Modeling free surface flows in a CFD context typically requires an incompressible approach along with a formulation to account for the air–water interface. Commonly, pressure-correction algorithms combined with the Volume of Fluid (VOF) method are used to describe these kinds of flows. Pressure-correction algorithms are segregated solvers, which means equations are solved in sequence until convergence is accomplished. On the contrary, the artificial compressibility (AC) method solves a single coupled system of equations. Solving at each timestep a single system of equations obviates the need for segregated algorithms, since all equations converge simultaneously. The goal of the present work is to combine the AC method with VOF formulation and prove its ability to account for unsteady flows of immiscible fluids. The presented system of equations has a hyperbolic nature in pseudo-time, thus the arsenal of the hyperbolic discretization process can be exploited. To this end, a thorough investigation of unsteady flows is presented to demonstrate the ability of the method to accurately describe unsteady flows. Problems of wave propagation on constant and variable bathymetry are considered, as well as a fluid structure interaction problem, where viscous effects have a significant impact on the motion of the structure. In all cases the results obtained are compared with theoretical or experimental data. The straightforward implementation of the method, as well as its accurate predictions, shows that AC method can be regarded as a suitable choice to account for free surface flows.

Flapping foils for augmenting thrust production have drawn attention as a means of assisting ship propulsion in waves due to their high efficiency rate compared to traditional screw propellers. However, they can also offer substantial resistance reduction when used as stabilizers. In this work, the aim is to investigate the feasibility of a symbiotic concept combining the ship’s propeller with a foil arranged at the ship’s bow at a fixed position operating as a trim-pitch stabilizer. The work presents results obtained from experiments conducted in the towing tank of the Laboratory of Ship and Marine Hydrodynamics of the National Technical University of Athens (LMSH NTUA), as well as results from an in-house CFD solver. The test cases focused on the resistance and the dynamic behavior of the wing–vessel configuration in calm water conditions and in head waves. All results were compared against the performance of a bare hull (without foil). The findings of this work are based both on numerical and experimental data and indicate that a bow wing in static mode can be used for trim-control of a vessel by altering the angle of attack leading to a possible drop in wave resistance both in calm water and in waves. In the latter case, utilizing the wing in head waves results in a significant reduction in the pitching and heaving responses of the vessel, which may lead to substantial enhancement of the propulsion performance.

The present paper proposes a new mixed-fidelity method to optimize the shape of ships using genetic algorithms (GA) and potential flow codes to evaluate the hydrodynamics of variant hull forms, enhanced by a surrogate model based on an Artificial Neural Network (ANN) to account for viscous effects. The performance of the variant hull forms generated by the GA is evaluated for calm water resistance using potential flow methods which are quite fast when they run on modern computers. However, these methods do not take into account the viscous effects which are dominant in the stern region of the ship. Solvers of the Reynolds-Averaged Navier-Stokes Equations (RANS) should be used in this respect, which, however, are too time-consuming to be used for the evaluation of some hundreds of variants within the GA search. In this study, a RANS solver is used prior to the execution of the GA to train an ANN in modeling the effect of stern design geometrical parameters only. Potential flow results, accounting for the geometrical design parameters of the rest of the hull, are combined with the aforementioned trained meta-model for the final hull form evaluation. This work concentrates on the provision of a more reliable framework for the evaluation of hull form performance in calm water without a significant increase of the computing time.

Moonpool-type floaters were initially proposed for applications such as artificial islands or as protecting barriers around a small area enabling work at the inner surface to be carried out in relatively calm water. In recent years, a growing interest on such structures has been noted, especially in relation to their use as heaving wave energy converters or as oscillating water column (OWC) devices for the extraction of energy from waves. Furthermore, in the offshore marine industry, several types of vessels are frequently constructed with moonpools. The present paper deals with the hydrodynamics of bottomless cylindrical bodies having vertical symmetry axis and floating in a water of finite depth. Two computation methods were implemented and compared: a theoretical approach solving analytically the corresponding diffraction problem around the moonpool floater and a computational fluid dynamics (CFD) solver, which considers the viscous effects near the sharp edges of the body (vortex shedding) as non-negligible. Two different moonpool-type configurations were examined, and some interesting phenomena were discussed concerning the viscous effects and irregularities caused by the resonance of the confined fluid.

Wave breaking is widely recognized as a very challenging phenomenon to emulate using numerical/computational methods. On that condition, the transition from modelling regular to irregular breaking waves is not trivial. Even though some issues are surpassed in CFD simulations, there still are two substantial problems to account for. The first one entails the proper generation of irregular waves in a numerical wave tank, while the second is the introduction of the turbulent regime of breaking in the solver. The present work addresses these two problems by employing the Stabilized $k-\omega$ SST model for turbulence closure and by proposing an efficient and accurate method for irregular wave generation. Apart from that, an artificial compressibility method is used for coupling the system of equations, which solves these equations in a non-segregated manner and overcomes problems pertaining to the existence of the interface in free-surface flows. The methodology is validated through the test case of irregular wave propagation over a submerged breaker bar and a piecewise sloped bottom, indicating the ability of the method to capture irregular breaking wave phenomena. Simulations are in fair agreement with experimental data regarding energy spectra and free surface time-series, while results suggest that the known over-prediction of turbulent kinetic energy (TKE) is significantly constrained by the stabilized $k-\omega$ SST model.

In this work a fully passive energy harvesting foil is studied computationally. An in-house 2nd order finite volume CFD solver, MaPFlow, is strongly coupled with a rigid body dynamics solver to investigate the foil operation under uniform and sheared inflow conditions with/without free surface. The mesh follows the airfoil motion using a radial basis function (RBF) mesh deformation approach. Initially, MaPFlow predictions are compared to experimental and numerical results available in the literature, where reasonable agreement is found. Next, one-phase simulations are considered for linearly sheared inflow for various shear rates. Results suggested that foil performance can be enhanced under sheared inflow conditions. Finally, two-phase simulations taking into account the free surface, for both uniform and sheared inflow, are considered. Predictions indicate a significant deterioration in performance of the system when the foil operates under the free surface due to the interaction of the shed vorticity with the free surface.