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The seakeeping ability of ships is one of the aspects that needs to be assessed during the design phase of ships. Traditionally, potential flow calculations and model tests are employed to investigate whether the ship performs according to specified criteria. With the increase of computational power nowadays, advanced computational tools such as Computational Fluid Dynamics (CFD) become within reach of application during the assessment of ship designs. In the present paper, a detailed validation study of several computational methods for ship dynamics is presented. These methods range from low-fidelity system-based methods, to potential flow methods, to high-fidelity CFD tools. The ability of the methods to predict motions in calm water as well as in waves is investigated. In calm water, the roll decay behavior of a fully appended self-propelled free running 5415M model is investigated first. Subsequently, forced roll motions simulated by oscillating the rudders or stabilizer fins are studied. Lastly, the paper discusses comparisons between experiments and simulations in waves with varying levels of complexity, i.e. regular head waves, regular beam waves and bi-chromatic waves. The predictions for all methods are validated with an extensive experimental data set for ship motions and loads on appendages such as rudders, fins and bilge keels. Comparisons between the different methods and with the experiments are made for the relevant motions and the high fidelity CFD results are used to explain some of the complex physics. The course keeping and seakeeping of the model, the reduction rate of the roll motion, the effectiveness of the fin stabilizers as roll reduction device and the interaction of the roll motion with other motions are investigated as well. The paper shows that only high-fidelity CFD is able to accurately predict all the relevant physics during roll decay, forced oscillation and sailing in waves. [ABSTRACT FROM AUTHOR]

Hull-form stochastic optimization methods are presented and evaluated for resistance reduction and operational efficiency (operability), addressing stochastic sea state and operations. The cost/benefit analysis of the optimization procedure is presented by comparison of four hierarchical problems, from stochastic most general to deterministic least general. The parent hull is a high-speed catamaran, with geometrical constraints for maximum variation of length, beam, draft, and displacement. Problem 1 is used as a benchmark for the evaluation of the other problem formulations and is defined as a multi-objective stochastic optimization for resistance and operability, considering stochastic sea state and speed, but limited to head waves. Problem 2 is a multi-objective stochastic optimization for resistance and motions at fixed sea state and speed. Problem 3 is a multi-objective deterministic optimization for resistance and motions using a single regular wave at fixed speed. Problem 4 is a single-objective deterministic optimization for calm-water resistance at fixed speed. The design optimization is based on hull-form modifications by the Karhunen-Loève expansion of a free-form deformation, URANS-based CFD simulations, regular wave approximations for irregular waves, metamodels and multi-objective particle swarm. The design optimization achieves an 8.7, 23, 53, and 10% average improvements for problems 1, 2, 3, and 4, respectively. Comparing to problem 1, problem 2, 3, 4 optimized designs have average performances 1, 2.1 and 1.7% worse, respectively. The most efficient problem, from the computational cost/benefit viewpoint, is problem 3. Nevertheless, problem 1 is needed to evaluate and compare the stochastic performance of the designs and finally assess the optimization cost/benefit. [ABSTRACT FROM AUTHOR]

In recent years, the immersed boundary method has been well received as an effective approach for the fully resolved simulations of particulate flows. Most immersed boundary approaches for numerical studies of particulate flows in the literature were based on various discrete delta functions for information transfer between the Lagrangian elements of an immersed object and the underlying Eulerian grid. These approaches have some inherent limitations that restrict their wider applications. In this paper, a sharp interface direct forcing immersed boundary approach based on the method proposed by Yang and Stern (Yang and Stern, 2012, "A Simple and Efficient Direct Forcing Immersed Boundary Framework for Fluid-Structure Interactions," J. Comput. Phys., 231(15), pp. 5029-5061) is given for the fully resolved simulations of particulate flows. This method uses a discrete forcing approach and maintains a sharp profile of the fluid-solid interface. It is not limited to low Reynolds number flows and the immersed boundary discretization can be arbitrary or totally eliminated for particles with analytical shapes. In addition, it is not required to calculate the solid volume fraction in low density ratio problems. A strong coupling scheme is employed for the fluid-solid interaction without including the fluid solver in the predictor-corrector iterative loop. The overall algorithm is highly efficient and very attractive for simulating particulate flows with a wide range of density ratios on relatively coarse grids. Several cases are examined and the results are compared with reference data to demonstrate the simplicity and robustness of out method in particulate flow simulations. These cases include settling and buoyant particles and the interaction of two settling particles showing the kissing-drafting-tumbling phenomenon. Systematic verification studies show that out" method is of second-order accuracy on very coarse grids and approaches fourth-order accuracy on finer" grids. [ABSTRACT FROM AUTHOR]

Immersed boundary methods have been widely used for simulating flows with complex geometries, as quality boundary-conforming grids are usually difficult to generate for complex geometries, especially when motion and/or deformation is involved. A major task in immersed boundary simulations is to inject the immersed boundary information into the background Cartesian grid, such as the inside/outside status of a grid point with regard to the immersed boundary and the accurate subcell position of the immersed boundary for a grid point next to it. Complex geometries in immersed boundary methods can be conveniently represented with triangulated surfaces placed upon underlying Cartesian grids in a Lagrangian manner. Regular, intuitive implementations using triangulations can be error-prone and/or cumbersome in dealing with robustness issues. In addition, they can be prohibitively expensive for high resolution simulations with complex moving/deforming boundaries. In this paper, a simple, robust, and fast procedure is developed for setting up complex triangulations in immersed boundary simulations. Central to this setup procedure are a ray casting and closest surface point computation algorithms. Several illustrative examples, including high resolution cases with Cartesian grids of up to 2.1 x 109 points and triangulations of up to 1.3 x 106 surface elements, are performed to demonstrate the robustness and efficiency of our procedure. [ABSTRACT FROM AUTHOR]

Abstract: A direct forcing immersed boundary framework is presented for the simple and efficient simulation of strongly coupled fluid–structure interactions. The immersed boundary method developed by Yang and Balaras [J. Yang, E. Balaras, An embedded-boundary formulation for large-eddy simulation of turbulent flows interacting with moving boundaries, J. Comput. Phys. 215 (1) (2006) 12–40] is greatly simplified by eliminating several complicated geometric procedures without sacrificing the overall accuracy. The fluid–structure coupling scheme of Yang et al. [J. Yang, S. Preidikman, E. Balaras, A strongly-coupled, embedded-boundary method for fluid–structure interactions of elastically mounted rigid bodies, J. Fluids Struct. 24 (2008) 167–182] is also significantly expedited by moving the fluid solver out of the predictor–corrector iterative loop without altering the strong coupling property. Central to these improvements are the reformulation of the field extension strategy and the evaluation of fluid force and moment exerted on the immersed bodies, by taking advantage of the direct forcing idea in a fractional-step method. Several cases with prescribed motions are examined first to validate the simplified field extension approach. Then, a variety of strongly coupled fluid–structure interaction problems, including vortex-induced vibrations of a circular cylinder, transverse and rotational galloping of rectangular bodies, and fluttering and tumbling of rectangular plates, are computed. The excellent agreement between the present results and the reference data from experiments and other simulations demonstrates the accuracy, simplicity, and efficiency of the new method and its applicability in a wide range of complicated fluid–structure interaction problems. [Copyright &y& Elsevier]

Numerical optimization of the initial design of a fast catamaran (high-speed sealift research model B, HSSL-B) has been carried out through a simulation-based design (SBD) framework, based on an advanced free-surface unsteady Reynolds-averaged Navier-Stokes (URANS) solver and a potential flow solver, and global optimization (GO) algorithms. The potential flow computational fluid dynamics (CFD) SBD was used to guide the more expensive URANS CFD SBD. The fluid-dynamic analysis of the flow past the catamaran proved that the use of the URANS solver was fundamental in dealing with the multihull interference problem. In the case investigated, the separation distance was small and the viscous flow quite distorted by the proximity of the hulls, so that only viscous solvers could correctly capture the flow details. Sinkage and trim effects, due to the high speed range and again to the small separation distance investigated, are also relevant. The initial HSSL-B geometry and three optimization problems, including single- and multiobjective optimization problems, proposed by designers from Bath Iron Works, were successfully optimized/solved, and finally an experimental campaign was carried out to validate the optimal design. A new verification and validation methodology for assessing uncertainties and errors in simulation-based optimization was used based on the trends, i.e., the differences between the numerically predicted improvement of the objective function and the actual improvement measured in a dedicated experimental campaign, including consideration of numerical and experimental uncertainties. Finally, the success of the optimization processes was confirmed by the experimental measurements, and trends for total resistance, sinkage, and trim between the original and optimal designs were numerically and experimentally verified and validated. [ABSTRACT FROM AUTHOR]

Abstract: A sharp interface Cartesian grid method for the large-eddy simulation of two-phase turbulent flows interacting with moving bodies is presented. The overall approach uses a sharp interface immersed boundary formulation and a level-set/ghost–fluid method for solid–fluid and fluid–fluid interface treatments, respectively. A four-step fractional-step method is used for velocity–pressure coupling, and a Lagrangian dynamic Smagorinsky subgrid-scale model is adopted for large-eddy simulations. A simple contact angle boundary condition treatment that conforms to the immersed boundary formulation is developed. A variety of test cases of different scales ranging from bubble dynamics, water entry and exit, landslide-generated waves, to ship hydrodynamics are performed for validation. Extensions for high Reynolds number ship flows using wall-layer models are also considered. [Copyright &y& Elsevier]

High-fidelity Reynolds-averaged Navier Stokes (RANS) simulations are presented for the ducted marine propulsor P5206, including verification and validation (V&V) using available experimental fluid dynamics data, and subvisual cavitation, and acoustics analysis using the modified Rayleigh-Plesset equation along the bubble trajectories with a far-field form of the acoustic pressure for a collapsing spherical bubble. CFDSHIP-IOWA is used with the blended k-ω/k-ε turbulence model and extensions for a relative rotating coordinate system and overset grids. The intervals of V&V analysis for thrust, torque, and profile averaged radial velocity just downstream of rotor tip are reasonable in comparison with previous results. The flow pattern displays the interaction and merging of the tip-leakage and trailing edge vortices. In the interaction region, multiple peaks and vorticity are smaller, whereas in the merging region, there is better agreement with the experiment. The tip-leakage vortex core position, size, circulation, and cavitation patterns for σi=5 also show good agreement with the experiment, although the vortex core size is larger and the circulation in the interaction region is smaller. The simulations indicate globally minimum Cp=-σi=-8.8 on the suction side of the rotor tip at 84% chord from the leading edge and locally minimum Cp=-6.4 in the tip-leakage vortex at 08% chord downstream of the trailing edge, whereas EFD indicates σi= 11 and the location in the tip-leakage vortex core 50% chord downstream of the trailing edge. Subvisual cavitation and acoustics analysis show that bubble dynamics may partly explain these discrepancies. [ABSTRACT FROM AUTHOR]

URANS capability for hydrodynamic performance and slamming of high speed planing crafts are assessed, using metrics specified for naval applications. Wedge drop validation studies using two different solvers, one single-phase and the other two-phase, shows good agreement with experimental data for time history of pressure gauges. Planing hull validation studies are carried out using the historical benchmark experiments of Fridsma. Simulation conditions include calm water in deep and shallow conditions with fixed and free to sinkage and trim motions, as well as regular and irregular head waves in deep water free to heave and pitch motions. For simulations in calm deep water free to sinkage and trim, grid studies show that refined grid density on the hull, especially over the spray root area, chines, and transom stern are necessary for accurate solutions. Detailed verification and validation studies are carried out with satisfactory results since monotonic convergence and validation are achieved for most of the variables. Additional assessment metrics are studied including wave rise, hull pressure distribution, surface streamlines, existence of porpoising, and main spray trajectory. The results are compared with empirical expressions with reasonably good agreement except for main spray trajectory. Resolving the details of the spray flow is beyond the scope of the current work since finer grid resolution and/or more advanced free surface modeling is required. For simulations in calm deep water with fixed sinkage and trim, the assessment metrics include drag and lift forces, pitch moment, drag-lift ratio, and center of pressure. The results agree well with empirical expressions. For simulations in shallow water, similar studies are carried out and reasonable comparison results are obtained. Seakeeping simulations in regular head waves are carried out and validated with experimental data for response amplitudes and phases, with good agreement. Simulations in irregular head waves are carried out for two different weight configurations and validated against experiments for mean and amplitudes of resistance, heave, pitch, and acceleration. Slamming load analysis is carried out, where both emerging and re-entering slams are detected, although the current experimental data lack slamming pressures for validation. More recent research provides additional studies for calm water motions, slamming pressures, and spray flow trajectory that supports current results. [ABSTRACT FROM AUTHOR]

The paper presents a methodology to reduce the dimension of design spaces in shape optimization problems, while retaining a desired level of geometric variance. The method is based on a generalized Karhunen–Loève expansion (KLE). Arbitrary shape modification spaces are assessed in terms of Karhunen–Loève modes (eigenvectors) and associated geometric variance (eigenvalues). The former are used as a basis in order to build a reduced-dimensionality representation of the shape modification. The method is demonstrated for the shape optimization of a high-speed catamaran, based on CFD simulations and aimed at the reduction of the wave component of calm-water resistance. KLE is applied to three design spaces with large dimensionality ( ≥ 20 ), based on a free form deformation technique. The space with the largest geometric variance is selected for dimensionality reduction and design optimization. N -dimensional design spaces are used, with N = 1 , 2, 3, and 4, retaining up to the 95% of the geometric variance associated to the original space. The correlation between the objective reduction achieved, the dimension N and the geometric variance of the reduced-dimensionality space is shown and found significant. [ABSTRACT FROM AUTHOR]

Abstract: Detached Eddy Simulation on a 13M grid is conducted for a KVLCC2 tanker hull form at drift angle 30°. The objective is to identify and analyze the vortical structures, instabilities, and turbulent structures with analogy to the vortex breakdown and helical instability analysis for delta wing at large angles of attack. One shear layer, one Karman-like vortex shedding, and three helical mode instabilities are identified. The after-body side vortex, fore-body side vortex, and after-body bilge vortex exhibit all characteristics for vortex breakdown. For helical instability in the wake of the vortex breakdowns, Strouhal number using the ship length or the distance along vortex core shows the same trend with delta wings. The turbulent kinetic energy peaks near the separation point at the bow and on the vortex core of all vortices. For the after-body side vortex, turbulent kinetic energy reaches local maximum right after the vortex breakdown and is intensified along the vortex core further downstream. The simulations at drift angle 0° show four unsteady aft-body vortices, significantly improve the prediction of integral variables and local flows. In addition to the three steady vortices identified before, the simulations at drift angle 12° show unsteady aft-body hairpin and after-body side vortices. [Copyright &y& Elsevier]

Abstract: The flow past an interface piercing circular cylinder at the Reynolds number Re=2.7×104 and the Froude numbers Fr=0.2 and 0.8 is investigated using large-eddy simulation. A Lagrangian dynamic subgrid-scale model and a level set based sharp interface method are used for the spatially filtered turbulence closure and the air–water interface treatment, respectively. The mean interface elevation and the rms of interface fluctuations from the simulation are in excellent agreement with the available experimental data. The organized periodic vortex shedding observed in the deep flow is attenuated and replaced by small-scale vortices at the interface. The streamwise vorticity and the outward transverse velocity generated near the edge of the separated region, which enforces the separated shear layers to deviate from each other and restrains their interaction, are primarily responsible for the devitalization of the periodic vortex shedding at the interface. The lateral gradient of the difference between the vertical and transverse Reynolds normal stresses, increasing with the Froude number, is the main source of the streamwise vorticity and the outward transverse velocity at the interface. [ABSTRACT FROM AUTHOR]

Abstract: Part II of this two-part paper presents the simulation results of the plunging wave-breaking generated by impulsive flow over a submerged bump fixed in a shallow water flume using the exact experimental initial and boundary conditions provided in Part I of this study. The overall plunging wave breaking process is described with major wave breaking events identified: jet plunge, oblique splash and vertical jet. These major events repeat up to four times before entering the chaotic breaking phase. The plunging wave breaking process of the simulations shows a similar time line as the experiments consisting of startup, steep wave formation, plunging wave, and chaotic wave breaking swept downstream time phases. Wave breaking profile, air entrainment, velocity, vorticity, flume bottom pressure, and energy are analyzed and compared with the experimental results. The simulations qualitatively predict all four time phases, all four plunging events and their sub-events. The wave profile and location at the maximum height is very close to the experiment results. The flume flow and velocity demonstrate the same flow trend as the experiments but with reduced velocity magnitudes. The simulations show similar bottom pressure to the experiments but with large oscillations, and the post-breaking water elevations are larger as compared to the experimental results. [Copyright &y& Elsevier]

Abstract: The plunging wave-breaking process for impulsive flow over a bump in a shallow water flume is described, which is relevant to ship hydrodynamics albeit for an idealized geometry since it includes the effects of wave–body interactions and the wave breaking direction is opposite to the mean flow. This paper consists of two parts, which deal with experimental measurements and numerical simulations, respectively. In Part I, ensemble-averaged measurements are conducted, including the overall flume flow, 2-D particle image velocimetry (PIV) center-plane velocities, turbulence inside the breaking wave, and bottom pressures under the breaking wave. A series of individual plunging wave-breaking tests were conducted, which all followed a similar time line consisting of startup, steep wave formation, plunging wave breaking, and chaotic wave breaking swept downstream time phases. The plunging wave breaking process consists of four repeated plunging events each with three [jet impact (plunge), oblique splash and vertical jet] sub-events, which were identified first using a complementary computational fluid dynamics (CFD) study. Video images with red dye display the plunging wave breaking events and sub-events. The wave profile at maximum height, first plunge, bump and wave breaking vortex and entrapped air tube trajectories, entrapped air tube diameters, kinetic, potential, and total energy are analyzed. Similarities and differences are discussed with the previous deep water or sloping beach experimental and computational studies. The numerical simulations using the exact experimental initial and boundary conditions are presented in Part II of this paper. [Copyright &y& Elsevier]