Damage Characteristics Of Transient Fluid-structure Interaction Of Warship Structures Subjected To Near-field Underwater Explosion

It is often fatal of warship subjected to near-field or contact underwater explosion,which threaten to the survivability of warship seriously,thus investigations on the load characteristics and damage mechanism are significant.Different from the mid-and far-field underwater explosion,the loads of near-field or contact explosion include shock waves,high-velocity fragments,water-gas jet induced by detonation,bubble jet,etc.,which are featured by high temperature,high pressure and high speed shocks,and accompanied by large deformation,tearing,attacking,and splashing and so on.The damage of near-field structure is often strongly nonlinear and unrecoverable.This study aims at the loads of near-field or contact underwater explosion,through improving the meshfree Smoothed Particle Hydrodynamics(SPH)method for fluid-structure interaction(FSI)and its coupling algorithm with mesh algorithms,e.g.Finite Element Method(FEM)and Boundary Element Method(BEM),combined with verifications by existing experimental and theoretical results,the load characteristics of near-field or contact underwater explosion and their damage laws or mechanism are studied.Firstly,the representative works on underwater explosion in recent years are briefly reviewed according to the latest search results.Furthermore,focusing on the researches of near-field loads of underwater explosion including shock waves,bubble pulsation,high-velocity fragment,mixed water-gas jet,the blind spots and shortcomings are summarized:the load of near-field shockwaves is vague and lack of convincing data support;there is no systematical references for the damage mechanism and protective design of high-velocity fragments;the understanding of mixed water-gas jet is still confined to early approximation solutions and controversial.Given the above drawbacks,the recent applications of meshfree SPH method on underwater explosion are reviewed.Its natural Lagrange and particle properties are advantages of dealing with such problems and provide an effective tool for further studies.In view of the accuracy,the interface,the stability and the efficiency of three-dimensional SPH method,the key issues in the applications of SPH method are systematically analyzed.Through these analyses,as for the numerical accuracy,the total Lagrangian and updated Lagrangian equations are determined for structures and fluids respectively;the moving least square(MLS)function is used as the approximation function of structures to ensure the completeness and stability further.Considering the boundary of different phases,the boundary treatments of fluid,solid and interfaces between different phases should be applied on the premise of the completeness of approximation function;in order to satisfy the non-reflection boundary,a damping sponge layer or a matching impedance layer proposed inthis paper can be arranged outside the boundary.Taking the numerical stability into account,the stress point may effectively improve the hourglass mode,and the choice of kernel function should avoid the stress instability;some damping measures are necessary for the FSI model.In view of the computation efficiency,the linked-list searching method with variable smoothing length and OpenMP parallelized directives can effectively improve the efficiency of three-dimensional SPH method.As for the nonlinear problems often occurring in near-field or contact underwater explosion,such as large deformation,tearing,etc.,based on the global model of SPH shell(SPS)invented by A Combescure,an integral model of SPH shell with higher accuracy is proposed and meanwhile the adaptive multi-line cracks processing technology are presented to deal with the nonlinearly dynamic analyses of elastoplastic damage and fracture of spatial thin-walled structures combined with the plastic damage theory of Lemaitre-Chaboche.Moreover,taking the cross components and stiffened plate structures into account,the simplified contact algorithm is derived based on the Lagrangian multipliers.A quasi-static analysis method of SPH shell is also proposed.As for the transient FSI problems of underwater explosion,a coupled SPS-BEM method is proposed to realize the numerical simulation with self-programmed FSI codes.The coupled model is only discretized into a layer of SPS particles and a layer of flow field boundary elements.The MLS function is applied to guarantee the compatibility conditions at the interface of fluid and structure.The coupled method is simple,operable and efficient,which have been verified by some standard FSI benchmarks.As for the loads and structural damage of near-field or contact underwater explosion,the peak pressure and impulse distributions of underwater explosion near free surface and seabed are studied respectively;furthermore,considering the case of contact explosion near elastoplastic warship structures,the loads near charge,which have great influences on the structural damage,are investigated by axisymmetric SPH method and improved treatments of symmetric axis;finally,the fully SPH FSI method and coupled SPH-FEM FSI method are put forward,and then based on the combined damage factor,considering the strain rate effect,the damage modes and failure mechanism of flat plate,stiffened plate and complex elastoplastic warship structures are investigated and partial results have been verified by experimental and empirical formulas.As for the damage and protection of warship structures subjected to high velocity fragments,the fully SPH FSI method is introduced to simulate the high velocity fragment of different incident velocity,different radii and different incident angles penetrating through the broadside liquid cabin,and the velocity of fragments,the pressure and deflection at the inner plate of liquid cabin are observed and discussed.Given the above analyses,the protective design of liquid cabin is carried out in terms of variable widths and variable liquid loadings,which aims to provide a reference for the liquid cabin design.As for the mixed gas-water jet induced by detonation and bubble jet caused by underwater explosion,the fully SPH FSI model is adopted to conduct simulations.Firstly the water jets of different shapes impact on rigid wall and elastic target plate are studied,and the distribution laws of peak pressure and time-averaged pressure are summarized and partially verified by existing results;further,under the assumption of isotropy,the state equation of homogeneous gas-water mixture is derived based on the impact dynamics theory,and then the mixed gas-water jets of different gas volume ratios impact on rigid wall and elastic target plate are simulated.Similarly,the distribution of peak pressure and time-averaged pressure are discussed.