A BEM-Level set domain decomposition for violent two-phase flows in ship hydrodynamics

In many practical circumstances relevant in ship hydrodynamics, complex water flow evolutions occur, involving large deformations of the free-surface, breaking and fragmentation. The resulting water-ship interactions may lead to dangerous structural loads and they can concern the safety and the stability of the vessel. The analysis of such problems is challenging both in terms of the capability to handle the physics involved, and of the CPU time and memory space needed for sufficiently accurate investigations. The latter become crucial when three-dimensional effects are accounted for. Here we present a numerical study to deal with these types of phenomena, their occurrence, evolution and possible structural effects. The used method is based on the Domain Decomposition (DD) philosophy where the problem is split into sub-problems, each one analyzed by the most efficient and suitable solver. In particular, a Boundary Element Method (BEM) is used in the fluid regions where the air-water interface can be modeled as a smooth surface and vorticity and viscosity effects are negligible. A Navier-Stokes (NS) solver, coupled with a Level-set (LS) technique for capturing the air-water interface, is applied in the fluid areas interested by breaking phenomena and vorticity generation. Since the air evolution becomes relevant in case of air entrainment, both liquid and gas phases are simulated by the BEM and NS-LS solvers. The most important feature of the DD method proposed here is the presence of an air-water interface in the exchange region between the different solvers. In this paper, the features of the present domaindecomposition strategy are described and the challenges connected with the coupling are deeply discussed. The numerical investigation highlighted the importance of a proper rational study when CFD methods are considered. In the present case a crucial aspect is represented by the domain composition (DC) step, where the information from one solver to the other have to be properly reconstructed and made consistent with the receiver sub-domain. Such aspects are detailed analyzed by using the dam-breaking problem as test case.