engleski

Multiscale nonlinear and viscous numerical modelling of wave impact loads

Numerical framework for simulation of wave impact loads on ships and offshore structures in ocean environment is developed in this study, where fully nonlinear, viscous, turbulent and compressible two–phase flow is considered. Number of numerical methods and models are developed, tested and applied to provide a clear, comprehensive, reliable, robust and efficient numerical procedure for assessing wave impact loads on marine structures, with emphasis on green sea loads. The computational efficiency of the framework is improved by developing enhanced hydro–mechanical coupling strategies which reduce the number of nonlinear iterations required for the fluid flow–rigid body motion coupling to converge. A detailed verification and validation of the present numerical model is performed in order to ascertain the accuracy and precision in calculating green sea pressure loads, where a novel geometrical Volume–of–Fluid method is used and evaluated. Furthermore, a two–phase flow model is developed where water is assumed to be incompressible, while air is modelled as ideal adiabatic gas. The abrupt change in fluid properties across the interface, mainly density and compressibility, is handled with the Ghost Fluid Method, while Volume–of–Fluid method is used for interface capturing. The Ghost Fluid Method allows a one–cell– sharp representation of the interface with respect to the density field as well as compressibility effects, thus accurate and conservative trapped air cushioning effects can be captured, which is the main goal of the present numerical model. A pressure based formulation and the assumption of isentropic compression/expansion results in a highly efficient method, where no notable overheads exist with respect to the incompressible version of the model. The model is thoroughly verified and validated for flows without significant compressibility effects, as well as events where trapped air compression effects are important, such as breaking wave impact and free fall impact. Finally, a complete procedure for assessing wave impact loads is conducted with the developed model for an Ultra Large Container Ship, where green water loads on a deck structure at the bow are sought. The procedure relies on linear frequency domain method to provide a long term distribution of ship response, which is in turn used to define a deterministic design wave. Two different approaches for defining the design wave are used and compared. The development conducted in this work is performed within the Naval Hydro Pack software library, which is based on collocated Finite Volume method–based Computational Fluid Dynamics software foam–extend, community driven fork of the open–source software OpenFOAM.

Wave impact loads ; Green sea loads ; Two–phase compressible flow ; Ghost Fluid Method ; Polyhedral Finite Volume method ; Naval Hydro Pack ; foam–extend

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