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Computational scientists frequently publish papers discussing various sources of uncertainty in numerical methods for computational fluid dynamics. The frequently discussed sources of uncertainty are round off error, discretization error, iterative error, and mathematical model uncertainty (i.e. uncertainty in turbulence modeling). While all of these sources of uncertainty are real and impact the results of a simulation, the authors have found through experience that the most critical element to achieving accurate simulation results for high-speed craft is the generation of a high quality mesh on which the numerical methods are solved. There are, in general, two requirements for a high quality mesh; refinement regions must be applied in regions where the physics of the flow are most significant, and the three dimensional model of the hull form must be appropriately defined and fair. The latter appears to be a major contributing factor to uncertainty in CFD simulations that is not often discussed in the literature. Further, in practice there are numerous sources of geometric uncertainty between a prescribed CAD geometry and physically constructed model. In this paper, simulations are performed on two models of GPPH, with and without an edge radius on the transom to evaluate the impact of including fine geometric details (like tooling radii) in a RANS CFD model. The results of the simulations show that the inclusion of the rounded edge leads to large simulation errors in resistance and running attitude. This work has concluded that inclusion of fine geometric details in a planing hull CFD model is not beneficial to the overall accuracy of the simulations relative to necessary design accuracy.

Advances in nonlinear modeling techniques have created opportunities for more robust modeling of planing hull dynamics than previous techniques relying on linear assumptions. These techniques rely on the imposition of complex, coupled forced motions on a hull. RANSE CFD provides a distinct advantage over experimentation when imposing complicated forced motions because mechanical limitations of the forced motion mechanism and uncertainty in the prescribed motion are eliminated, though the accuracy of the simulations needs to be validated. In this work, a series of sinusoidal forced heave experiments on a planing craft are used to validate the force response predicted by simulation for the same forced motion. The accuracy of the predicted force response is evaluated relative to the experiments with the experimental setup uncertainty considered. Within the experimental setup uncertainty, the force response is predicted well by RANSE CFD and is found to be reasonably accurate. The dynamic trim angle is found to have a major impact on the dynamic force response with variations on the order of half a degree having substantial impacts on the measured forces.

This paper introduces a method to improve maneuvering models for AUVs that experience relatively tight turning radii (on the order of 2–3 vehicle lengths). By augmenting the original virtual planar motion mechanism (VPMM) datasets used in coefficient regression with high-angled steady-state CFD tow-tank data, maneuvering coefficients are adjusted to better reflect the craft's behavior under tight turning conditions. To verify the concept, two AUVs are tested, with one AUV experiencing relatively tight 2–3 vehicle-length turn radii at 10○ and 20○ rudder deflection, and the other AUV experiencing more typical 4–6 length turn radii. Both AUVs are on the order of 5.5m and modelling accuracy is expected to increase for the tighter-turning vehicle. Virtual steady-state tow-tank datasets with 5○, 10○, and 15○ hull-angles are used as the large drift angle data for coefficient refinement. By including large drift-angle data in the coefficient regression datasets, the turn-circle radius accuracy for the tighter-turning vehicle improves by upwards of 18%. The accuracy for the other vehicle, which does not experience such drift angles in maneuvering, decreases by up to 14%. Prediction accuracy is estimated by comparing lumped parameter model (LPM) against free-body CFD simulations which utilize a time domain unsteady RANSE solver under the action of deflected control surfaces (the same used to simulate the planar motion mechanisms tests). Our coefficient-refinement method improves maneuvering accuracy for vehicles which experience the large drift angles used in the coefficient refinement process.

A hydroelastic study of a deformable catamaran is presented. Two-way coupling has been implemented between a RANS (Reynolds Averaged Navier-Stokes) multiphase finite volume flow solver and a finite element method for a highly flexible hull structure. The presented cases consider the hull is advancing at a constant speed in head waves. Different levels of stiffness are tested and deformation time series are recorded. Simulated motion response appears to be very sensitive to the stiffness of the hull which is responsible for a steady and unsteady structural deformation. The deformable hull does not help improve vessel motions. The largest deformations occur at the transom. The bow and the aft part of the boat acquire a combination of permanent and dynamic deformations. Both of them have the same effect, a variation of the effective length of the boat. As a consequence of the shortening of the hull the pitch restoring moment and the heave resorting force will be reduced. So there will have to be bigger heave and pitch movements to compensate similar forces.

Growing interest in marine autonomous systems such as underwater gliders and autonomous underwater vehicles (AUVs) has led to the increase of research on hydrofoil performance at low Reynolds numbers, bringing forward the issue of simulating laminar to turbulent transition using computational fluid dynamics (CFD). Most conventional turbulence closures in practical Reynolds-averaged Navier-Stokes (RANS) simulations are incapable of predicting transition with excessive turbulence generation near the leading edge, deriving from their eddy viscosity assumption. Having the potential of inherently predicting transition by solving the near wall flow accurately, three Low-Re RANS closures with anisotropy consideration are selected and evaluated for their ability to capture both separation-induced transition and natural transition. The empirical γ − Re θ transition model is also assessed in this work. Comparison with published experiments shows the capability of the four turbulence closures to capture separation-induced transition accurately. Predicting natural transition remains more challenging: two Low-Re closures exhibit the potential to capture natural transition only at medium grid finesse levels, whereas the γ − Re θ model evidences challenges in reproducing the correct flow physics. The lag elliptic blending model provides the most accurate solution owing to its consistent grid convergence, high accuracy for predicting the hydrodynamic coefficients, and ability to reproduce the correct transition physics.

A method to derive a nonlinear model of the vertical plane hydrodynamic forces and moments on a planing hull using forced motion Unsteady RANSE CFD simulations and nonlinear system identification is investigated. The use of forced motion simulations on the fully three-dimensional hull form allows three-dimensional effects to be included in the nonlinear force model which have previously been neglected by strip theory or approximated by 2D+T theory. Nonlinear system identification allows the most significant dynamic terms in the hydrodynamic model to be included in the model without over regressing the response. The process of identifying the nonlinear model is investigated in detail to determine the best collection of forced motion simulations to derive an adequate model. This methodology is intended to provide higher fidelity predictions of vertical plane motions of planning hulls than previous methods while requiring a lower computational expense than a comprehensive CFD evaluation in waves. The final nonlinear model, developed for the Generic Prismatic Planing Hull (GPPH), is validated against static and dynamic simulations not included in the system identification and is found to produce a promisingly accurate model of the hydrodynamic forces and moments When used to predict the heave force and trimming moment of a forced coupled heave and trim motion, the selected nonlinear model is found to predict the heave force and trimming moment with an average absolute error of 3.9% of the maximum force and 5.5% of the maximum moment respectively.

Small Waterplane Area Twin Hulls (SWATHs) are known to have superior seakeeping performance but higher resistance compared to equivalent catamarans or mono-hulls. A way to improve their resistance characteristics is to use unconventional hull forms parametrically defined and optimized by CFD methods. This study builds on previous SWATH optimization studies proposing a comprehensive, systematic investigation on the effect of different shapes and canting angles of the struts. For the first time we demonstrate the importance of considering the shape of the strut that is fully parametrized in our study. The effect of the design speed on the best shape is addressed through a multi-objective optimization targeting the minimum total resistance at two very different speeds, namely the cruise and slow transfer speeds. Optimum hull shapes are discussed in terms of maximum resistance reduction, together with the predicted free waves patterns.

A computational framework to design a new family of unconventional super cavitating (SC) hydrofoils with optimized hydrodynamic performance is developed. A low-order boundary element method is used to solve for the steady potential flow over the hydrofoil predicting its hydrodynamic characteristics, including the vapor-cavity interface. Shape variations are obtained by an ad hoc parametrization scheme by composite B-spline curves whose control points represent the design variables to the hydrodynamic optimization problem. The accuracy of the Computational Fluid Dynamics (CFD) tools is also preventively validated on the experimental characteristics of a conventional SC hydrofoil. A computational test case is performed to maximize the efficiency of a SC hydrofoil accounting for both shape and angle of attack variations. The new hydrofoil leads to 40% improvement on the lift over drag ratio compared to the initial reference shape. This result is confirmed by high-fidelity unsteady multiphase viscous solver.

This paper details the design and simulation of a novel position control mechanism for marine operations or inspection in extreme, hostile, or high-speed turbulent environments where unprecedented speed and agility are necessary. The omnidirectional mechanism consists of a set of counter-rotating blades operating at frequencies high enough to dampen vibrational effects on onboard sensors. Each rotor is individually powered to allow for roll control via relative motor effort and attached to a servo-swashplate mechanism, enabling quick and powerful manipulation of fluid flow direction in a hull’s coordinate frame without the need to track rotor position. The mechanism inherently severs blade loads from servo torques, putting all load on the main motors and minimizing servo response time, while exploiting consistent blade momentum to minimize the corresponding force response time. The mechanical design and kinematic analysis of each subsystem is presented, followed by kinematic and hydrodynamic analysis of the hull and surrounding fluid forces during various blade maneuvers. Special maneuvers are verified using Computational Fluid Dynamic (CFD) software. Finally, a controller is constructed with decoupled parameters for each degree of freedom.

Approximation techniques for the coefficients that parameterize the hydrodynamic forces acting on a maneuvering underwater vehicle are presented and discussed. While captive tests yield the highest fidelity coefficients, they also incur significant costs that are prohibitive in early vehicle design phases. We present a method, requiring only a vehicle's geometric profile, to predict a vehicle's hydrodynamic coefficients. The method is applied to the SATLP vehicle. Using computational fluid dynamics simulations, we assess the uncertainty of the approximate method, and show that the method yields hydrodynamic coefficients that satisfactorily approximate a vehicle's dynamic behavior, suitable for preliminary vehicle design analysis.

The influence of vessel size on the performance of a second generation Autonomous Surface Vessel (ASV) in irregular head waves is investigated. The hull of the SWATH type ASV is optimized using a combination of high fidelity hydrodynamic numerical models to achieve minimal propulsion powering requirement as well as lower motions in waves compared to state of the art catamaran type (CAT) surface vessels currently referred to as first generation. Autonomous Underwater Vehicles (AUV) assistance at sea requires the ASV to possess exceptional seakeeping qualities which hull optimization procedures aim to achieve. However, the performance of the ASV may be sensitive to the most probable wave period of the ambient environment due to the asymptotic peaks typical of SWATH vessel responses in long waves. It is the objective of this study to assess the seakeeping behavior of five different sizes of the ASV at sea to determine the limiting sea state for each vessel size. Parameters such as the significant heave and pitch motions are used to examine the operability of the vessels in five sea states. The results highlight the importance of selecting appropriate vessel size based on the predominant weather conditions of the location.

Capitalizing on the experience and technology developments gained with underwater autonomous vehicles, the current research frontier in the field of autonomous marine vehicles has moved from under water to the sea surface, i. e., autonomous sea surface vehicles. Current and future perspectives of these types of autonomous vehicles are given in Sect. 13.1, with particular attention paid to US navy current interests. These were initiated a decade ago and still actively drive developments in this sector. The ability to design craft specialized for particular tasks to reach the best performance at sea, going beyond size and operational/safety limitations currently imposed by manned ships, offers unique opportunities to the naval architect. In this respect, the basic naval architecture principles that drive the selection of a type of hull with respect to operational requirements are given in the Sect. 13.3. The selection of the type of hull is a preliminary essential activity for the successful design or acquisition of an autonomous surface craft. Finally, as a practical example, Sect. 13.4 summarizes the main results of extensive research done at the MIT-iShip lab to develop a new class of autonomous unmanned surface vehicles, based on a highly specialized and optimized design of an unconventional SWATH (small waterplane area twin hull) hull, able to achieve superior performance and operational capabilities in real sea state conditions.

The propeller design is an activity which nowadays presents ever increasing challenges to the designer, involving not only the usual mechanical characteristics fulfillment and cavitation erosion avoidance, but also other cavitation side effects, such as radiated noise and/or pressure pulses. Moreover, in some cases propeller characteristics have to be optimized in correspondence to very different functioning points, including considerably off-design conditions, hardly captured by conventional design methods. In the present paper, a recently presented method, based on the coupling between a multiobjective optimization algorithm and a panel code, is applied to the design of a CPP propeller at different pitch settings, with the aim of reducing the cavitating phenomena and, consequently, the resultant radiated noise. Particular attention has been devoted to the slow speed (low pitch) condition, obtained at constant RPM, and characterized by considerable radiated noise and vibrations related to face cavitation. Numerical results are validated by means of an experimental campaign, testing both the original and the optimized geometry in terms of cavitation extent and radiated noise. Experimental results confirm the numerical predictions, attesting the capability of the method to assess propeller functioning characteristics, thus representing a very useful tool for the designer in correspondence of challenging problems.

In the present paper an experimental and numerical analysis of an unconventional CLT propeller is carried out. Two different numerical approaches, a potential panel method and an RANSE solver, are employed. Cavitation tunnel experiments are carried out in order to measure, as usual, thrust, torque, and cavity extension for different propeller working points. Moreover, LDV measurements are performed to have a deep insight into the complex wake behind the propeller and to analyze the dynamics of generated tip vortexes. The numerical/experimental analysis and comparison of results highlight the peculiarities of this kind of propellers, the possibility to increase efficiency and reduce cavitation risk, in order to exploit the design approaches already well proven for conventional propellers also in the case of these unconventional geometries.

In this paper we investigate the resistance and seakeeping effects of an inverted bow by comparing the motions of an existing combatant hull, the Oliver Hazard Perry class frigate (FFG-7), with a modified version of the hull with an inverted bow. The bow of the FFG-7 was redesigned by developing a set of basic curves that define the parametric surfaces of the new shape. Two 1/80th scale models were tested for resistance in calm water and seakeeping in both regular waves and irregular head seas. The differences between the FFG-7 and the inverted bow responses are characterized in terms of pitch and heave motions and vertical accelerations.

The next generation of Autonomous Surface Vessels (ASV) will be optimized for optimum performance at sea. The design of an innovative unconventional SWATH-ASV family, patented by the author and under further investigation at the MIT innovative Ship design lab is described in its main features. The new unconventional design of the hull is the key to superior resistance, powering and seakeeping performance. The achievement of these excellent results is a direct consequence of the extensive use of new computational fluid dynamic tools integrated into full parametric geometry optimization methods. The main modeling capabilities of these numerical methods are presented. As a result, the new ASV-SWATH is able to demand 20% to 40% less propulsion power than conventional catamarans at high speeds and it is subject to significantly lower motions in waves: 50% less heave and 60% pitch in a wide range of incoming wave frequencies. The new ASV platform is the ideal, yet missing, element to realize a true persistent and portable network of AUVs at sea. The characteristics of onboard systems for AUV launching/recovering and wireless recharging are finally discussed and their feasibility assessed in the light of the seakeeping predictions performed both for the AUV and the ASV in the same sea state.

We present a CFD characterization of a new type of super-cavitating hydrofoil section designed to have optimal performance both in super-cavitating conditions and in sub-cavitating conditions (including transitional regime). The basic concepts of the new profile family are first introduced. Lift, drag and cavity shapes at different cavitation numbers are calculated for a new foil and compared with those of conventional sub-cavitating and super-cavitating profiles. Numerical calculations confirm the superior characteristics of the new hydrofoil family, which is able to attain high lift and efficiency both in sub-cavitating and super-cavitating conditions. Numerical calculations are based on a multi-phase fully turbulent URANSE solver with a bubble dynamic cavitation model to follow the generation and evaporation of the vapor phase. The new profile family, initially devised for ultra-high speed hydrofoil crafts, may result useful for diverse applications such as super-cavitating or surface-piercing propellers or high-speed sailing boats.

The objective of this work is to investigate the performance of two pairs of negative dihedral surface-piercing (SP) hydrofoils designed especially for an unmanned surface vessel with a top speed of 120 knots in sea state two. Physical modeling of a 1/6-scaled model of the SP hydrofoil was conducted at the free-surface cavitation tunnel at the Technical University of Berlin (TUB). The SP hydrofoil feature a new type of super-cavitating profile with an annex tapered trailing edge to achieve good efficiencies in foil born conditions (60–120 knots, super-cavitating/ super-ventilated regimes), as well as at take-off speeds (25–40 knots, wetted and/or partial-cavitating regimes). Preliminary results showed interesting anomalies in the trends of the measured forces with respect to the cavitation number and angle of attack for a wide range of inflow speeds. Details of the experimental study are presented along with numerical predictions obtained using finite volume RANSE solver with a volume of fluid technique to allow for a mixture flow with air/vapor and water phases. Explanation of the anomalies in the hydrodynamic performance is given.

We present a comparison of performance of an all-electric-ship (AES) simulator using two different approaches to control the induction machine in forward as well as in cash astern operations. The first approach uses a constant-slip control while the second uses direct-torque control. We also investigate the use of proper breaking resistors required for minimal power dissipation during the crash astern operation. The AES simulator describes a medium voltage DC system (MVDC) for the USS DDG-51 Arleigh Burke-Class destroyer.

The paper presents the main characteristics of an innovative platform which has been conceived and designed to extend the operational capabilities of current unmanned surface vehicles in terms of platform stability in waves and of powering requirement at a relatively high speed. The main idea which rules the project is the realization of a small autonomous surface unit (about 6 m in length) capable of undertaking several tasks in the marine environment even with moderate rough sea conditions. The designed vessel has the ability to locate, recover, and launch other members of the autonomous fleet (like AUVs or other underwater devices) and at the same time could carry out a surveillance service of the surrounding areas. To manage these tasks, the vehicle is designed to provide a fairly good autonomy which is needed to face intermediate-range missions (100 nautical miles). The choice of a small waterplane area twin hull (SWATH) form has been motivated by its excellent properties of seakeeping qualities, combined with a non-conventional low resistance underwater hull shape, currently under patenting process, which is able to reduce to a minimum the resistance of the vessel especially at higher speeds. To obtain the most efficient profile of the underwater bodies, a systematic optimization with an automatic procedure based on a parametric definition of the geometry, a state-of-the-art computational fluid dynamics (CFD) flow solver, and a differential evolution global minimization algorithm have been created and used. As expected, all the final CFD computations on the best design have demonstrated the superior efficiency of the developed unconventional SWATH technology with respect to different alternatives of current hull typologies.

Impact pressures, due to slamming, are of particular importance in assessing local and global structural strength. Knowledge of pressure distribution and slamming forces, experimental or numerical, in the forefoot and aftbody of a ship can be used in the evaluation of global wave-induced loads, for example, whipping bending moment. Impact pressures and slamming forces for a bow section, impacting upright and at two angles of heel are presented in this article. The relevant impact loads are predicted using a range of two-dimensional potential and viscous flow methods and compared with available experimental measurements.

The paper presents a computational method developed for the design of contra-rotating propellers of high-speed craft propulsors. The method is based on Lerbs’ lifting line theory, but with some new peculiarities introduced to adapt the method, originally devised by Morgan for slow speed propellers. The differences introduced in the method regard mainly the interaction effects between the two propellers and the criteria used to select the thickness/chord distribution to match the cavitation and strength constraints. In fact, a new fully numerical method, based on the exact lifting line model of the propellers, is used to recalculate the mutual induction factors used by Morgan (1960) to consider the interference of flow induced by each propeller to the other. After the description of the theoretical and numerical method, an example of practical application of the design method to the case of a stern-podded drive for a fast-planing boat is presented and critically discussed through the analysis of t...

This paper presents a comprehensive approach to simulate the propulsion system behavior during ship maneuvers. The behavior of the ship system is simulated by means of a dynamic simulation model, which was already validated for the performance prediction of ship propulsion plants with the ship traveling in a straight line. The dynamic model has now been extended to consider the ship maneuverability in the horizontal plane. In order to analyze the different maneuvers of a ship, the simulation model has been applied to a propulsion system consisting of a twin shaft arrangement with controllable pitch propellers, driven through reduction gears by turbocharged medium speed diesel engines. The accuracy of the simulation procedure is illustrated in the paper through comparison of full-scale results for different maneuvers such as the turning cycle, zig zag and spiral.

Sloshing describes the movement of liquids inside partially filled tanks, generating dynamic loads on the tank structure. The resulting impact pressures are of great importance in assessing structural strength, and their correct evaluation still represents a challenge for the designer due to the high level of nonlinearities involved, with complex free surface deformations, violent impact phenomena and influence of air trapping. In the present paper, a set of two-dimensional cases, for which experimental results are available, is considered to assess the merits and shortcomings of different numerical methods for sloshing evaluation, namely two commercial RANS solvers (FLOW-3D and LS-DYNA), and two academic software (Smoothed Particle Hydrodynamics and RANS). Impact pressures at various critical locations and global moment induced by water motion in a partially filled rectangular tank, subject to a simple harmonic rolling motion, are evaluated and predictions are compared with experimental measurements. 2012 Copyright Taylor and Francis Group, LLC.