The generation and propagation of gravity waves dominates the far field of the vessel and can be simulated with efficient inviscid potential-flow methods. In the vicinity of the vessel, the diffraction of the travelling wave field becomes significant. Accordingly, the predictive ability to mimic wave diffraction determines the accuracy of the computed forces and motions. Since viscous effects are force crucial, inviscid approaches are deemed to be inappropriate. On the contrary, viscous CFD approaches, e.g. RANS methods, have proven capabilities to predict the response of floating bodies in turbulent flow. RANS methods are however afflicted with prohibitive computational expenses due to the associated grid requirements for an accurate propagation of incident waves towards the ship. Moreover, successive wave reflections from the far-field boundaries are difficult to manage and usually restrict the exploitable part of a transient simulation.
Simulations are usually performed on fine grids upstream of the vessel. Aft of the vessel, the grids are drastically streched towards the downstream direction in order to generate a suffient amount of numerical diffusion. As a result, any wave elevation is damped before the outlet is reached. Such methods are inappropriate for vessels manoeuvring in waves. Here, the location of inlet and outlet regions might change during the simulation, which requires a unique boundary condition for all far-field boundaries. FreSCo's approach is to implicitly force the simulation towards far-field seaway conditions along all far-field boundaries of the domain.
Animated results displayed on top pertain to simulations of a Wigley hull in head waves. The rectagular domain spans three hull lengths in width and breadth. All boundaries refer to FreSCo's unique seakeeping boundary condition which supports efficient simulations of vessels manoeuvring in waves and large relative changes of wave directions in hazardous conditions.
The second example refers to the predicted drag forces on a cuboid in head waves. Results are compared for the simulation in a compact domain using the present unique seakeeping condition with a simulation using traditional boundary conditions in conjunction with a numerical beach domain downstream of the cuboid.
A fair agreement between the predicted forces is seen for the two apporaches, which validates the quality of the employed unique seaway boundary conditions.
The below mentioned third example illustrates the turning circle of a series60 hull in waves, with emphasis given to the roll and pitch motion.
Dipl.-Ing. Katja Wöckner
Prof. Dr.-Ing. Thomas Rung
Dipl.-Ing. Manuel Manzke