Study On SPH Method For The Simulation Of Object-Free Surface Interactions

In abominable sea state,violent interactions between objects and the fluid surface make the hydrodynamic problem very complex.For example,during the violent interactions between non-linear water waves and floating structures,the wave slamming load may threaten the structure's local strength and in the meanwhile the impact force also induces large overall motions of six degrees of freedom(6-DOF)of the structure;surface ships generate non-linear rolling and splashing waves when sailing in high Froude numbers;in the water entry of rigid bodies,large slamming force leads to intense spray jets on the water surface.After the rolling of the water surface in the above phenomena,the entrapped air bubbles further make the violent fluid-structure interactions more complex.Different computational fluid dynamics(CFD)methods have been proposed for solving these challenging hydrodynamic problems.The CFD methods can be divided into two categories:mesh-based methods and mesh-free methods.In the first category,the mesh distortion in Lagrangian methods and free surface tracking in Eulerian methods have often caused many troubles in the simulation of free surface flows with large surface deformations;while in the second category,mesh-free methods are very suitable for the simulation of problems with large material deformations and even fragmentations.In this work,a mesh-free method named Smoothed Particle Hydrodynamics(SPH)method has been enhanced and applied for simulations of violent interactions between objects and free surface.The present work can offer technological supports for the development of numerical tanks in the future.In chapter 2,SPH theory and improved numerical techniques are introduced firstly.On these bases,for the study of floating body and water wave dynamics,a coupling algorithm in the framework of SPH method for the interactions between fluid and structure motions of 6-DOF is presented in chapter 3.In addition,the numerical water wave tank with a numerical wave maker and a wave damping zone is developed for the modelling of propagations of non-linear water waves.With an in-house SPH code,different challenging hydrodynamic problems are simulated,including the non-linear interactions between the focusing wave and floating body,green water impact,dynamic sinking process of three dimensional(3D)damaged structures and splashing bow waves generated by high speed ships.The SPH results are validated by either experimental data or other numerical solutions.All the results have demonstrated the superiority of SPH method in dealing with the fluid dynamics with large free surface deformations and large-amplitude motions of solid wall boundaries.In traditional SPH methods,numerical instabilities are often induced by the negative pressure.In addition,the computational efficiency in traditional SPH method is limited since Adaptive Particle Refinement(APR)is difficult to be implemented.In order to tackle the weaknesses of traditional SPH methods,in chapter 4 of the present work,a novel SPH model named?~+-SPH has been developed.The new SPH model further improves the accuracy of traditional SPH method and is capable of avoiding the non-physical oscillations in the pressure and velocity fields as well as avoiding the tensile instability in the fluid region characterized by negative pressure.After a wide validation through CFD benchmarks,the apparent improvements of the new?~+-SPH model with respect to the traditional?-SPH have been demonstrated.Thanks to the implementation of the APR technique,a multi-resolution?~+-SPH model has been proposed and the numerical accuracy in local fluid regions is improved while the computational cost for the overall simulation is significantly reduced.Based on the self-developed multi-resolution?~+-SPH model,along with some careful corrections for the boundary implementation,different water entry problems are modeled in chapter 4.It is worth noting that this is the first time the water entries and final-stage movements of 3D cylinders are modeled with the using of adaptive particle refinements.In order to investigate the effect of air phase in the water entry problem,the?~+-SPH model is further modified into a multiphase SPH model,with which the water entry of a wedge is modeled considering the effect of the entrapped air bubble.All the SPH results are validated by either experimental data or other numerical solutions and good agreements are obtained.In the flows around small-scale objects,the viscous effects cannot be neglected.Therefore,in chapter 5 of the present work,a viscous multi-resolution?~+-SPH model has been developed for the modelling of flow around different bluff bodies and also the body-induced free surface wave breaking.A new numerical technique named Tensile Instability Control(TIC)has been proposed to prevent the non-physical flow void incepted by the tensile instability in strong negative pressure regions.With the proposed TIC technique,viscous flows around bluff bodies in high Reynolds numbers can be modeled.Thanks to the adaptive particle resolution,very fine particle resolution can be arranged close to the body surface and therefore the turbulent flow details inside the boundary layer can be predicted.In the present work,flows around different NACA foils and cylinders are modeled and the SPH results are validated either by experimental data or by reference solutions of Diffused Vortex Hydrodynamics(DVH)method and Finite Volume Method(FVM).On these bases,the fish-like swimming problem is simulated and the results suggest that the viscous multi-resolution?~+-SPH model may be suitable for applications in biological fluid mechanics.Traditional SPH models are mostly applied to fluid-solid interactions,in which the solid bodies only perform rigid body movements while the effect of structure deformation on the fluid evolution is rarely considered.In chapter 6 of the present work,an efficient and accurate fluid-structure coupling algorithm has been proposed to couple the multi-resolution?~+-SPH model for the fluid modelling and the Total-Lagrangian SPH(TLSPH)method for the structure modelling.In this way,a new Fluid-Structure Interaction SPH(FSI-SPH)model is developed for FSI problems.After a number of benchmark validations including vortex induced vibrations and dam-breaking flow impacting the elastic plate,the FSI-SPH has been shown to be accurate in solving FSI problems with large structure deformations.Marine hydrodynamics is also often related to many complex gas-water flows which are often encountered during the exploitation of marine resources such as combustible ice.The accurate simulation of the complex gas-water flow with complex interfaces is not trivial for CFD methods.In chapter 7 of the paper,based on the principle of virtual work,governing equations for multiphase SPH based on volume approximation are derived.In addition,accurate viscous and surface tension force formulations are recommended and a modified time integral method is proposed for better numerical stability.Finally a robust multiphase SPH model is developed.A number of rising bubble benchmarks are adopted for the testing of the accuracy and stability of the multiphase SPH model and good numerical results are obtained.On these bases,3D single rising bubbles,bubble-bubble and bubble-free surface interactions are simulated and validated.All the numerical results demonstrate great superiority of the present multiphase SPH model in dealing with complex gas-water flows with interface fragmentations.