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This paper addresses the problem of guidance and control of an underwater vehicle. Guidance and control is achieved by introducing a control architecture with three modules: path generation, path f...

This article presents progress on modeling bubble entrainment and transport around ships using hybrid Reynolds-averaged Navier-Stokes/large eddy simulation (RANS/ LES) methods. Previous results using a Boltzmann-based polydisperse bubbly flow model show that LES perform better than RANS in predicting transport of bubbles to depth, a very important process to predict bubbly wakes. However, standard DES-type models fail to predict proper turbulent kinetic energy (TKE) and dissipation, needed by bubble entrainment, breakup, and coalescence models. We propose different approaches to obtain TKE and dissipation in LES regions and evaluate them for cases of increasing complexity, including decay of isotropic turbulence, a flat plate boundary layer, and the flow in the wake of the research vessel Athena. An exponential weighted average is used to estimate statistics and obtain the averaged quantities in regions with resolved turbulence. The TKE is satisfactorily predicted in the cases tested. A modified ω equation in the SST model is proposed to implicitly compute the dissipation, showing superior results than the standard DES models, although further improvements are necessary. A hybrid RANS/LES approach is proposed, which focused at conserving total TKE as the flow crosses RANS/LES interfaces, as previously performed for zonal approaches but attempting a DES-like detection of regions suitable for LES, critical for large-scale computations of bubbly flows involving complex geometries. A general form of a dynamic forcing term is derived to transfer the modeled TKE to resolved TKE with a controller to guarantee proper conservation of the energy transferred. It was verified that the model is not sensitive to grid size or time step. Improvements to DDES and the proposed TKE-conserving hybrid RANS/ LES method show encouraging results, although remaining challenges are discussed.

We present simulations and experiments of the generic submarine Joubert BB2 performing standard turn, zigzag, and surfacing maneuvers in calm water at depth. The free-sailing experiments, performed at Maritime Research Institute Netherlands (MARIN), are unique in that they present an open dataset for the community to benchmark maneuvering prediction methodologies. Computations were performed with explicitly gridded sailplanes, tail planes, and propellers using a dynamic overset technique. This study analyzes a 20-degree turning maneuver with vertical control commanding the stern planes and a 20/20 zigzag maneuver with vertical control commanding both sail and stern planes, both of them at a nominal speed of 10 knots, and a 20-degree rise maneuver with horizontal control at 12 knots. The results show that computational fluid dynamics can predict well motions and speeds for free-sailing conditions, but controller commands are harder to replicate. Computations of the rise maneuver with surfacing compare well with experiments, including a crashback maneuver to stop the submarine. Introduction Simulation of free-running submarine maneuvers is demanding in terms of computational fluid dynamics (CFD) capabilities and computational resources. Full 6-DoF motions, moving appendages, control algorithms, and scalable performance are some of the requirements to execute free-running maneuvering simulations. Open literature data for validation of submarine maneuvers are very limited and consist mostly of static conditions (see for instance rotating arm [Toxopeus et al. 2012] and static drift [Roddy 1990; Toxopeus 2008] data for Defense Advanced Research Projects Agency (DARPA) Suboff and static drift [Quick & Woodyatt 2014] for the Joubert hull form [Joubert 2006]). A new dataset has been recently made available to the research community for the generic submarine model BB2 (Overpelt et al. 2015), based on the Joubert hull form.

The airwake of a tumblehome ship geometry in uniform wind is investigated experimentally through dye visualization, particle image velocimetry, and laser Doppler anemometry, and numerically using a...

We describe the implementation of several recently developed boundary layer transition models into the overset computational fluid dynamics code, REX, developed at the University of Iowa, together with an evaluation of its capabilities and limitations for naval hydrodynamics applications. Models based on correlations and on amplification factor transport were implemented in one- and two-equation Reynolds-averaged Navier-Stokes turbulence models, including modifications to operate in crossflow. Extensive validation of the transition models implemented in REX is performed for several 2- and 3-dimensional geometries of naval relevance. Standard tests with extensive available experimental data include flat plates in zero pressure gradient, an airfoil, and sickle wing. More complex test cases include the propeller, P4119, with some experimental data available, and the generic submersible, Joubert BB2, with no relevant experimental data available, to validate the transition models. Simulations for these last two cases show that extensive regions of laminar flow can be present on the bodies at laboratory scale and field scale for small vessels, and the potential effects on resistance and propulsion can be significant. 1. Introduction Progress for prediction of attached, fully turbulent flows for practical aerodynamic and hydrodynamic applications has reached a relatively mature plateau. However, according to a recent comprehensive review of pacing items (Slotnick et al. 2014), the single largest hurdle for incorporating computational fluid dynamics (CFD) into the design process in the near future is the ability to accurately predict turbulent flows with boundary layer transition and separation. Transition can impact skin friction, heat transfer, noise, propulsion efficiency, and maneuverability. This is especially true at model scale and for small craft such as unmanned or autonomous surface and underwater vehicles.

We present a study of the response of the generic Joubert BB2 geometry self-propelled near the free surface, using a dynamic overset methodology with discretized propeller and control surfaces. A grid study was performed for sail top depth 0.702 m . Even keel simulations at depths ranging from − 6.9 m to 40 m were conducted in calm water and regular waves representative of sea state 2 to 7. An autopilot acting on sail and tail planes, ballast and trim tanks was used to simulate controlled self-propulsion at several depths in calm water and waves. The even keel study shows that near surface operation causes considerable vertical forces and pitching moments, and an increase in required propeller thrust to achieve self-propulsion, decreasing efficiency. The boat responds to waves mainly through the added mass forces produced by the wave-induced velocity field, causing an exponential decay of the wave influence with depth and with decreasing wave amplitude. In controlled free running simulations very near the surface the boat responds to the forces observed in even keel, requiring compensation through the trim tanks to prevent high bow down pitch. Hull/free surface interaction causes fluctuations in the wake, resulting in a broadband response of propeller thrust.

We study the stratified bubbly wake of the research vessel Athena R/V during an overshoot maneuver using a polydisperse two-fluid model. The motions of ship, rudders and propellers are handled with a dynamic overset approach. The demands of the resolution of the flow and the small time step imposed by the rotating propellers make this problem very demanding and challenging. We discuss the dynamic response of the ship, focusing on trajectory, motions, forces and moments. Vortical structures and their effects on bubble and density transport are studied. Efforts are then focused on studying the bubbly flow and density distribution, and how vortices and flow field affect bubble entrainment, density and bubble transport, and size distribution. Results indicate that the void fraction and maximum depth in the wake increase during the maneuver, as do the disturbances to the stable stratification. Bubble entrainment during the maneuver increases in the transom stern but decreases around the hull, resulting in more bubbles in the wake. Due to transport of bubbles entrained around the hull into the inboard side of the turn, the resulting wake is also wider than in straight self-propulsion operation.

Unresolved pressure fluctuations at the sub-grid scale (SGS) level of large eddy simulation (LES) or Reynolds-averaged Navier–Stokes computations affect cavitation inception predictions, as SGS low pressures are simply ignored. We present a framework to take the unresolved SGS flow into account. Representing the SGS flow as canonical turbulence, in this paper, homogeneous isotropic turbulence (HIT), the pressure fluctuations, and transport and cavitating behavior of nuclei in such turbulence can be evaluated from direct numerical simulations (DNS) and used to create a model of cavitation inception that accounts for SGS fluctuations. To accomplish this, nuclei of different sizes were transported in DNS of HIT using their pressure history to drive the Rayleigh–Plesset equation that simulates bubble dynamics. In this way, expected average cavitation frequencies were tabulated for a range of SGS Taylor scale Reynolds numbers ([Formula: see text]), nucleus size, turbulent kinetic energy dissipation rate, and mean pressure. The model uses this table to estimate the cavitation event rate in each cell of a computational fluid dynamics solution. Inception can then be predicted by comparing the total cavitation rate with the detection criterion. The model is first assessed on two cases of HIT (at [Formula: see text] = 240 and 324) by comparing the pressure statistics, which it predicts in LES runs using the SGS cavitation model against the statistics of DNS. Then, a high [Formula: see text] (1660–1880) HIT flow is simulated using LES, and cavitation events are compared against experimental data. The inception model successfully predicts the inception pressure and the cavitation rates in the flow.

A numerical approach to incorporate the compressibility of bubbles to a two-phase solver with incompressible liquid is presented. The scheme introduces a bubble mass density equation and the ideal gas law (though other thermodynamic relations can also be used) to compute the void fraction for use in the continuous phase equations. Pressure and velocity are strongly coupled on a collocated grid scheme by obtaining face velocities through nonlinear interpolation from nodal values using an algebraic equation. Tight coupling is achieved for pressure and void fraction by careful treatment of the bubble transport and compressibility terms in the resulting pressure Poisson equation. The novel proposed implicit strategy prevents numerical instabilities even at high void fractions and highly compressible bubbles. In addition, the proposed method can capture acoustic waves for void fractions between 0.1% and 90% with errors in speed of sound of less than 2% respect to the speed of sound of isothermal compressible bubbles and water mixtures. 1D, 2D and 3D numerical tests, including the bubbly flow around a surface ship, are performed to demonstrate robustness and range of applications of the proposed scheme. The approach is shown to be stable at high void fractions when other schemes fail.

This paper investigates a coupled computational fluid dynamics (CFD)/potential propeller code approach to simulate maneuvers of ships. While this approach is successful in submarines, the concept h...

We propose a method to visualize vortex cores based on manipulation of the pressure field produced by isolated vortices in incompressible flow. Under ideal conditions the function $D=2|\nabla P|/\nabla^2 P$ yields an approximate distance to vortex centerlines. As opposed to local methods to identify coherent structures, isosurfaces of $D$ produce a field of vortex tubes equidistant to the vortex core center which, ideally, are independent of vortex intensity or size. In contrast to other line-vortex identification methods, which typically rely on algorithms to detect vortex core lines and frequently need complex implementations, the proposed method can be computed from the local Eulerian velocity and pressure fields as easily as vortex identification methods such as the $Q$ and $\lambda_2$ criteria. $2|\nabla P|/\nabla^2 P$ results in the exact distance to the core center for a Rankine vortex and is in general valid for the region of a vortex where there is pure rotation, yielding an approximation to the distance farther from the core in other simple one-dimensional vortex models. The methodology performs well in all tests we attempted, though limitations are presented and discussed. The method is demonstrated for a canonical Burgers vortex, a Bodewadt vortex, homogeneous isotropic turbulent flow, the wake of a propeller, a heaving plate, a turning containership and the airwake of a surface combatant. The proposed method helps to better visualize vortical flow fields by displaying vortex cores, complementing methods like $Q$ and $\lambda_2$ which display vortical volumes., Comment: 15 pages, 9 figures. Submitted to Journal of Fluids Engineering

The study of bubble’s behavior in turbulent flows is fundamental to the understanding of many engineering applications that are concerned with bubbly/two-phase flow. In turbomachinery, for example, tiny gas nuclei present in the liquid may grow to macroscopic size if the instantaneous pressure dips below the vapor pressure for a time long enough to incite cavitation events. In this paper, the Lagrangian pressure statistics of finite sized bubbles in homogeneous isotropic turbulence is investigated using highly-resolved direct numerical simulations of the Navier-Stokes equations at Reλ = 150. A modified Maxey-Riley equation is used for Lagrangian tracking of bubbles in the turbulence field. The Lagrangian pressure statistics (probability density function, frequency of low-pressure events and their duration) are analyzed as functions of the bubble size. The overall picture that emerges is consistent with finite-sized bubbles being driven towards vortex cores, resulting in an average pressure further below the mean value and longer and more frequent low-pressure events as the considered size is increased.

We propose a novel polydisperse cavitation model based on population balances, which seeks to provide the bubble size distribution in addition to the void fraction, enabling natural prediction of transitions from large bubbles to small cavities in regions of massive condensation, or from small bubble to large cavities in regions of strong evaporation. The mass transfer rate is modeled from the asymptotic solution of the Rayleigh-Plesset equation with modifications for the polydisperse approach. Existing breakup and coalescence models, which do not include effects of mass change, are adapted and new models are developed to account for evaporation and condensation. In addition, homogenous and heterogeneous nucleation models are incorporated to treat the activation and size distribution of nuclei. The model is evaluated against data for the Delft twisted foil, showing satisfactory results for the pressure distribution, forces, and shedding frequency. Though no experimental data for the bubble size distribution is available, bubble sizes in different regions of the flow predicted by the model indicate the right trends observed in videos and photographs of the experiments. Refinements to the model based on experiments are necessary, especially to evaluate the ability to properly predict void fraction and bubble size distribution.

A high-fidelity simulation framework is presented to investigate wind turbine aero-servo-elastic behavior, coupling dynamic overset computational fluid dynamics (CFD) and multibody dynamics (MBD) approaches. The Gearbox Reliability Collaborative (GRC) project gearbox was up-scaled in size and installed in the NREL 5-MW offshore wind turbine to demonstrate drivetrain dynamics. Generator torque and blade pitch controllers were implemented to simulate operational conditions of commercial wind turbines. Interactions between wind turbulence, rotor aerodynamics, elastic blades, drivetrain dynamics at the gear-level and servo-control dynamics were studied. Results show that gear contact causes dynamic transmission error within the drivetrain, and results in a decreased turbine thrust and rotational speed. The generator torque controller optimizes efficiency below rated wind speed, while the blade pitch controller properly regulates the turbine near rated power and generator speed at higher than rated wind speed under both uniform and turbulent winds. The pitch controller effectively reduces turbine thrust, blade tip deflections, and velocity deficit of the wake, benefiting both stand-alone turbines and wind farms. The tool and methodology developed show promise to study complex aerodynamic/mechanic systems, being the first time a complete wind turbine simulation includes CFD of the rotor/tower aerodynamics, wind turbulence, elastic blades, gearbox dynamics and feedback control.

A framework for the development of models of bubble entrainment in free surface turbulent flows is presented. The framework uses mechanistic processes to model the stages involved in the entrainment of bubbles due to interaction of turbulence with a free surface. Entrainment is modeled as a chain of events for a bubble that is formed at the free surface, then pulled into the fluid against buoyancy, and interacts with vortices that break it up into smaller bubbles, or with other bubbles by coalescence. The main entrainment mechanism is modeled as a vortex/free surface interaction process that can entrain bubbles if the vortices located a given distance from the surface are strong enough. This approach overcomes limitations of approaches where the entrainment is determined only by turbulence parameters, which in the case of objects interacting with a free surface entrain bubbles on the boundary layers irrespective of the distance to the free surface. Depending on the computational fluid dynamics approach used to solve the flow, these processes may need different levels of modeling; more resolved approaches like large-eddy simulation with a volume of fluid method of the free surface will require less modeling complexity than a less resolved RANS method, since some of the involved processes of entrainment are directly accounted for. In this paper a standard RANS approach is used with the free surface modeled using a single-phase level set method, and models are presented for each of the relevant processes to produce a complete mechanistic model of turbulent bubble entrainment. The model was calibrated and tested for two relevant problems: a 2D + T breaking wave in model scale, and the full scale bubbly flows around the US Navy Research Vessel Athena. For the second case a grid study is carried out to analyze grid convergence performance of the model. Comparisons with experimental data show that the model predicts well location and magnitude of entrainment.

A methodology is presented to predict density stratified flows in the near-field of naval vessels. The approach uses a single-phase level set method for the free surface, a dynamic overset technique to handle motions and controllers for self-propulsion and maneuvering. The density is solved with a higher-order transport equation coupled with momentum and mass conservation. Turbulence is implemented with a k − e / k − ω based Delayed Detached Eddy Simulation (DDES) approach modified to add density gradients. Evaluation tests were performed for a two-dimensional square cavity, including grid and time step studies, and the stratified flow past a sphere, showing good agreement with available data. The stratified flow was studied for a self-propelled ship and a submarine. Density, velocity, pressure and turbulent quantities at the exit plane of the near-field contain a description of the relevant scales of the flow and can be used to compute the far-field stratified flow. It is shown that, as is the case of surface waves, the generation of internal waves requires energy that results in an increase in resistance. Moreover, the presence of a density interface against the hull results in a thickening of the boundary layer, just as in a solid/free surface juncture flow.

CFD simulations are conducted for zero-speed damaged passenger ship SSRC in calm water and waves with 6DOF motions including flooding procedure in calm water, roll decay in calm water and motions in regular beam waves for various wavelengths. The simulations model the 6DOF soft spring experimental mount, the one- and two-room flooding compartment configurations, including both intact and damaged conditions. For flooding and roll decay, simulations show ability to predict the trend of increases in roll period and damping due to flooding, as reported in ITTC. The damping magnitudes were often under-predicted with large errors while the roll period and compartment water height were well predicted. Two-room compartment simulation showed three times larger damping than one-room compartment cases whereas the roll period was similar for both conditions. For wave cases, all motions show primarily 1st order responses, except for parametric roll condition which shows large ½ harmonic responses for the intact ship. The 2nd order responses are small for both damaged and intact ship. The larger roll period and damping for the damaged ship shift the peak of responses to smaller wave frequency and reduce the amplitude of responses. The average error is often large for 1st order intact ship pitch and damaged ship surge and pitch. The errors are larger for most ½ and 2nd order responses. Large errors could be partially due to the complex mounting system in the experiment. Overall, current CFD results show better predictions than those reported for potential flow solvers even though the computational cost is larger.

A study of a 10/10 vertical zigzag maneuver of the generic submarine Joubert BB2 is presented. CFD simulations, involving moving planes and rotating propeller using a dynamic overset technique, are performed at constant propeller rotational speed in model scale on four systematically refined grids and compared against experimental data for depth, pitch, pitch rate, vertical drift, vertical velocity, absolute velocity, and propeller thrust and torque coefficients. A grid study is also performed for the self-propelled approach condition before starting the maneuver, used to obtain the propeller rotational speed, controller vertical and horizontal commands, and boat attitude. The maneuver is also analyzed in full scale at 6, 10 and 15 knots. Results indicate that CFD matches very well the experimental data, even for the coarsest grid, with the propeller RPS converging slowest in grid. By comparing model and full scale results, it can be concluded that model scale simulations are a good surrogate for full scale for parameters related to motions, but as expected forces are strongly affected by scale and the propeller operational point changes considerably. Analysis of forces and moments affecting vertical control from hull, sail and stern planes, propeller and hull help understand the response of the vehicle.

The behavior of the pressure along the trajectories of finite-sized nuclei in isotropic homogeneous turbulence is investigated using direct numerical simulations at Reλ = 150. The trajectories of nuclei of different sizes are computed by solving a modified Maxey–Riley equation under different buoyancy conditions. Results show that larger nuclei are more attracted to the vortex cores and spend more time at low-pressure regions than smaller nuclei. The average frequency of pressure fluctuations toward negative values also increases with size. These effects level off as the Stokes number becomes greater than 1. Buoyancy, characterized by the terminal velocity w, counteracts the attraction force toward vortex cores while simultaneously imposing an average vertical drift between the nuclei and the fluid. Computational results indicate that weak vortices, associated with moderately low pressures, lose their ability to capture finite-sized nuclei if w ≥ u′. The attraction exerted by the strongest vortices on the largest of the considered nuclei, on the other hand, can only be overcome by buoyancy if w ≥ 8u′. The quantitative results of this study are shown to have a significant impact on modeling cavitation inception in water. For this purpose, the Rayleigh–Plesset equation is solved along the nuclei trajectories with realistic sizes and turbulence intensities. The simulations predict cavitation inception at mean pressures several kPa above vapor pressure.

Insight into the effects of motions and waves on a ship airwake can be used to provide information regarding safe conditions for at sea flight operations and pilot training. We present a study on the effects of waves and motions on a ship's airwake and a helicopter operating above the flight deck using full scale computational fluid dynamics simulations. The ONR Tumblehome Ship (ONRT) geometry is analyzed in wind and regular head waves corresponding to nominal sea states 3 and 6. In order to separate the effects of waves and motions, simulations are conducted for both sea states with combinations of: waves and motions, waves and no motions, no waves with motions, and no motions or waves. A triple velocity decomposition is conducted in order to quantify changes in the airwake due to motions and waves. The aerodynamic loads on a helicopter hovering in the airwake are studied with one-way and two-way coupling approaches. The one-way coupled analysis uses the velocity field data from the full scale airwake simulations along with disk actuator theory to calculate thrust fluctuations for three different rotor sizes operating above the flight deck. In the two-way coupled study a helicopter loosely based on the Sikorsky SH-60 Seahawk hovering above the flight deck of the ONRT is simulated, including dynamic overset grids of the main rotor, tail rotor and fuselage. This approach captures the interaction between the rotor downwash and the ONRT airwake. The study shows that for the lower sea state 3 the motions and waves have little effect on the airwake behavior, exhibiting little change with respect to the baseline case without motions and waves. At sea state 6 the flow field is modified significantly, which also affects the forces on the helicopter. Force fluctuations show main peaks at the wave encounter frequency and the blade passage frequency, as well as higher harmonics. The one-way coupling approach shows that larger rotors effectively filter out small scale fluctuations while smaller diameter rotors are affected by smaller flow structures.

The hydrodynamic characteristics of a waterjet-propelled amphibious craft traveling in displacement and planing modes are studied via full-scale experiments and numerical simulations in three ways. First, variations of pitch, resistance, and vertical rise for speeds from 2 to 18 knots are investigated. As speed is incrementally increased, pitch and resistance increase geometrically before maximizing in the transition region. In this region, the computations underpredict pitch, yet resistance matches the steady-state thrust measurements well. Second, maneuverability in displacement mode at 4 knots is investigated with a zigzag maneuver. Time variations of heading, pitch, yaw rate, roll, and roll rate predicted by computational fluid dynamics (CFD) for the zigzag maneuver are in good agreement with the experimental results. Third, the dynamic transition from displacement to planing mode is investigated by abruptly increasing the thrust beyond the craft's peak resistance. Beyond the resistance maximum, the vehicle experiences a sharp increase in speed and drop in pitch angle. Experiment and simulation conclude similarly with an abrupt cut to propulsion, resulting in a quick deceleration, which is overcome by its own wake. Simulated kinematics of the deceleration are remarkably accurate, suggesting valid computations of the transient drag, wake generation, and vehicle response to waves.

An experimental and computational fluid dynamics (EFD and CFD) study of the appended notional submarine DARPA Suboff fitted with the 7-bladed E1658 propeller and operating near the free surface is presented. The flow measurements were taken using particle image velocimetry with a multi-camera configuration in the Large Free Surface Cavitation Channel at INM. Single-phase level set numerical simulations with an overset approach were performed for three advance coefficients ( J = 0.5 , 0.65 , 0.82 ) at three shaft depths ( z / D = 1 , 1.5 , 2 ). The results reveal that the presence of the hull and the interaction with the free surface strongly affect the inflow and wake of the propeller, producing higher local advance coefficient and blade loads near the surface. Strong free surface fluctuations at small shaft depth also cause instability and breakdown of the propeller tip vortices in the near field. CFD simulations show elliptic instability before vortex breakdown. Comparison between CFD with EFD shows that CFD matches overall trends well, mainly for the phase-averaged flow field, but underpredicts wake fluctuations and thus displays more coherent phase-averaged vorticity. In addition, CFD results also underpredict tip vortex strength and predicts vortex instabilities farther downstream from the propeller plane due to the unresolved upstream turbulence.

The KCS container ship on a zigzag maneuver in shallow water is studied experimentally and numerically. The approach conditions are Fr=0.095 and H/T=1.2. Experiments were performed in the shallow water towing tank at Flanders Hydraulics Research for the workshop on verification and validation of ship maneuvering simulation methods (SIMMAN, 2014). CFD simulations were initially performed blind at the nominal rudder rate of 16.8°/s using direct discretization of moving rudder and propeller, including the tank bottom but neglecting walls. Grid studies were conducted for self-propulsion and the zigzag maneuver at the nominal rudder rate using grids of up to 71.3 million points. A simulation was then executed on the medium grid at the actual rudder experimental rate achieved (8.35°/s), and results analyzed. The grid study suggests that forces, moments and motions can be well predicted with coarser grids, and that medium grid results are very close to the fine grid, but flow details do not converge for the levels of refinement used. The verification results in large grid uncertainties, even though time evolutions of different variables show good grid convergence. CFD predictions match satisfactorily the experimental results for most variables, but under-predict yaw and yaw rate.

Part 2 of this two-part paper presents URANS ship–ship validation results against the experimental data studied in Part 1. The simulation matrix is designed based on the Part 1 results to focus on most significant effects, as well as to complement the data to further explain the physics involved. Calm water validation results show that the error values are comparable to the previous single-ship URANS studies. Single-ship (Hope only and Bobo only) calm water hull pressure distributions are compared to ship–ship solutions, providing insight into the physics of the induced effects based on the regions of decreased or increased hull pressure. Regular wave results achieve error values comparable to those for previous single-ship URANS validation studies. Also compared to the previous use of the same data for different simulation tools other than URANS, the current results achieve significantly lower error values. Ship–ship results in waves are compared to single-ship simulations, showing that the presence of the second ship induces resonant effects for most of the variables, and induces increased amplitudes for forces and moments at high frequencies. An overtaking validation simulation in regular head waves is carried out, which shows URANS capability with reasonably good agreement to the experiment.

This paper presents a study of the effects of the interaction of the free surface with a marine propeller operating close to it, including the effects of depth and cross flow. Extensive simulations using the naval hydrodynamics computational fluid dynamics solver REX were conducted to study the open water characteristics, transient blade loads, and flow behavior of propeller 4661, with comparison to fully submerged experimental data at 0° and 30° shaft inclination with respect to the incoming flow. Deeply submerged computations and experiments show that the cross flow results in an increase in thrust and torque and lower efficiency relative to uniform inflow. Near the surface, computations show that effects on thrust and torque increase more dramatically as the propeller load increases. Furthermore, the presence of the free surface breaks the symmetry resulting in highest blade force losses when the blade is near top dead center. As the propeller approaches the surface, the amplitude of the higher harmonics of blade force increases. A demonstration calculation for a self-propelled submarine sailing deep and near the surface shows that operating closer to the surface results in higher propeller loads and lower efficiency.

An implementation of the dynamic overset grid technique into naoe-FOAM-SJTU solver developed by using the open source code OpenFOAM is presented. OpenFOAM is attractive for ship hydrodynamics applications because of its high quality free surface solver and other capabilities, but it lacks the ability to perform large-amplitude motions needed for maneuvering and seakeeping problems. The implementation relies on the code Suggar to compute the domain connectivity information (DCI) dynamically at run time. Several Suggar groups can be used in multiple lagged execution mode, allowing simultaneous evaluation of several DCI sets to reduce execution time and optimize the exchange of data between OpenFOAM and Suggar processors. A towed condition of the KRISO Container Ship (KCS) are used for static overset tests, while open-water curves of the KP505 propeller and self-propulsion and zig-zag maneuvers of the KCS model are exercised to validate the dynamic implementation. For self-propulsion the ship model is fitted with the KP505 propeller, achieving self-propulsion at Fr=0.26. All self-propulsion factors are obtained using CFD results only, including those from open-water curves, towed and self-propulsion conditions. Computational results compare well with experimental data of resistance, free-surface elevation, wake flow and self-propulsion factors. Free maneuvering simulations of the HSVA KCS model appended with the HSVA propeller and a semi-balanced horn rudder are performed at constant self-propulsion propeller rotational speed. Results for a standard 10/10 zig-zag maneuver and a modified 15/1 zig-zag maneuver show good agreement with experimental data, even though relatively coarse grids are used. Grid convergence studies are performed for the open-water propeller test and bare hull KCS model to further validate the implementation of the overset grid approach.

Full scale bubbly flow experiments were performed on a 6 m flat bottom survey boat, measuring the void fraction, bubble velocity and size distributions as the bubbles naturally entrained at the bow of the boat interact with the boat’s boundary layer. Double-tip sapphire optical probes capable of measuring bubbles down to 50 μm in diameter were specifically designed and built for this experiment. The probes were positioned under the hull at the bow near the bubble entrainment region and at the stern at the exit of the bottom flat plate. Motorized positioners were used to vary the probe distance to the wall from 0 to 50 mm. The experiments were performed in fresh water (Coralville Lake, IA) and salt water (Panama City Beach, FL), at varying velocities with most data analysis performed at 10, 14 and 18 knots. The results indicate that the bubbles interact significantly with the boundary layer. At low velocity in fresh water, bubble accumulation under the hull and coalescence are evident by the presence of large bubbles at the stern. At high speeds bubble breakup dominates and very small bubbles are produced near the wall. It is also observed that salt water inhibits coalescence, even at low boat speeds. The void fraction increases with speed beyond 10 knots and peaks near the wall. Bubble velocities show slip with the wall at all speeds and exhibit large RMS fluctuations, increasing near the wall.

Summary A methodology for improved robustness in the simulation of high void fraction free surface polydisperse bubbly flows in curvilinear overset grids is presented. The method is fully two-way coupled in the sense that the bubbly field affects the continuous fluid and vice versa. A hybrid projection approach is used in which staggered contravariant velocities at cell faces are computed for transport and pressure–velocity coupling while the momentum equation is solved on a collocated grid arrangement. Conservation of mass is formulated such that a strong coupling between void fraction, pressure, and velocity is achieved within a partitioned approach, solving each field separately. A pressure–velocity projection solver is iterated together with a predictor stage for the void fraction to achieve a robust coupling. The implementation is described for general curvilinear grids detailing particulars in the neighborhood to overset interfaces or a free surface. A balanced forced method to avoid the generation of spurious currents is extended for curvilinear grids. The overall methodology allows simulation of high void fraction flows and is stable even when strong packing forces accounting for bubble collisions are included. Convergence and stability in one-dimensional (1D) and two-dimensional (2D) configurations is evaluated. Finally, a full-scale simulation of the bubbly flow around a flat-bottom boat is performed demonstrating the applicability of the methodology to complex problems of engineering interest. Copyright © 2015 John Wiley & Sons, Ltd.

A high fidelity approach for wind turbine aero-elastic simulations including explicit representation of the atmospheric wind turbulence is presented. The approach uses a dynamic overset computational fluid dynamics (CFD) code for the aerodynamics coupled with a multi-body dynamics (MBD) code for the motion responses to the aerodynamic loads. Mann's wind turbulence model was implemented into the CFD code as boundary and initial conditions. The wind turbulence model was validated by comparing the theoretical one-point spectrum for the three components of the velocity fluctuations, and by comparing the expected statistics from the CFD simulated wind turbulent field with the explicit wind turbulence inlet boundary from Mann model. Extensive simulations based on the proposed coupled approach were conducted with the conceptual NREL 5-MW offshore wind turbine in an increasing level of complexity, analyzing the turbine behavior as elasticity, wind shear and atmospheric wind turbulence are added to the simulations. Results are compared with the publicly available simulations results from OC3 participants, showing good agreement for the aerodynamic loads and blade tip deflections in time and frequency domains. Wind turbulence/turbine interaction was examined for the wake flow. It was found that explicit turbulence addition results in considerably increased wake diffusion. The coupled CFD/MBD approach can be extended to include multibody models of the shaft, bearings, gearbox and generator, resulting in a promising tool for wind turbine design under complex operational environments.

This article presents two approaches to simulate maneuvers of a model radio-controlled submarine. In the direct simulation approach, rudders, stern planes, and propellers are gridded and treated as moving objects using dynamic overset technology. The second approach couples the overset computational fluid dynamics (CFD) solver and a potential flow propeller code, with both codes exchanging velocities at the propeller plane and wake, body forces, and propeller forces and moments, whereas rudders and stern planes are still explicitly resolved. It is shown that during the maneuvers, the range of advance coefficients does not deviate much from the design point, making a coupled approach a valid choice for standard maneuvering simulations. By allowing time steps about an order of magnitude larger than for the direct simulation approach, the coupled approach can run about five times faster. The drawback is a loss of resolution in the wake as the direct propeller simulation can resolve blade vortical structures. Open water propeller curves were simulated with both the direct propeller approach and the coupled approach, showing that the coupled approach can match the direct approach performance curves for a wide range of advance coefficients. Simulations of a horizontal overshoot maneuver at two approach speeds were performed, as well as vertical overshoot and controlled turn maneuvers at high speed. Results show that both CFD approaches can reproduce the experimental results for all parameters, with errors typically within 10%.

The dead water problem, in which under certain conditions a vessel advancing in a stratified fluid experiences a considerable increase in resistance respect to the equivalent case without stratification, was studied using computational fluid dynamics (CFD). The advance of a vessel in presence of a density interface (pycnocline) results in the generation of an internal wave that in the most adverse conditions can increase the total resistance coefficient by almost an order of magnitude. This paper analyses the effects of stratification on total and friction resistance, the near field wake, internal and free surface waves, and resistance dynamics. Some of these effects are reported for the first time, as limitations of previous efforts using potential flow are overcome by the use of a viscous, free surface CFD solver. A range of densimetric Froude numbers from subcritical to supercritical are evaluated changing both the ship speed and pycnocline depth, using as platform the research vessel athena. It was found that the presence of the internal wave causes a favorable pressure gradient, acceleration of the flow in the downstream of the hull, resulting in thinning of the boundary layer and increases of the friction resistance coefficient of up to 30%. The total resistance presents an unstable region that results in a hysteretic behavior, though the characteristic time to establish the speed–resistance curve, dominated by the formation of the internal waves, is very long and unlikely to cause problems in modern ship speed controllers.

Damping of waves in moonpools has been investigated. Several different geometries of the moonpool have been considered, which are being mainly characterised by a piston motion of the free surface in the moonpools, while some sloshing effects are visible. A numerical damping approach has been implemented into a viscous solver to simulate the effect of perforated bulkheads without the necessity to create a complex grid. The simulations have been compared with respective model tests. Two different porosities of the bulkhead have been experimentally investigated. The simulations have been conducted for a variety of Froude numbers, moonpool layouts and in calm water as well as waves. A dependency of the surface elevation natural frequency on the ship speed has been found. Furthermore, the numerically determined surface elevation inside the moonpool has been compared with respective experimental data. Based on the surface elevation, the resulting damping for the different layouts is determined and discussed.

A fully coupled approach to large deformation fluid–structure interaction (FSI) using a nonlinear finite element (FEM) solver and a URANS/DES overset solver is presented. Since the relationship between strains and displacement cannot be treated as linear at large deformation problems, a nonlinear FEM treating geometric nonlinearity is applied instead of a linear modal analysis for a structure solver. For large deformation the reinitialization of the overset hole-cutting, which is carried out by default only at the beginning of the computation, is performed automatically at each time step. The approach uses the gluing method to transfer the forces and displacements on non-matching grids for fluid and structure domains. A linear FEM solver is applied to deform the outer boundary of the boundary layer grids which wrap around the deformable geometries. The deformation of interior points in the boundary layer grid is obtained using linear interpolation. Three cases of rolling tanks partially filled with fluid with an elastic bar clamped to bottom or top are simulated and compared with experiments and other numerical simulation results. The simulation results of the presented method show good agreement with the experiments for bar deformation and free surface elevation.

Simulations of zigzag maneuvers of the Postdam Model Basin (SVA) Korean container ship (KCS) model with moving rudder and rotating propeller are presented. Free running KCS presents considerable challenges to simulate the moving semi-balanced horn rudder which presents tight gaps with the rudder root. These difficulties are overcome using a dynamic overset technique with a hierarchy of bodies. To better resolve propeller/rudder interaction a Delayed Detached Eddy Simulation turbulence model based on Menter’s SST is used. Two types of maneuvers are simulated, the standard 10/10 zigzag maneuver and the modified 15/1 zigzag maneuver. Both simulations are performed at model scale in deep, calm water, for approach velocities corresponding to a Froude number 0.26. The self-propulsion at approach speed is achieved using a proportional–integral speed controller which acts on the propeller rotational speed with the ship free to surge, heave, roll and pitch. Once the ship achieves self-propulsion the propeller rotational speed is frozen and the ship is free to move in six degrees of freedom (6DOF) while the maneuver starts. The results show that direct Computational Fluid Dynamics (CFD) simulations of maneuvers with a moving rudder and rotating propeller are feasible and the comparisons between computations and experiments are highly satisfactory in both cases, but the computational cost is still high for many applications. In analyzing the flow physics, it is found that the rudder has asymmetric behavior caused by the asymmetry introduced by the single rotating propeller.

Prediction of the bubble size distribution in the wake of a ship is important to analyze its acoustic signature. To achieve CFD simulation of dynamic ships with moving control surfaces and rotating propellers in waves, a robust implementation is paramount. In this work a mass conserving multigroup discretization strategy of the Boltzmann transport equation for polydispersed bubbly flows is presented, as well as an analysis of available breakup and coalescence models. Modifications of the discrete equations for the fixed pivot method at the boundaries are introduced that guarantee exact bubble mass conservation. The role of the time stepping scheme in the conservation of mass and number of bubbles is discussed. Though the conservation properties of the discrete system of equations are satisfied provided they are solved exactly, in practice an iterative procedure must be used since the ODE’s are non-linear. Three iterative schemes are proposed and they are analyzed in terms of robustness and efficiency. Breakup, coalescence and dissolution models are analyzed from the numerical point of view. Available models of breakup and coalescence are studied finding appropriate choices for ship applications. Other models are appropriate as well, but are more costly numerically. As appropriate for ship applications, an extension to the model of Prince and Blanch for salt water is proposed and analyzed. The final model is tested against experimental data and computations by other researchers, and convergence properties in bubble size discretization is studied. It is found that for salt water the final steady state is dependent on the initial condition since there is a range of sizes for which coalescence and breakup are both negligible.

This paper presents a simple experiment that can be executed in an undergraduate laboratory to observe the boiling curve, including the transition and film boiling regimes. The experiment involves submerging an instrumented copper sphere into liquid nitrogen and measuring the evolution of the temperature of the sphere. Besides providing excellent visualization of the film, transition and nucleate boiling regimes, from the measurements the boiling curve can be obtained. Quantitative results and uncertainty analysis are presented for the critical heat flux and minimum heat flux. A paradox is presented when insulation is added to the sphere, since it can prevent the creation of a vapor film, maintaining the more efficient transition and nucleate boiling heat transfer regime and thus cooling the copper sphere faster than with no insulation. As expected, when the copper sphere is insulated further, heat transfer diminishes, offsetting the gains made by maintaining nucleate heat transfer. The experiment also evaluates temperature gradients inside the sphere and the validity of the assumption of uniform sphere temperature. This experiment is suitable for a senior-level experimental class or as a part of a heat transfer course.

Simulations of the submarine propeller E1619 using the overset flow solver CFDShip-Iowa V4.5 are presented. Propeller open water curves were obtained for two grids for a wide range of advance coefficients covering high to moderately low loads, and results compared with available experimental data. A verification study was performed for one advance coefficient (J=0.71) on four grids and three time step sizes. The study shows that grid refinement has a weak effect on thrust and torque but very strongly affects the wake. The effect of the turbulence model on the wake was evaluated at J=0.4 comparing results with RANS, DES, DDES and with no turbulence model, showing that RANS overly dissipates the wake and that in the solution with no turbulence model the tip vortices quickly become unphysically unstable. Tip vortex pairing is observed and described for J≤0.71, showing multiple vortices merging for higher loads. The wake velocities are compared against experimental data for J=0.74, showing good agreement. Self-propulsion computations of the DARPA Suboff generic submarine hull fitted with sail, rudders, stern planes and the E1619 propeller were performed in model scale and the resulting propeller performance analyzed.

The motions and added resistance of KVLCC2 at Fr=0.142 and 0.25 with free and fixed surge in short and long head waves are predicted using URANS and validated against EFD datafor Fr=0.142. Verification studies show the results are fairly insensitive to the grid size and time step. CFD indicates no significant difference between free and fixed surge while EFD pitch motion and added resistance are affected. Added resistance was largest when the bow relative motion has largest amplitude and is about 180° out of phase with the waves. The decomposition of forces and moments works well for first harmonics but not for higher harmonics originated from radiation in long waves and diffraction in very short waves. Maximum responses occur near the resonance condition and near the maximum wave excitation force which is at λ/L=1.33 and long waves for surge/pitch and heave, respectively. Potential flow predictions for motions and the added resistance are further from the data than CFD. Local flow analyses show that added resistance is mainly induced by high pressure on the upper bow which is correlated with bow relative motion. The unsteady wave pattern is analyzed and the wake flow is compared with PIV measurements.

Part 2 of this two-part paper presents the analysis and validation results of local flow characteristics for a surface combatant Model 5415 bare hull under static and dynamic planar motion mechanism simulations. Unsteady Reynolds averaged Navier–Stokes (URANS) computations are carried out by a general-purpose URANS/detached eddy simulation research code CFDShip-Iowa Ver. 4. The objective of this research is to investigate the capability of the code in relation to the computational fluid dynamics-based maneuvering prediction method. In the current study, the ship is subjected to static drift, steady turn, pure sway and pure yaw motions at Froude number 0.28. The free surface, three dimensional vortical structure and, the validation of two dimensional local flow quantities together with the available experimental data are of the interest in the current study. Part 1 provides the verification and validation results of forces and moment coefficients, hydrodynamic derivatives, and reconstructions of forces and moment coefficients from resultant hydrodynamic derivatives.

Part 1 of this two-part paper presents the verification and validation results of forces and moment coefficients, hydrodynamic derivatives, and reconstructions of forces and moment coefficients from resultant hydrodynamic derivatives for a surface combatant Model 5415 bare hull under static and dynamic planar motion mechanism simulations. Unsteady Reynolds averaged Navier–Stokes (URANS) computations are carried out by a general purpose URANS/detached eddy simulation research code CFDShip-Iowa Ver. 4. The objective of this research is to investigate the capability of the code in regards to the computational fluid dynamics based maneuvering prediction method. In the current study, the ship is subjected to static drift, steady turn, pure sway, pure yaw, and combined yaw and drift motions at Froude number 0.28. The results are analyzed in view of: (1) the verification for iterative, grid, and time-step convergence along with assessment of overall numerical uncertainty; and (2) validations for forces and moment coefficients, hydrodynamic derivatives, and reconstruction of forces and moment coefficients from resultant hydrodynamic derivatives together with the available experimental data. Part 2 provides the validation for flow features with the experimental data as well as investigations for flow physics, e.g., flow separation, three dimensional vortical structure, and reconstructed local flows.

Simulations of the National Renewable Energy Laboratory (NREL) phase VI wind turbine using dynamic oversetgridtechnologyarepresented.Thesimulationsareperformedinaninertialframeofreferencewith the rotor consisting of the blades and hub. The geometries of the tower and nacelle are approximate but includedinthecomputation.Computationsoftheeffectofwindspeed(5,10,15and25m/s)ata fixedblade pitch angle of 3 � with constant rotational speed using unsteady Reynolds-Averaged NaviereStokes (RANS) and Detached Eddy Simulation (DES) turbulence models, both showing little difference in the averaged forces and moments. However, significant improvements in the transient response are seen when using DES. The effect of angle of attack is evaluated by dynamically changing the pitch from � 15 � to 40� at

The mechanisms of broaching in following regular waves are studied by analysis of free model computations. Simulations of the fully appended ONR Tumblehome model DTMB 5613 are performed with the ship hydrodynamics code CFDShip-Iowa v4.5 and validated against experiments of an auto-piloted, self propelled model ship. Appendages include bilge keels, skeg, shafts and struts, moving rudders to control heading, and rotating propellers for self-propulsion. Proportional and proportional–integral autopilots are used. Once validated against experimental data for the proportional controller, the flow field and forces and moments on the hull and individual appendages are analyzed in detail to identify the mechanisms leading to the broaching event. It was found that several reasons contribute to the inability of the ship to overcome the hydrostatic yaw moment caused by the approaching wave. However, it is shown that the use of a slightly better autopilot prevents broaching under identical operating conditions, presenting an opportunity to extend the safe operating envelope of the ship.