Study On Prediction Method Of Dynamic Behavior And Propeller Strength Under Propeller-Ice Contact

Icebreaker is an indispensable means for countries to implement polar strategies,and its research and construction have been paid more and more attention by researchers in recent years.When icebreaker navigates in the ice-covered waters,with the motion of ship the ice pieces will approach to the propeller and then the interaction such as blocking effect,impact,milling and extrusion between propeller and ice will happen.Especially during the propeller-ice contact conditions including milling and impact,extreme ice load will exert on the propeller blades.In general,the ice contact loads are at least an order higher than the hydrodynamic loads during the propeller-ice interaction.The ice contact loads are so high that may cause deformation and damage of blades.Besides,since the ice contact loads are high frequency,their induced vibration will transmitted to the hull structure through the shaft,which may lead to local damage of ship structure.Thus,it is necessary to study on dynamic behavior and propeller strength during propeller-ice contacts,which can contribute to safety navigation of icebreaker in the polar environment.In previous studies,more attention has been paid to theoretical analysis and model experiment,but the study of numerical method is limited.Since the implementation of propeller-ice contact experiment is very difficult,many actual operational conditions cannot be carried out.A lot of mechanics for propeller-ice contact haven't been revealed.However,numerical method can overcome these shortcomings and simulate the cases that model experiment could not implement.Therefore,numerical method will be developed to study dynamic behavior during propeller-ice contact,which can help to research mechanics of propeller-ice contact,form the ability to analyze ice propeller strength,and support the design of ice propeller.In the introduction,the developments of ice mechanics,propeller-ice contact and ice propeller strength have been reviewed.For the review of ice mechanics,it is found out that peridynamics is suitable to simulate the large scale deformation and fracture of materials.Propeller-ice contact has been reviewed from theoretical analysis,numerical simulation and experiment investigation.The numerical method for propeller-ice contact is still in the early stage,and there still exist a lot of problems to be solved.For the review of ice propeller strength,although some scholars have carried out this research,the means are relatively limited.Peridynamics is the foundation for the study of dynamic behavior for propeller-ice contact.The motion equation of peridynamics is derived systematically,and the numerical solution method of peridynamics is introduced from the discretion of motion equation,particle search algorithm,calculating correction and boundary processing.Mechanical properties of ice are discussed,including complex mechanical characteristics of ice under different tensile and compressive loads.Considering failure characteristic under impact loads,material failure criterion of ice and the calculation equation of ice load are derived.Then,the numerical model of ice impact is developed based on peridynamics and its code is written independently.To validate the stability and feasibility of the model,the cases of ice sphere impact problem and ice cylinder impact problem are simulated.The effect of ice particle spacing on calculation results are studied,and the numerical results are compared with experimental data.Considering the sophisticated spatial structure of propellers,the surface of a propeller is discretized into small quadrilateral panels with the idea of panel method.Ice piece is considered as elastic brittle model and discreted into particles.The ice failure process of propeller-ice contact is simulated by the peridynamic method.To reflect the physical reality during calculation,a continuous contact detection algorithm has been developed,which can be used to prevent the interpenetration of a propeller body and ice material particles.The equations to calculate contact force and moment during propeller-ice contact are derived.Combined peridynamics with the contact detection algorithm,the numerical model of propeller-ice contact is developed.Propeller-ice milling mode is the most threat to propeller structure.The real condition of propeller-ice milling is simplified and its numerical model is developed.The effect of mesh method and number of propeller,ice material spacing on calculating results is analyzed,which can help to make sure the stability of the numerical model.The feasibility of the numerical model is validated by comparing the numerical results with the experimental data.Based on the numerical model,the dynamic behaviors of propeller-ice milling are simulated.The ice failure process,transient ice loads and bearing force are calculated.Based on the numerical model of propeller-ice contact,the dynamic behaviors of propeller-ice impact are studied for different propeller rotation direction,velocity of ice,size of ice and shape of ice.The motion and failure mode of ice and the transient ice loads are analyzed.It is found out that the ice cube will contact with the blade twice due to its rotation,and the ice sphere will contact with the blade only once.Considering real situation of propeller-ice impact,the numerical model of the impact between propeller and multiple ice pieces is developed,which can consider the interaction between propeller and ice and between different ice pieces.The second and fourth quadrants of propeller ice contact can be regard as special impact condition,and their behaviors have been studied.Considering the elastic deformation of propeller blades,the numerical method to calculate structural dynamic response of propeller-ice contact is developed by combining peridynamics with Finite Element Method(FEM),and its code is written independently.The code is applied to research on the structural dynamic response of propeller-ice milling condition.Ice load pressure,stress and deformation distribution on propeller blade over time are calculated.Numerical results show that ice load pressure is distributed in leading edge and outer radius of propeller.The stress is concentrated on the contact area as the blade enter or exit the ice block.When the propeller blade contact with the ice block fully,the stress is concentrated on the central section of the root.Then the code is applied to research on the structural dynamic response of propeller-ice impact condition.For the case of ice sphere,the situations of the ice contacting with leading edge,trailing edge an tip of blade are studied,which show that the stress and deformation mainly concentrate on the contact edge.Based on ice rule on propeller strength in IACS URI3 Rule,static strength assessment method of ice class propeller is established by FEM.For the blade strength assessment,mesh independence analysis and verification of the method are carried out.The blade stress and deformation distributions under five load cases are studied,and then the strength of the overall blade is checked.Then,the strength assessment method of blade edges under the concentrated ice load is established.The blade stress and deformation distributions under different loading positions are studied,and then the strength of the overall blade is checked.