Numerical Study Of Water Entry Prob-Lem Using A CIP-based Model

Water entry is a complex multiphase flow problem with short duration.It involves solid,liquid and gas,which has great scientific and engineering research value.However,due to the short duration of impact(order of milliseconds),the large instantaneous slamming pressure and the complicated fluid-structure interaction,the flow mechanisms are yet to be studied.Taking into account the economic efficiency,adaptability and comprehensive information on flow field,numerical simulation has become an important method to study water entry.Thus in this paper,an improved CIP-ZJU numerical model is used to study the water entry of cylinder and wedge.The evolution,characteristics and impact factors of flow field distribution in the vicinity of the object are analyzed.Considerable attention is paid into kinematic and hydrodynamic characteristics of moving body.The paper firstly describes the improved CIP-ZJU numerical model.In the improved CIP-ZJU model,the finite volume method based on Multi-Moment grid is used to discretize the viscous Navier-Stokes equations.The R-CIP-CSL2 method only needs to use the value of one grid point and the integral of its adjacent grid points to achieve third-order accuracy.Therefore,the RCIP-CSL2 method is applied to solve the convection term of the NS equation to improve the accuracy and efficiency.The centre difference scheme is employed to calculate the diffusion term.Due to the great instantaneous impact pressure during water entry,an improved implicit projection method is used to obtain the coupling of pressure and velocity fields.The SOR algorithm is used to solve Poisson equations twice to improve the stability and accuracy.The tangent of hyperbola for inter-face capturing with slope weighting(THINC/SW)is adopted to capture the free surface/interface.In order to avoid grid update and improve computational efficiency,an immersed boundary method is applied to simulate the motion of bodies.Then the improved CIP-ZJU model is adopted to study the water entry of symmetric wedge and horizontal cylinder to verify the accuracy of the model.The influence of initial impact velocity and deadrise angle on the motion,impact force and pile-up coefficients of wedge and the pressure and velocity fields of fluid,especially the peak value of pressure on wedge surface and its position are analyzed.The influence of the impact velocity and mass of the horizontal cylinder on the motion,impact force,free surface evolution and distribution of flow fields is analyzed as well.The added mass force is an important factor for the analysis of underwater motion.In this paper,according to VonKarman's theory,a method to calculate the added mass force during constant water entry of wedge is proposed.Besides,the variation of added mass force is analyzed.In the field of ocean engineering,the wind,waves and currents always cause asymmetric impact.Due to the different deadrise angles on both sides,the interaction of the fluid on both sides is more significant for the water entry of asymmetric wedges.The asymmetric water entry problem is more difficult than symmetric water entry.In this paper,The effects of initial deadrise and inclination angles on the hydrodynamics of wedges,the pressure and velocity fields for fluid near the wedges are analyzed.The evolution of the peak value of pressure on wedge surface and its position during asymmetric water entry is explored.The water entry of twin wedges and twin horizontal cylinders are studied based on sea bed installation of jacket platform,slamming of catamaran and the slamming during two ships in par-allel.Because the interaction between the bodies and the motion of bodies needs to be considered during water entry,it is more difficult than single body water entry.Therefore,the effects of the distance between the two bodies,the velocity and the mass of bodies on the kinematic and hy-drodynamic characteristics of twin bodies are analyzed.The distribution of the flow field in the vicinity of the bodies is analyzed,especially the mechanism of the extreme pressure value.The numerical simulation of the body water entry in waves has to reproduce the evolution of waves and the interaction of moving bodies.As a result,it is much more difficult than body impacting calm water surface.Therefore,the wedge water entry in waves is studied.Based on the viscous N-S equation,the momentum source term derived from the Boussinesq equation is applied to the wavemaker.The wave damping by sponge is applied to the numerical wave tank.The influence of wave parameters and the position of wedge impacting water surface on wedge hydrodynamics is discussed.The distribution of the flow field in the vicinity of the wedge is analyzed.Besides,the influence of the wedge on the wave transformation is studied as well.