Fluid And Structure Impact Through Similarity Solution And Time Stepping Method

Fluids/structures impact problems have a wide range of important applications, including ship slamming, wave impact on offshore platforms, plunging wave on coastal structures, emergency landing of airplanes on the sea, as well as impact of super-cooled large droplets and ice lumps in the aeronautical settings, and applications in sports. Review and assessment of estimation techniques on fluids/structures impact are presented. The state of the art of prediction method is presented; Difficulties and key technologies are discussed.Impact usually lasts for a very short period of time and the effects of the viscosity of the liquid are usually ignored. As a result, the velocity potential can be introduced, which satisfies the Laplace equation when the compressibility of the liquid is ignored. A major difficulty is however the boundary conditions on the free surface, which are fully nonlinear and are imposed on a surface which is unknown and is part of the solution itself. The gravity effect on the flow is ignored based on the assumption that the ratio of the incoming speed of the liquid to the acceleration due to gravity is much larger than the time scale of interest. Based on the above assumptions we have developed the corresponding mathematic equations together with the numerical methodology.For a two dimensional rigid wedge colliding with water wedge at constant speed, similarity solution method is used, because there is no length scale. In other words, the spatial variables and the time variable could be combined. The problem of this similarity flow is solved by the boundary element method together with an analytical solution in the jet zone based on the shallow water approximation. Especially the convergent results are achieved through iteration for the integral form of the free surface boundary conditions. Various results are provided for the wave elevation, pressure distribution and force at different deadrise angles and at different direction of oblique entry. The effects of asymmetry and horizontal speed on these results are investigated. In particular, negative pressure near the tip of the solid wedge is observed and discussed.The water entry problem of single wedge or twin-wedges through free fall is then studied. Firstly the vertical entry of single or twin-wedges are considered, where the effects of mass, entry speed and deadrise angle of a single wedge have been discussed, and the interaction between twin-wedges has been observed. Furthermore the water entry problem of a wedge through free fall in three degrees of freedom is studied; In particular, the effect of the rotational velocity is taken into account, which seems to have been neglected so far. Extensive investigation has been made on the coupling of motions in three degrees of freedom. Similarity solution has been adopted as the initial flow pattern, and these problems are solved in a stretched coordinate system and the impact process is simulated based on the time stepping method. Auxiliary function method has been used to decouple the mutual dependence between the body motion and the fluid flow. The developed method is verified through results from other simulation and experimental data for some simplified cases and further validation are made through mass conservation and energy conservation.For the three dimensional axisymmetric hydrodynamic impact of cone structures and axisymmetric liquid columns, the problem is converted to quasi two-dimensional one and is solved by axisymmetric boundary element method in the axisymmetric coordinate system. This has simplified the solution procedure significantly. The constant water entry of a cone has been studied through similarity solution similar to the two-dimensional wedge, the integral free surface conditions have been adopted and the results are achieved through iteration. Various results of free surface profile and pressure distribution with different deadrise angle are presented. In particular comparisons with two dimensional wedges are presented to investigate the three dimensional effect.Then further researches are made on the coupled motion of the collision between cone structures and a liquid cone column or ellipsoid droplet. The free fall motion of cone structures is also studied, and various results on free surface profile and pressure distribution plus the acceleration and velocity at different entry distance are presented. Again the auxiliary function has been introduced to decouple the motion of the cone structures and the fluid flow. Then numerical simulations are made on the impact of liquid cone columns or sphere droplet. The results of the collision of liquid cone column at constant velocity are compared with those obtained from similarity solution and good agreement is found. The collisions of liquid column or sphere have been investigated at either constant velocity or through coupling analysis through time stepping method.