Simulation Of Mid-high Vibro-acoustic In Ship Cabins And Research Of Its Transmission Path

Due to the deepening of marine development,the power level of power plants for ships and offshore platforms is increasing,and vibration and noise caused by machinery is becoming prominent in personnel accommodation.The International Marine Organization(IMO)and Classification Societies have carried out strict noise regulations.This,in turn,put forward new requirements for ship design and construction.Therefore,vibro-acoustic simulation contributes to noise reduction at design stage,which control measures adopted in the beginning,help to avoid the costly redesign or refit.The mid-high frequency band is known as the most important component of cabin noise,hence research on noise sources,energy transmission paths and reduction methods plays an important role in shipbuilding and national defense industry.However,the degrees of freedom(DOFs)of ship structure model are usually considerably large and the vibro-acoustic computational accuracy is poor,which making the research mostly remains in the qualitative analysis.In the meaning time,with the uncertainty of mid-high frequency problems,the efficiency of traditional analysis methods is very low,which unable to meets the demand for large ship simulation.There is an urgent need for new prediction methods.In order to overcome the deficiency of traditional methods,some valuable research and explorations on mid-high frequency vibro-acoustic analysis and optimization of ship accommodation performed in this thesis.The main research and conclusions of this thesis as follows:(1)Generalized vibro-acoustic analysis methods for mid-high frequency and selection basis.The Finite Element-Statistical Energy Analysis(FE-SEA)method and Statistical Energy Analysis(SEA)method were generalized to investigate middle frequency and high frequency problems in complex system separately.The effective bands of the two methods were distinguished by the ratio of feature size and internal wavelength.Then,the FE-SEA hybrid model and SEA model of a ship were established to meet the requirments of mid and high frequency vibro-acoustic problems separately.(2)Partial FE subsystem approach and Local Mode Perturbation(LMP)approach for hybrid model to improve the efficiency in middle frequency problems.In order to overcome low efficiency of FE-SEA method in large-scale issues,partial structures near sources were modeled as deterministic subsystems,and the secondary ones for statistical subsystems.LMP approach was proposed to analysis the Coupling Loss Factors(CLFs)of strong coupling structures,so that the DOFs of the whole system were limited within tolerable accuracy loss.(3)Source determination of accommodation.According to the bench and field test data,simulation on the ship system at mid-high frequency from 200～8000 Hz band was carried out to analysis the effectiveness of vibro-acoustic radiation from the main engine,gen-sets and propeller separately.The results show that main engine and gen-set are the principal sources,while the propeller can be ignored in cabin noise analysis.(4)Graph methods of Transmission Path Analysis(TPA)in mid-high frequency band.By regarding the subsystem as vertices and the couplings as segments,the SEA system transferred into the SEA weighted digraph,which the weight of each segment was derived from loss factors.The deviation algorithm of graph theory used for another purpose to solve the K dominant paths,which carry maximum energy of all paths,from the vibro-acoustic sources to targets.In addition,the centrality measures used to assess the importance of individual subsystem for energy flow on the system scale.(5)Determination of the graph analysis domain to minimize complexity.The unidirectional energy transmission was evaluated by vibro-acoustic temperature,which was defined by the mode energy,mode number and wave number of subsystems.Subsystems of higher temperature than target were identified as the path analysis scope,so that vertices and segments in graph are significantly reduced,the complexity of path analysis is the same.(6)Noise control measures based on mid-high frequency simulation and path analysis.A practical application of transmission path analysis from engine room to adjacent cabins was included to demonstrate the effectiveness and commonality of the combination of graph theory and SEA method.The results reveal that critical structures in vibration energy transmission are the input and path vertices,which are corresponding to the inner bottom plates of main engine room and auxiliary room,and the vertical bulkhead between them.Noise control measures of blocking mass applied on the couplings provides promising Sound Power Level(SPL)reduction by 5.59 dB(A)in crew spaces.As the conclusion,this thesis not only overcome some problems in engineer application of FE-SEA and SEA method,but also provide a validate choice for transmission path analysis in mid-high frequency domain.Combination of SEA and graph theory inspired ingenious ways to analysis energy transfer in complex systems and evaluating measures for local subsystem on system scale.The research obtained a series of practicable engineer results,provide guidance for vibro-acoustic design of ships.