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High-fidelity simulations of wave breaking processes are performed with a focus on the small-scale structures of breaking waves, such as bubble/droplet size distributions. Very large grids (up to 12 billion grid points) are used in order to resolve the bubbles/droplets in breaking waves at the scale of hundreds of micrometres. Wave breaking processes and spanwise three-dimensional interface structures are identified. It is speculated that the Görtler type centrifugal instability is likely more relevant to the plunging wave breaking instabilities. Detailed air entrainment and spray formation processes are shown. The bubble size distribution shows power-law scaling with two different slopes which are separated by the Hinze scale. The droplet size distribution also shows power-law scaling. The computational results compare well with the available experimental and computational data in the literature. Computational difficulties and challenges for large grid simulations are addressed.

This paper discusses the maneuvering performance of the Office of Naval Research Tumblehome (ONRT) surface combatant model in waves. Maneuvering tests in calm water and regular waves using free-running model were conducted at the IIHR wave basin, The University of Iowa. Ship trajectories and 6DOF motions/velocities were obtained for all tests. Course keeping tests were performed in 8 variable headings for three wave lengths and harmonic analysis were done for measured variables. The 1st harmonic amplitude of pitch and heave have maximum value in head waves and beam waves, respectively. Other amplitudes except for these two have maximum value at stern quartering waves. Zigzag and turning tests were performed in calm water and in waves. Maneuvering characteristic parameters such as 1st and 2nd overshoot angles, advance and tactical diameter in calm water were evaluated based on the International Maritime Organization (IMO) criteria. Maneuvering characteristic parameter of zigzag and turning in waves were also evaluated and compared to those of S-175 and Very Large Crude Carrier (VLCC). It is confirmed that ONRT satisfies IMO criteria and has higher maneuverability than S-175 and VLCC due to its slender hull with the twin-rudder twin-screw.

The KCS container ship was investigated in calm water and regular head seas by means of EFD and CFD. The experimental study was conducted in FORCE Technology’s towing tank in Denmark, and the CFD study was conducted using the URANS codes CFDSHIP-IOWA and Star-CCM+ plus the potential theory code AEGIR. Three speeds were covered and the wave conditions were chosen in order to study the ship’s response in waves under resonance and maximum exciting conditions. In the experiment, the heave and pitch motions and the resistance were measured together with wave elevation of the incoming wave. The model test was designed and conducted in order to enable UA assessment of the measured data. The results show that the ship responds strongly when the resonance and maximum exciting conditions are met. With respect to experimental uncertainty, the level for calm water is comparable to PMM uncertainties for maneuvering testing while the level is higher in waves. Concerning the CFD results, the computation shows a very complex and time-varying flow pattern. For the integral quantities, a comparison between EFD and CFD shows that the computed motions and resistance in calm water is in fair agreement with the measurement. In waves, the motions are still in fair agreement with measured data, but larger differences are observed for the resistance. The mean resistance is reasonable, but the first order amplitude of the resistance time history is underpredicted by CFD. Finally, it seems that the URANS codes are in closer agreement with the measurements compared to the potential theory.

Unsteady Reynolds averaged Navier–Stokes (URANS) computations of standard maneuvers are performed for a surface combatant at model and full scale. The computations are performed using CFDShip-Iowa v4, a free surface solver designed for 6DOF motions in free and semi-captive problems. Overset grids and a hierarchy of bodies allow the deflection of the rudders while the ship undergoes 6DOF motions. Two types of maneuvers are simulated: steady turn and zigzag. Simulations of steady turn at 35° rudder deflection and zigzag 20/20 maneuvers for Fr  = 0.25 and 0.41 using constant RPM propulsion are benchmarked against experimental time histories of yaw, yaw rate and roll, and trajectories, and also compared against available integral variables. Differences between CFD and experiments are mostly within 10 % for both maneuvers, highly satisfactory given the degree of complexity of these computations. Simulations are performed also with waves, and with propulsion at either constant RPM or torque. 20/20 zigzag maneuvers are simulated at model and full scale for Fr  = 0.41. The full scale case produces a thinner boundary layer profile compared to the model scale with different reaction times and handling needed for maneuvering. Results indicate that URANS computations of maneuvers are feasible, though issues regarding adequate modeling of propellers remain to be solved.

This paper describes a novel sharp interface approach for modeling the cavitation phenomena in incompressible viscous flows. A one-field formulation is adopted for the vapor-liquid two-phase flow and the interface is tracked using a volume of fluid (VOF) method. Phase change at the interface is modeled using a simplification of the Rayleigh-Plesset equation. Interface jump conditions in velocity and pressure field are treated using a level set based ghost fluid method. The level set function is constructed from the volume fraction function. A marching cubes method is used to compute the interface area at the interface grid cells. A parallel fast marching method is employed to propagate interface information into the field. A description of the equations and numerical methods is presented. Results for a cavitating hydrofoil are compared with experimental data.

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.

An overview is provided of CFDShip-Iowa modeling, numerical methods and high performance computing (HPC), including both current V4.5 and V5.5 and next generation V6. Examples for naval architecture highlight capability and needs. High fidelity V6 simulations for ocean engineering and fundamental physics describe increased resolution for analysis of physics of fluids. Uncertainty quantification research is overviewed as the first step towards development stochastic optimization.

Numerical simulations of an open-water propeller are performed using CFDShip-Iowa. The propeller, originally designed by Mercury Marine for a 21 feet high-speed planing hull, is scaled to match a 42 feet hull configuration. Three advance ratios (J = 0.8, 1.1, and 1.4) and two cavitation numbers (σ = 0.274 and 1.095) are considered in the computations, and the results are compared with those obtained from the commercial CFD solver STAR-CCM+. For the fully wetted conditions without cavitation, the overall trends of the computed thrust (Kt), torque (Kq), and propeller efficiency (η) with respect to the advance ratios are similar. The computed Kt, Kq, and η with cavitations generally agree with the STAR-CCM+ results except for η at σ = 0.274, where the latter shows a much higher value for J = 1.4. For σ = 1.095, the cavitation patterns and overall pressure distributions are similar for both codes. For σ = 0.274, the cavitation is more violent for CFDShip-Iowa than STAR-CCM+. CFDShip-Iowa shows better preservation of the cavities and blade-to-blade interactions, which are not captured in the simulations using STAR-CCM+, since a single blade with periodic boundary conditions are used.

A method that can be used to perform self-propulsion computations of surface ships is presented. The propeller is gridded as an overset object with a rotational velocity that is imposed by a speed controller, which finds the self-propulsion point when the ship reaches the target Froude number in a single transient computation. Dynamic overset grids are used to allow different dynamic groups to move independently, including the hull and appendages, the propeller, and the background (where the far-field boundary conditions are imposed). Predicted integral quantities include propeller rotational speed, propeller forces, and ship’s attitude, along with the complete flow field. The fluid flow is solved by employing a single-phase level set approach to model the free surface, along with a blended k − ω / k − ɛ based DES model for turbulence. Three ship hulls are evaluated: the single-propeller KVLCC1 tanker appended with a rudder, the twin propeller fully appended surface combatant model DTMB 5613, and the KCS container ship without a rudder, and the results are compared with experimental data obtained at the model scale. In the case of KCS, a more complete comparison with propulsion data is performed. It is shown that direct computation of self-propelled ships is feasible, and though very resource intensive, it provides a tool for obtaining vast flow detail.

For accurately evaluating the risk of broaching for a ship in stern quartering waves, the wave forces calculated with a slender body theory under the low encounter frequency assumption are often corrected empirically with the help of captive model experiment or computational fluid mechanics (CFD). However, the hydrodynamic explanation of such empirical correction was not sufficient so far. Thus, this paper attempts to provide an answer to this deficiency. First, we visualise the vortices shed from the hull surface using existing CFD results for the ONR tumblehome topside vessel. Then we extend Hamamoto’s slender body theory, in which the interaction between the shed vortex and the hull sections is taken into account for calm water, to the case in astern waves and apply it to the current case by using the CFD-based vortices strength and position. As a result, it is confirmed that additional hydrodynamic forces due to vortices shed from the hull surface other than transom are a major component of the empirical correction for supplementing the conventional slender body theory, which covers the Froude–Krylov force and hydrodynamic lift from the transom.