High-Resolution Numerical Simulation of Turbulent Interfacial Marine Flows.

Abstract

An important aspect of designing offshore structures and seagoing vessels is an accurate prediction of the loads associated with wave impacts. In regions near the shore or during storms at sea, breaking waves are a common occurrence and the loading caused by their impact is typically more severe than in the case of regular non-breaking waves. Present methods for numerically predicting the impact forces use potential-flow methods with empirically-derived coefficients or relatively low-order methods in the computational-fluid dynamics (CFD) family. The potential-flow methods usually cannot simulate wave breaking and thus correction factors are necessary to account for slamming-like impacts that may occur due to plunging breakers. In some applications of the CFD tools, turbulence models are used to approximate the turbulent wave-breaking process in an effort to improve the prediction of the flow. The present work expands the understanding of the turbulence-interface interaction using highly-resolved numerical simulations to improve the CFD modeling capabilities in marine applications. The complex behavior of turbulence in the proximity of a deformable interface separating two incompressible phases is studied using two variants of CFD: direct numerical simulations (DNS) and large-eddy simulations (LES) that require modeling of the turbulence closure terms. Canonical flows are studied with DNS to determine the influence of the information typically not resolved by lower-order CFD methods and to establish the hierarchy of the modeling terms present in the governing equations. The relative magnitude of the convective and the interfacial subgrid terms are found to be significant and thus not negligible for a plunging-breaking wave flow. A scale-similarity-based model is proposed and implemented in the LES solver to include the effects of the unresolved flow features associated with the presence of the interface. The model is found to successfully approximate the subgrid behavior in multiphase flows with sufficient spatial and temporal resolution. The multiphase LES framework is extended to the study of breaking waves impinging on an offshore platform and the importance of the subgrid modeling to an accurate prediction of forces on the structure in demonstrated.