The paper presents velocity measurements, using particle image velocimetry, as well as a reconstruction of hydrodynamic pressures for the analysis of fast ships. Stereoscopic PIV measurements with a towed underwater PIV system are conducted during towing tank tests to obtain the velocity field in the bow region of a fast ship at speeds up to Fr=0.8. While the model is kept at a fixed trim and sinkage, multi-plane PIV measurements with a total of 68 measurement planes are conducted to reconstruct a volumetric representation of the time-averaged velocity field in the bow region. The obtained velocity field is subsequently used for a volumetric description of the time-averaged hydrodynamic pressure field. In addition to these captive runs, forced oscillation tests are conducted. During these tests, the flow field is recorded in three successive planes to obtain a local phase-averaged description of the velocity and its gradients for the reconstruction of the phase-averaged hydrodynamic pressure field. The postprocessing procedure for the pressure reconstruction, including the solution of the Poisson equation, is implemented into the open-source CFD package OpenFOAM. For the detection of the free surface and the ship hull, an automated procedure is presented. Experimental results are finally compared to results from numerical simulations. Results show that the PIV method is capable of capturing the flow characteristics in the bow region of a fast ship. In addition, it can be used together with the pressure Poisson equation to obtain the hydrodynamic pressure field. However, large out-of-plane velocities require a large dynamic range, which limits the resolution of local effects close to the ship hull.

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Wind propulsion for commercial ships has been identified as a key component in the energy transition for the maritime industry. The sailing hybrid ship will operate with leeway (drift) angles to produce a lateral force known as sideforce, for steady operation under sail. In this paper, experimental results for the sailing performance of ships fitted with bilge keel appendages are presented. Systematic variations in appendage height, length, and position were tested, including several special cases (multiple bilge keels). The appendage typology is shown to mitigate the strong ‘destabilizing’ yaw moment that is characteristic of wind-assisted commercial vessels and to promote the non-linear sideforce component. The working principal for bilge keels—promotion of flow separation—can be employed to specify the separation location for components of the vessel vortex wake to improve the sailing performance of the ship.@en

A Reynolds-averaged Navier Stokes computational fluid dynamics (RANS-CFD) package will be one of the primary tools used during the development of a performance prediction program for wind-assisted commercial ships. This paper describes the simulation verification exercise, performed in support of the experimental validation presented in Part 1 of this two-part series describing the RANS-CFD method employed in this research. The predominance of large-scale separated flow structures in the wake of the sailing ship, an artefact of sideforce production necessary for sailing, points to a careful verification exercise and estimate for the numerical uncertainty to support the systematic investigation of wind-assisted ship hydromechanics and meshing guidelines within the available computer resources. Methods for CFD uncertainty quantification are defined and implemented for verification cases at leeway angles equal to 0^ᵒ, 6^ᵒ, and 9^ᵒ. Analysis for four sets of grids with different meshing strategies and for varying time steps results in a grid definition and time step for simulation validation. Numerical uncertainty as adopted in Part 1 for validation is defined. Finally, the meshing strategy for full-scale simulation is described, as used for the production runs of the Delft Wind Assist Series.

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Flettner rotors are nowadays becoming a widespread solution for wind-assisted propulsion. To increase the fuel savings of the ship on which they are installed, multiple devices are typically used. However, in the performance estimate of these hybrid ships, it is currently assumed that Flettner rotors operate independently, regardless of the number of devices employed and their relative position on the ship's deck. The present investigation deals with a wind-tunnel experimental campaign aimed at understanding the aerodynamic interaction effects on the performance of two similar Flettner rotors. The study indicates that the aerodynamic performance of the two Flettner rotors is affected by their interaction, and, generally, this is most noticeable when the devices are set closer to each other and when they are aligned with the wind direction. It is demonstrated that, depending on the apparent wind direction, the layout of the Flettner rotors on the ship's deck has a remarked influence on the driving and heeling force coefficients of the entire rig. Lastly, the velocity ratio is found to play a key role in the determination of how the interaction affects the Flettner rotor aerodynamic performance.

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The present study describes the application of the particle image velocimetry (PIV) technique for the reconstruction of hydrodynamic pressures and loads on a ship model from measured velocity fields during towing tank tests. As an alternative to conventional pressure and force measurement techniques the method simultaneously pictures the velocity field and captures the dynamic aspect of the flow. The presented measurements are conducted in the transom region of a generic hull of a planing vessel which is equipped with an interceptor to create a stagnating flow, associated with a high pressure peak. The flow close to the hull is captured with an underwater stereoscopic PIV system and the pressure peak in front of the interceptor is reconstructed from time-averaged velocity fields. Results show the effect of different interceptor heights on the pressure distribution in the center-plane of the model. Further, a 3D flow field is reconstructed from scanning PIV measurements to analyze the lift reduction due to the finite span of the interceptor. The spatial variation of the measurement uncertainty is analyzed and propagated to the pressure field uncertainty and the potential of the method is further evaluated by comparison with numerical results from steady Reynolds Averaged Navier-Stokes (RANS) simulations.

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Small high-speed craft are the most vulnerable to the severity of the sea: achieving a design which pairs good performance and acceptable levels of safety is not a trivial task. The seakeeping and manoeuvrability of these vessels play a crucial role in following sea conditions: dynamic instabilities, namely broaching-to and surf-riding, are more than a rare eventuality and threaten the survivability of the vessel and the life of the mariners. This study investigates the effects of the steering qualities on the broaching-to behaviour of a high-speed craft when it is sailing in following and stern-quartering waves. The motions and loads of the vessel are simulated by means of a 3D time domain blended potential flow boundary element method (BEM), validated using captive model tests in regular waves carried out at the Seakeeping and Manoeuvring Basin (SMB) of MARIN. The hull directional stability and turning ability of the high-speed craft were artificially modified, showing that an increase in the directional stability as well as in the effectiveness of the steering can be beneficial to avoid the inception of broaching-to, but they have different consequences on the dynamics of the vessel's loss of control.

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A Reynolds-averaged Navier Stokes computational fluid dynamics (RANS-CFD) package will be one of the primary tools used during the development of a performance prediction program for wind-assisted commercial ships. This paper is Part 1 of a two-part series describing the RANS-CFD method adopted for this study. The modelling challenge presented by large separated flow structures in the wake of a sailing ship points to a conscientious validation study. A validation data set, consisting of hydrodynamic forces acting on the ships sailing with a leeway angle, was collected at the Delft University of Technology towing tank facility, for bare-hull and appended cases. Appended cases were designed to represent a broad range of appendage typologies: Rudder, Bilge-keels, Skeg, and Barkeel. A validation statement is made for simulations for the entire bare-hull series and for appended geometries, excepting the Bilge-keel case. The simulation method is described in Part 2, including the assessment of the numerical uncertainty.

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The linear relationship between the pressure and the relative wave elevation on the hull surface is a prominent factor to be reconsidered in relation to the uncertainty of the added resistance. An evaluation method is proposed to access the nonlinear relationship between the hull pressure and the relative wave elevation, which has a decisive influence on the added resistance evaluation. This method is used to experimentally investigate the effect of bow-wave breaking of the fast displacement ship in waves. The results show that the nonlinearity between relative wave elevation and hull surface pressure due to the plunging breaking of a bow wave is intuitively detected using the proposed analytical tool. The effect of bow-wave breaking is deduced by comparing the integral of the local pressure. This study provides important insight into the nonlinear relationship between relative wave elevation and added resistance. In addition, the findings provide a better understanding of the process of plunging breaking of bow waves. The procedure of plunging type of bow-wave breaking is defined in three stages considering the relationship between pressure and wave height: bow-wave developing stage, pile-up and breaking stage, and bow-wave absent stage.

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The Flettner rotor is attracting increasing attention as a viable technology for wind-assisted ship propulsion. Nonetheless, the influence of the Reynolds number on the aerodynamic performance of rotating cylinders is still unclear and under debate. The present study deals with a series of wind-tunnel experiments on a large-scale Flettner rotor in which the forces and pressures acting on the cylinder were measured for Reynolds numbers as large as Re=1.0⋅10 ⁶ . The rotating cylinder used in the experimental campaign had a diameter of 1.0 m and span of 3.73 m. The results indicate that the lift coefficient is only affected by the Reynolds number in the critical flow region and below velocity ratio k=2.5. Conversely, in the velocity ratio range 1<k≤2.5, the drag coefficient is markedly influenced by the Reynolds number over the entire range of flow conditions analyzed. The power coefficient scales with the cube of the tangential velocity and it appears to be insensitive to the Reynolds number or whether the cylinder is spun in an air stream or in still air.

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Experiments on a large-scale Flettner rotor were carried out in the boundary-layer test section of Politecnico di Milano wind tunnel. The rotating cylinder used in the experimental campaign (referred to as Delft Rotor) had a diameter of 1.0 m and span of 3.73 m. The Delft Rotor was equipped with two purpose-built force balances and two different systems to measure the pressure on the rotor’s outer skin. The goal of the experiments was to study the influence of different Reynolds numbers on the aerodynamic forces generated by the spinning cylinder. The highest Reynolds number achieved during the experiments was.

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The manoeuvring characteristics of high speed craft are greatly influenced by the hydrodynamic loads generated by the asymmetrical underwater hull shape when the vessel heels. In order to provide an insight into this aspect of the manoeuvring of high speed craft, captive model experiments were conducted in the model towing tank at the Delft University of Technology. The experiments were divided in two main phases. In the first phase, the heel-sway, heel-yaw coupled linear coefficients and hydrodynamic heel moment were measured using static heeled model measurements over a range of speeds. The second stage of the experiments examined the influence of different running trim attitudes on the values of the manoeuvring coefficients. The results from three running trim conditions were compared.@en

Simulating the hydrodynamics of deformable, floating structures using a partitioned strategy poses a major challenge when the ratio of the added mass to the structural mass is considerate. Existing computational procedures for fluid-structure interaction become less efficient or even unstable. In these situations, it is advisable to modify the coupling to allow the fluid to respond better to the solid motions. A simultaneous solution of the equations governing fluid and solid-body would be a stable choice but is often not feasible. Usually the numerical problems are taken care of with subiterations between fluid and structure, but their convergence can be slow. In this paper we present a more powerful, quasi-simultaneous approach, which tries to mimic a fully simultaneous coupling in an affordable way. It makes use of a simple approximation of the body dynamics, based on the (6 DOF) solid-body modes and the main elastic modes of the structure. The method will be demonstrated in offshore practice, with a falling life boat, a floating CALM buoy, an elastic membrane and a rubber gate.@en

The physical level of interaction between fluid and structure can be either one-way or two-way depending on the direction of information exchange at the interface of fluid and solid. The former can be solved by a partitioned approach and weak coupling. In problems involving two-way fluid-structure interaction, using a partitioned approach and strong coupling, sometimes stability restriction is encountered. This is an artificial added mass effect, which is independent of the numerical time step. Unfortunately an accurate and efficient method to deal with all the different levels of interaction is scarce. Conventionally, relaxation is applied to remedy this problem. The computational cost is directly related to number of sub-iterations between fluid and structural solver at each time step. In this study, the source of this instability is investigated. A discrete representation of a basic added mass operator is given and instability conditions are assessed. A new method is proposed to relax this restriction, the idea essentially is to remove the instability source from the structure and move it to the fluid and solve it monolithically with the fluid. We call this an interaction law. An estimate of the structural response is derived from structural mode shapes. As a test case, a 2D dam break problem interacting with an elastic vertical flexible beam is selected. The interaction of fluid with the beam undergoes several stages. The breaking waves on the beam can increase the added mass drastically, therefore the added mass ratio increases as well. In such a cases, the asset of interaction law is better elaborated, while the stability condition requires very high relaxation without interaction law, but the relaxation can be lowered by only using first five beam mode shapes. As a consequence, the number of sub-iterations reduces by one order. The numerical observations confirm the reduction in computational time due to utilization of the interaction law.@en

In Heavy Marine Transport, it is common practice to drytransport large and heavy floating offshore structures. In general, loading and discharge of these floating cargoes on- and from heavy transport vessels is done at sheltered locations, like harbors, where sea-state and swell conditions are insignificant. Often these locations are at large distance from operating fields of the offshore structures, which means that the structures need to be towed from- or to these fields. To save time and costs, it is beneficial to perform the loading and discharge operations in the field. This necessitates a reconsideration of the maximum allowable wave conditions such as to perform the loading- and discharge operations within specified time frame whilst ensuring safety of crew, cargo and heavy transport vessel. Therefore accurate prediction tools to determine the relative motions between heavy transport vessel and cargo are required. In the past, studies have shown that standard prediction tools over-estimate relative vertical motions compared to model tests and practical experience. This paper discusses the prediction of relative vertical motion, which is dominated by the phenomenon squeeze flow. From model tests and CFD calculations non-linear hydrodynamic loads related to squeeze flow are recognized. Standard linearized solutions do not cover the non-linear loads and therefore result in a lack of accuracy in predicting the relative vertical motions. Linearized solutions assume small motion amplitudes with respect to characteristic dimensions of the flow problem; in the case of squeeze flow this assumption is not valid as the motion amplitude may be in the same order as the gap between cargo bottom and the deck of the heavy transport vessel. With linearized potential solvers it is found that the added mass is strongly dependent on the gap height, which verifies the analytical work of Molin et al. [1]. Following the work of Molin, the change in added mass due to changing gap height gives a large contribution to the non-linear hydrodynamic load. Additionally, a second important contribution is related to the relative vertical velocity, recognized as a drag component. As such, a non-linear formulation is found which can be used in a time-domain approach. This formulation requires gap-height dependent added mass as found using the linearized potential solver. As potential solvers are known to have difficulty dealing with the small gap, different methods have been investigated. Results of model tests and CFD calculations are shown, which are used to tune the non-linear formulation. Tuning is done by adapting the drag component. Furthermore, results of a multi-body problem based on standard linear hydrodynamics and the non-linear formulation are compared.@en

In Heavy Marine Transport it is common practice to drytransport large and heavy floating offshore structures. In general, loading and discharge of these floating cargoes on- and from heavy transport vessels is done at sheltered locations like harbors where sea-state and swell conditions are insignificant. Often these locations are at large distance from operating fields of the offshore structures, which means that the structures need to be towed from- or to these fields. To save time and costs, it is beneficial to perform the loading and discharge operations in the field. This necessitates a reconsideration of the maximum allowable wave condition such as to perform the loading- and discharge operations within specified time frame whilst ensuring safety of crew, cargo and heavy transport vessel. Since precise positioning of the cargo on the HTV cribbing beams is of importance to support the cargo on its structural strong points, the allowed relative horizontal motion during loading or discharge operations is limited to a fraction of the width of these cribbing beams. When increasing the maximum allowable wave conditions, relative horizontal motions between heavy transport vessel and cargo easily exceed these limits if only the standard handling equipment is used. Also, the loads in the handling equipment may exceed safe limits. This paper presents two methods including complementary equipment to reduce- and limit the relative horizontal motions. The first method is based on increasing the stiffness of the con-nection between cargo and heavy transport vessel. This means that there is a transition from a soft (standard handling) system with a low natural frequency to a stiff (clamping) system with high natural frequency. During this transition the system natural frequency will coincide with the wave frequent excitation force. Resonant behavior during the transition is avoided as the complementary equipment also employs a damping force. The second method is based on a closed-loop controller applied to the desired relative horizontal position. The resulting desired load to control the relative horizontal motion is then allocated to several line tension actuators. Contradictory to well-known Dynamic Positioning systems which control low frequent motions, motion control during offshore loading and discharge is performed on wave frequent behavior. This implies that the line tension actuators also need to deliver loads within a wave frequent time-frame. In fact, the peak tension needs to be obtained within a quarter of a wave period. System design and simulation results are presented. Depending on the cargo type, different solutions and operational aspects are discussed. Simulations are done for a typical cargo where both methods to reduce the relative horizontal motions are utilized.@en

In the last forty years wave drift loads have been calculated with methods based on the near-field theory (hull pressure integration, Pinkster [4]) and/or the far field method (linear momentum theory). Both methods use linear theory and through its formulation ignore the ship's hull form above the mean water line. It is evident that in survival sea-states the small motion assumptions are violated and the hull form above the mean water line can affect the motion characteristics of the ship and the drift loads. In order to get more insight in this effect, SBM has conducted a systematic model test campaign at the TU Delft using an Aframax size tanker. The campaign included tests with two different bow shapes: the original bow with flare, and a wall-sided bow. Horizontal loads on the complete vessel and a section of the bow only were measured accompanied by measurements of the ship motions and relative wave heights. Measurements were performed for various wave heights and periods. Numerous repeat tests were conducted to establish the confidence level of the measurement data. Measurements have shown motions and relative wave heights are dependent on wave height. It was suggested that viscous damping may play a part in this. The relative wave height in high waves is affected by bow shape; namely the finite draft, the flare and the bulb. How this departure from linear theory affects the forces on the vessel should be investigated further.@en

The numerical modeling of wave propagation in offshore applications is a formidable challenge even today when we have highly capable numerical schemes and computational power at our disposal. The waves travel in a huge and unbounded domain while the phenomena of interest are only around the structures. Therefore, the infinite domain is truncated via artificial boundaries. This results in a compact computational domain and a residual infinite domain. One of the bottlenecks is what type of boundary condition should be imposed on these introduced boundaries. It should be chosen such that the solution in the small truncated domain coincides with that in the original infinite domain.@en

A Reynolds-Averaged Navier Stokes computational fluid dynamics (RANS-CFD) package will be one of the primary tools used during the development of a performance prediction program for Wind-Assisted commercial ships. The modelling challenge presented by large separated flow structures in the wake of the sailing ship points to a conscientious validation study. A validation data set, consisting of hydrodynamic forces acting on the ship sailing with a leeway angle, was collected at the Delft University of Technology towing tank facility, for bare-hull and appended cases. Four hull geometries were selected to represent of the Delft Wind-Assist Systematic Series. Appended cases were designed to represent a broad range of appendage topologies: Rudder, Bilge-keels, Skeg, and Barkeel. The direct validation exercise for the bare-hull case was successful, with the validation level for the sideforce equal to 9.5% (fine mesh: 9M cells). An extended validation statement is made for simulations for the entire series. This exercise was successful for leeway angles equal to 훽훽=[3표표,6표표]. The validation level (base mesh, 3M cells) for each force component is:푢푢푋푋′=12%, 푢푢푌푌′=17%, 푢푢푁푁′=10%. The validation for appended geometries was not regarded as successful, with the exception of the Rudder case. The numerical uncertainty is the dominant contribution for the validation level, motivating a proportionate refinement of the grid. Here, it is sufficient to achieve parity with other contributions to the uncertainty within the larger context of the project.@en