and numerical methods. Analytical method is suitable for the underwaternoise prediction of simple structure (such as cylindrical shell), while it is not competent whenthe structure is slightly complex. Engineering estimation method can be used for theunderwater noise prediction of ships with similar forms; however, significant error will occurif gaps between the predicted ship and motherboard are huge. Numerical methods, especiallythe presence of Finite Element Method (FEM), Boundary Element Method (BEM),FEM/BEM, and Statistic Energy Analysis method, have provided abundant approaches forunderwater noise prediction of ships. However, due to the complexity of underwater acousticradiation of ship structure, there are still many basic and fundamental problems requires to besolved in the numerical noise prediction, especially the underwater noise prediction with lowand middle frequencies.The present situation of studies on the underwater prediction of ship structural bornenoise are overviewed firstly in this paper, and then the general situation of the development ofship structural borne noise prediction are represented systematically. Fundamental problemsand difficulties in the underwater acoustic prediction of ship structural borne noise areanalyzed. Analysis shows that underwater noise prediction of ship structural borne noise isvery difficult due to the involving of the coupling of fluid-structure-sound andstructure-fluid-sound-air. For the numerical prediction of ship structure available, there arestill various shortages in the research of numerical method of ship structural borne noiseprediction, determination of equipment exciting loads to ship hulls, model simplification ofnumerical prediction model of ship structural borne noise. With problems above, based onengineering requirements on numerical prediction of ship structural borne noise, this paperexamines the underwater noise prediction method with low and middle frequencies of shipstructure. Study lays special emphasis on the discussions of predicting principles of truncatedmodel of ship structural borne noise prediction, exciting loads from equipment to ship hulls,model simplification of ship structural borne noise, time-frequency integrated predictionmethod and infinite element method. Based on discussion above, a systematical truncatedmodel method of ship structural borne noise prediction is proposed. Combining with the reality of noise prediction of a ship and an underwater vehicle, the paper explores feasibilitiesof t truncated model prediction method of ship structural borne noise. Characteristics ofunderwater sound radiation of the ship and the underwater vehicle is preliminarily analyzed inthe hope of providing references for ship’s acoustic quantative prediction and relatedresearches.Aiming at overcome shortages of ship structural borne noise prediction, such as largescales of calculation, low solving efficiency, narrow bands of forecasting, this paper proposesa truncated model method to analyze ship structural dynamics based on the analyzing chainstructural dynamics. Combining underwater noise radiation theory, this paper gives outtruncated method of fluid domain of ship structural noise prediction model. On that basis, thepaper proposes truncated model method of ship structural borne noise prediction, which canreduce the scale of calculation, and increase the solving efficiency and forecast frequencybands. Examples of underwater acoustic radiation prediction of cabin structure shows that thetruncated model method can effectively reduce the scale of calculations, avoid numericalcalculation error caused by factors such as modeling of equipment, and increase predictionfrequency bands of ship structural borne noise. This method has large potential applications inlow and middle frequency prediction of ship structural noise especially when there arenumbers of equipment, which can reduce the scale of calculations effectively, increaseforecast frequency bands and solving efficiency.This paper also carries out research on the input of ship structural borne prediction-exciting loads from equipment to ship. The determination of excitation load from equipmentto ship hulls is analyzed. Based on the equipment endogenous characteristics and parameteridentification theory, the paper proposes the identification method for equipment endogenouscharacteristics with the bench test, and thereby establishes the excitation load transformingmethod from bench tests to ship hulls. Based on the coupled vibration mechanism ofequipment and hull structures, the paper proposes a simplified calculating method for theexcitation from equipment to hull structures. Based on the unitary theory of force boundaryand motion boundary, the analogical method of motion boundary is proposed to solvedifficulties that equipment excitation loads of vibration acceleration is not easy to exert innumerical computing. Examples have proved the effectiveness of moving boundaryanalogical method, which solves problems of providing input loads for noise prediction ofships.In the aspects of model simplifying of ship structural noise predicting, based on structurepower analysis and acoustic radiation theory, this paper proposes the traveling-wave methodthat can use local cabin model instead of the entire ship model for ship structural borne noiseprediction. The applicable conditions, further simplifications of the prediction model of shipstructural noise are discussed. Through underwater acoustic radiation truncated model ofcircular structure, the paper verifies the validity of cabin truncated model of ship structuralborne noise, and solves the difficulties of using local cabin instead of the entire whole shipmodel in the prediction of ship structural borne noise. For the problems in the prediction available of ship structural noise, such as lowefficiency, the disability to consider non-linear factors and take into account of low-frequencyvibration and high-frequency wave coupling factors of ship structure, the paper proposes thetime-frequency integrated prediction method of ship structural noise, and carries out thecorresponding numerical verification. The research shows that time-frequency combinedmethod can not only smooth difficulties in the prediction of ship structural noise, but alsoanalyze the interested noise frequency in detail to determine the dominant pass componentand dominant pathway of ship structural noise of the concerning frequency, which providesrapid support for the prediction, assessment and noise control of ship structural noise. For theproblems existing in external fluid field truncation and extraterritorial far-field acousticradiation of ship structural noise, the paper proposes the application of acoustic infiniteelement method in ship structural noise prediction model, in the hope of reducing the modelsize and improving the calculation accuracy, and further realizing the fast prediction ofextraterritorial far-field acoustic radiationFinally, a systematic truncated model method of ship structural borne noise prediction isproposed to standardize the flow of ship structural borne noise prediction based on thesummarization of studies above in this paper. Combining with the demands of ship structuralborne noise prediction of ships and underwater vehicles, this paper gives out specific stepsand flows of the application of truncated model method of ship structural borne noiseprediction. The underwater acoustic radiation characteristics of a ship and underwater vehicleis analyzed, with respects to provide method supports for engineering applications oftruncated model method of ship structural borne noise prediction...