Numerical solution of 3D highly nonlinear water entry problems

Yang, Qingyong (2012) Numerical solution of 3D highly nonlinear water entry problems. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (18MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

Ships or offshore structures may experience severe water impact problems in the harsh environments, such as cargo sloshing, slamming and green water on deck. These impact loads can cause serious structural damage and are of considerable concern to the stability and survivability of ships or offshore structures. All of these forces are associated with highly nonlinear free surface flows. -- The thesis presents the numerical solution of slamming problems for 3D bodies entering calm water symmetrically and asymmetrically, with prescribed entry velocities and free-fall motions. The highly nonlinear slamming problems are governed by the Navier-Stokes equations and are solved by a Constrained Interpolation Profile (CIP)-based finite difference method on a fixed Cartesian grid. The CIP method is employed for the advection calculations and a pressure-based algorithm is applied for the non-advection calculations. For the pressure computation, a Poisson-type equation is solved at each time step by the Conjugate Gradient iterative method. The solid body and free surface interfaces are captured by density functions. A panel-based method is developed to capture the interfaces of 3D bodies. The motion of a body is described in terms of six degrees of freedom. -- Validation studies of the present method were carried out for several 3D bodies entering calm water symmetrically and asymmetrically with prescribed velocities and free-fall motions. Water entries of 3D bodies with prescribed velocities were first studied. For the water entry of a 3D wedge, 3D flow effects were investigated. 3D flow effects tend to cause a reduction in slamming force. The computed slamming forces are in good agreement with experimental results. For the sphere entering calm water obliquely, the computed vertical and horizontal slamming forces in general agree well with experimental results. The simulations were further carried out for a couple of bodies with complex geometry. For the water entry of a 3D ship section, pressures near the knuckles were under-predicted by the numerical method. The slamming force on a 3D flared body was also computed by the present numerical method, and the predicted slamming forces are in good agreement with the experimental results. The maximum slamming force coefficients of a planing hull with different pitch and roll angles were computed by the present numerical method and compared with these by the 2D strip theory. The 2D results are slightly greater than the 3D solutions. -- The studies were then extended to 3D bodies entering calm water with free-fall motions. The predicted motion of the half-buoyant cylinder with free-fall motion agrees well with the experimental data. For the neutrally buoyant cylinder, reasonable agreement is obtained, except at one experimental value which obviously deviates from the other data. The complicated free surface elevations during water entry of cylinder were simulated by the present numerical method. They are visually in good agreement with the photographs taken from the experiments. The present numerical method over-predicts the velocity ratios for water entry of a catamaran, especially for large drop heights. Velocity, acceleration, as well as vertical and horizontal hydrodynamic forces as a function of time were predicted by the present numerical method for the asymmetric water entry of a ship section. A satisfactory agreement with experimental drop test results is demonstrated.

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