Flooding Characteristics Of Damaged Cabin Of Ship Structures Subjected To Near-field Underwater Explosion

Near-field underwater explosions or contact explosions may cause great damages to ships,and the flooding of damaged ships will reduce the ship's buoyancy and stability,posing a direct threat to the ship survivability.Meanwhile,near-field underwater explosions may also induce overall rigid motions of ships,which influence the flow characteristics inside the cabin.A complex fluid-structure interaction involving the ship hull,the inner and outer fluid as well as complex environmental loads will take place.The researches on the dynamic responses of damaged ships subjected to underwater explosions have great significances to the evaluation of the ship remaining survivability and the policy of sea rescues.What's more,it can provide references for the cabin optimization designs and ship unsinkability designs.The paper firstly reviews the characteristics of shockwave loads of near-field underwater explosions and the structural damage to ship,the process of a damaged ship flooding and the application of SPH method in the flooding of damaged ships.Based on the above-mentioned reviews,it can be found the great progress has been made in these problems.For instance,the understanding of near-field underwater explosion shock loadings is clearer,and the dynamic response characteristics of the cabin during the flooding process have been obtained preliminarily.What's more,the SPH method has been improved and applied to the flooding process of damaged cabins.However,the research on the flooding process of damaged cabins subjected to near-field underwater explosions is still relatively rare.In such problems,there are several complex processes,such as the propagation of underwater explosion shockwaves in multiple media,the large deformation and tearing of the structure,the slamming,flipping and splashing when the flooding against the bulkhead,the convection and the dopant of multi-phase media near the damaged hole and the rigid velocity by underwater explosions.These complex processes above may bring great difficulties and challenges to the traditional solutions.Firstly based on the coupling Euler-Lagrange(CEL)method,the characteristics of near-field shockwaves and its structural damage are studied.Then the damage characteristics are treated as an initial condition for the research of the flooding process.Finally,based on an the improved meshfree Smoothed Particle Hydrodynamics(SPH)method,the characteristics of the flooding hydrodynamics and dynamic responses of a damaged ship are studied considering various nonlinear factors,including multi-DOF motions of ships,wave loads and sloshing,etc.Firstly,the weakly compressible SPH(WCSPH)model is established in order to solve the fluid-structure coupling problem,and the key issues of the WCSPH method which including the treatments of the accuracy,the stability and the interface are analyzed.To reduce the calculation errors caused by the nuclear approximation,a high-accuracy kernel function is selected.Meanwhile,the continuity correction of the kernel function is made to ensure the completeness so that it can resolve the pressure noises caused by the weakly compressible assumption,and the dummy particles are applied so as to dispose the movement of the boundary in the problem of the fluid-structure interaction.On the base of the above work,a SPH numerical model with a high accuracy and stability is established to deal with violent FSI problems.Meanwhile,the effectiveness of the model have been verified.Furthermore,based on the Euler-Lagrange coupling method,the ripple effects of underwater explosion shockwaves in multiple media are studied,and the influences of the thickness of the structure and the back-side media is summarized;and then,the structure-acoustic coupling method is used to study the structural damage of near-field underwater explosions which is treated as an initial condition for the research of the flooding of damaged ships.Afterwards,a fully SPH FSI numerical model which combined with the slice theory is proposed to solve the flooding of damaged cabins.Firstly,the flooding process of the hole at different locations is simulated,and the flow characteristics inside the damaged cabin are investigated.Considering the rigid motion of ships subjected to near-field underwater explosions,the transient SPH FSI model is established to get the induced velocity varying with different charges,and then the effect of induced velocities on the dynamic responses of damaged cabins are discussed.Considering the effect of wave loads on the dynamics responses of ship motions,a wave model is established firstly to forecast wave loads,and the accuracy and stability of the coupling model are verified compared with the theoretical and experimental results;moreover,the dynamic responses of intact and damaged ships in waves at different frequencies and wave heights are studied,and the flow characteristics during the convective process near the opening and the motion of damaged ships are analyzed.Finally,the slamming characteristics of the inner flooding and the motion of damaged ships are summarized for the ship broadside in head and back waves.Considering the free surface inside the tank,the coupling motion of the free surface sloshing and the cabin motion are studied firstly,then the laws of impact pressure on the tank under the forced motions with different excitation amplitudes and excitation frequencies are discussed;further,the effects of liquid ratios and the inner baffle on the dynamic responses of damaged cabins are studied Meanwhile,a two-phase SPH model was introduced to consider the effect of air flows,and the accuracy of this model was verified by the benchmark of dam-break which show better agreements with experimental results.What's more,the flooding process of a damaged cabin is explored by the two-phase SPH FSI model.Considering the coupling factors of the flooding,the wave loads and the inner free surfaces,the fully SPH FSI model based on slice theory is adopted.The effect of multiple nonlinear loads on the ship motion and the slamming characteristics during the flooding are analyzed.Finally,a three-dimensional SPH for fluid-structure interactions is established and the experiments of two scaled ship models with different openings sinking into water are carried out.The comparisons show good agreements between the experiment and the simulation which verify the accuracy and feasibility of the three-dimensional parallel program.On this basis,the spatial motions of the models with openings at different locations are studied,which lays a foundation for the further researches on the sinking process of vessels which have more complex cabins.