Ship In Calm Water Research Articles

Study on the Resistance of a Large Pure Car Truck Carrier with Bulbous Bow and Transom Stern

The resistance of a large Pure Car Truck Carrier (PCTC) with a bulbous bow and a transom stern is evaluated in the present paper. Several cases at nine different ship speeds in calm water are simulated and results are compared with the experimental measurements. The maximum relative error is 0.93% at a Froude number (Fr) of 0.209. The total resistance coefficient of the ship in calm water shows a parabolic trend with increasing Fr, and it reaches a minimum value at Fr = 0.1794. Furthermore, the cases of the ship in regular waves with six different wavelengths and three wave heights are simulated. It is observed that the total resistance exhibits a quadratic relationship with the wavelength when the wave height is fixed. The wave-making resistance increases with the increase in wave height at any fixed wavelength, and it reaches a maximum value when the wave-length is 1.2 times the ship length (Lpp). Additionally, we also investigated the resistance in three different sea states at four different speeds. When the significant wave height of irregular waves is the same as regular waves, the wave-making resistance under irregular waves is much smaller than that of the regular waves. All of these results indicate that the bulbous bow and transom stern can reduce the wave-making and residuary resistances, which can provide a useful reference for the subsequent design and manufacturing of related ships.

On the performance of different Deep Reinforcement Learning based controllers for the path-following of a ship

A set of continuous state-action space-based deep reinforcement learning algorithms are used for the path following of a ship in calm water and waves. The mathematical model of a KVLCC2 tanker represents the ship dynamics. The mathematical model includes the hull force, rudder force, propulsion force, and external wave forces. Look ahead distance-based guidance algorithm called Line of Sight (LOS) is used for computing the Cross Track Error (CTE) and Heading Error (HE). The reward function is designed based on HE and CTE. The created Environment is trained with four different Deep Reinforcement Learning (DRL) agents named Proximal Policy Optimization (PPO), Deep Deterministic Policy Gradients (DDPG), Twin-Delayed Deep Deterministic Policy Gradients (TD3), and Soft-Actor Critic (SAC). Common Neural Network architecture is used for all four agents. Yaw rate, HE, and CTE serve as input to the Neural Network, and the rudder deflection rate (δ°) corresponds to the action space (output). Computation time, average cross-track error, and rudder actuation are computed and compared for path-following scenarios. DDPG performs better with a minimum average CTE for all the simulated cases. However, SAC demands minimum rudder control effort to achieve the tasks. Finally, the trained agents are validated using Hardware In-Loop (HIL) simulation.

A Unified Internal Flow Model with Fluid Momentum for General Application in Shipflooding and Beyond

This paper deals with the development and validation of a unified internal flow model (UIF) for the prediction of fluid behaviour in a network of 3D floodable cells such as an internal ship compartment subdivisions. The UIF model is incorporated in a generic time-domain ship-motion simulation environment. The flow model uses cell-averaged fluid momentum to account for dynamic (inertia) effects in compartments. A compartment is typically given the size of a ship compartment on board. The fluid solver can keep track of the air pressure in and air flow through compartments under isothermal assumption. Simulation results are compared against experimental data from four different configurations: a single tank draining experiment, a two compartment down-flooding experiment, an oscillating water column experiment under pressure, and a transient flooding experiment with a cruise ship in calm water. The general validity of the UIF model is demonstrated through these four examples. The newly developed UIF model overcomes the shortcomings seen in the steady Bernoulli-based simulations. Its application leads to a more accurate prediction of the floodwater progression in the ship, and it captures the fluid dynamics seen in the oscillating water column device very well, which is not possible using the steady Bernoulli approach. The general applicability of the UIF solver for internal fluid response in maritime application is thereby demonstrated. The effect of the internal compartmentalisation of a cruise ship and the effect of viscous roll damping on the transient roll response is discussed.

Numerical study on transom stern ventilation and resistance of high-speed ship in calm water

Numerical study on transom stern ventilation and resistance of high-speed ship in calm water

Data driven control based on Deep Q-Network algorithm for heading control and path following of a ship in calm water and waves

A reinforcement learning algorithm based on Deep Q-Networks (DQN) is used for the path following and heading control of a ship in calm water and waves. The rudder action of the ship is selected based on the developed DQN model. The spatial positions, linear velocities, yaw rate, Heading Error (HE) and Cross Track Error (CTE) represent the state-space, and a set of rudder angles represents the action space of the DQN model. The state space variables are in continuous space and action spaces are in discrete space. The decaying ϵ-greedy method is used for the exploration. Reward functions are modeled such that the agent will try to reduce the Cross Track Error and the Heading Error. Based on the literature available, the L7 model of a KVLCC2 tanker is used for testing the algorithm. The vessel dynamics are represented using a 3DoF maneuvering model that includes hydrodynamic, propeller, rudder and wave forces. The wave disturbances are calculated from the second-order mean drift forces . The environment is assumed to have the Markov property. The CTE and HE are calculated based on the Line of Sight (LOS) Algorithm. The effect of Pre-trained weights on different heading actions is investigated based on the exploration threshold. The DQN is trained and tested for heading control and path-following in calm water and different wave headings.

Numerical investigation into the resistance performance for the damaged DTMB 5415 ship in calm water and regular head waves

ABSTRACT This paper demonstrates the feasibility of Computational Fluid Dynamics (CFD) to predict the motion responses of the damaged ship and the total resistance in calm water and regular head waves. The head waves are described with a fifth-order Stokes scheme. Two benchmarking studies are done to respectively validate the accuracy of the applied numerical approach in predicting the total resistance and ship's motions. By comparing the total resistance of the intact ship with motions and the damaged ship, it can be obtained that the increased total resistance due to the flooding flow almost comes from the viscous pressure resistance. Moreover, added resistance generated by head waves is a considerable factor affecting the resistance performance of the ship. The increased value can be several times even more than ten times that of the upright-floating intact ship in calm water.

A unified seakeeping and manoeuvring model with a PID controller for path following of a KVLCC2 tanker in regular waves

Heading control of ships in calm water has been investigated by several authors using different control techniques. However, the study on heading control of ships in actual sea conditions is very limited. This paper attempts to investigate the heading control in regular waves using a PID controller. A 6 DoF unified seakeeping and manoeuvring numerical model is integrated with a PID controller. A crude oil carrier (KVLCC2) prototype is used in the numerical study. The second order wave mean drift forces are calculated based on Salvesen’s method and affect the horizontal motions. The wave exciting forces/moments and the restoring forces in the vertical planes are calculated for the exact wetted surface area. Empirical relations available in the existing literature are used to compute the propeller thrust, control forces/moments and hydrodynamic forces. PID controller gains Kp,Kd and Ki changes for different sea states depending on the external wave force. Hence these gains are pre-calculated for different sea states by trial and error. The performance of the controller under the wave action is then studied for path-following application.

Representation of small passenger ferry maneuvering motions by practical modular model

Maneuvering motions of a ship in calm water are studied through the concept of MMG model. Governing forces are defined by the use of available empirical formulae that require only main ship particulars as input variables. In order to validate the calculation tool, a full-scale sea experiment was carried out in Osaka Bay using a 17-m twin-screw passenger ferry. Test execution and data measurement were performed through the utilization of an autopilot control unit and satellite compass. The result of a straight running test confirms the acceptable accuracy in addressing the surge motion problem. Reasonable agreement between simulation and experiment is also confirmed for 5∘/5∘ and 10∘/10∘ zig-zag tests despite the strong environmental disturbance. The current model can generally represent the subject ship maneuvering motions and is promising for the application to other ship hulls.

Numerical simulations of zigzag maneuver of free running ship in waves by RANS-Overset grid method

The twin-screw fully appended ONR Tumblehome ship model is numerically studied for the standard 10/10 zigzag maneuver. Simulations are carried out using naoe-FOAM-SJTU, a CFD solver developed on open source platform, OpenFOAM. Overset grid technique and 6DoF module with a hierarchy of bodies are applied to solve the motions of the free-running ship with twin rotating propellers and turning rudders. The self-propulsion point is using the constant value obtained by the previous CFD simulation of the twin-screw self-propelled ship in calm water. Simulations of the standard zigzag maneuver in both calm water and different incident waves are considered. The present CFD results agree well with the available experimental measurement for both ship motions and the main parameters of zigzag maneuver. This indicates that the direct simulations for ship zigzag maneuver in waves utilizing present methods are feasible and reliable. In addition, it is found that unsteady effects of the hydrodynamic loads on the free-running ship during zigzag maneuver in waves is noticeable. Furthermore, flow visualizations in different incident waves are presented so as to give a better understanding of the wave influences on free-running ship maneuver.

Numerical study of hydrodynamic interaction between two ships in calm water

Numerical study of hydrodynamic interaction between two ships in calm water

Development of a numerical simulation tool for efficient and robust prediction of ship resistance

In this paper, a two-phase flow solver HiFoam® has been developed based on the OpenFOAM® to predict resistance of a ship in calm water. The VOF method of OpenFOAM® was reviewed and a simple flux limiter was introduced to enhance the robustness of the solver. The procedure for predicting ship motion was modified by introducing Quasi-Steady Fluid-Body Interaction (QS-FBI) with least square regression to improve the efficiency. Other minor factors were considered as well in terms of the efficiency and robustness. The HiFoam was applied to KCS and JBC simulations to validate its efficiency and accuracy by comparing the results to experimental data and STAR-CCM+. The HiFoam® was also applied to various ships and it showed good agreements to the experimental data.

Total resistance prediction of an intact and damaged tanker with flooded tanks in calm water

This paper presents the prediction of the total resistance of an intact and damaged ship model using the computational fluid dynamics (CFD) technique. The study is performed on the model of a tanker with a large hole in the bottom of the hull. The damage is based on statistical data on ship grounding accidents and the chosen hole size and location in the midship area represents its plausible size and location due to grounding. Reynolds-Averaged Navier-Stokes (RANS) equations with the volume of fluid (VOF) surface capturing technique are employed to solve the flow around the steadily advancing model of a damaged ship in calm water. The experiments, both on an intact and a damaged ship model that were carried out in the towing tank of the Brodarski Institute in Zagreb, Croatia, are used to evaluate the results. The numerical results are in a good agreement with the experimentally obtained results. The significant average increase of 27% in total resistance due to the altered flow around the hole and inside the flooded tanks can be observed for the analysed case. The study shows that the proposed CFD model and settings provide a good prediction of the total resistance together with the flow both around the damaged hull and inside the flooded tanks of the damaged tanker.

Numerical and Experimental Analysis of Added Resistance of Ships in Waves

Two Reynolds-Averaged Navier–Stokes (RANS) based field methods numerically predicted added resistance in regular head waves for a 14,000 TEU containership and a medium size cruise ship. Long and short waves of different frequencies were considered. Added resistance was decomposed into diffraction and radiation force components, whereby diffraction forces were obtained by restraining the ship in waves and radiation forces by prescribing the motions of the ship in calm water. In short waves, the diffraction part of total resistance was dominant as almost no ship motions were induced. In long waves, the sum of diffraction and radiation forces exceeded total resistance, i.e., the interaction of these two force components, which caused the reduction of total resistance, needed to be accounted for. Predictions were compared with model test measurements. Particular emphasis was placed on the following aspects: discretization errors, frictional resistance as part of total added resistance in waves, and diffraction and radiation components of added resistance in waves. Investigations comprised two steps, namely, a preliminary simulation to determine calm water resistance and a second simulation to compute total resistance in waves, always using the same grids. Added resistance was obtained by subtracting calm water resistance from total averaged wave resistance. When frictional resistance dominated over calm water resistance, which holds for nearly all conventional ships at moderate Froude numbers, high grid densities were required in the neighborhood surrounding the hull as well as prism cells on top of the model's surface.

URANS simulations for a flooded ship in calm water and regular beam waves

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 finite pointset method for the numerical simulation of free surface flow around a ship

In this paper, we present a Finite pointset method (FPM) for the numerical simulation of free surface flow around a ship in calm water. It is a Lagrangian and meshless particle scheme which is applied to the projection method for the incompressible governing equations. This requires the solution of Poisson problems in each time step, so a moving least squares (MLS) interpolants is used for the spatial derivatives in order to discretize the Poisson equation with pressure-Dirichlet condition of free surface flow in meshless structure. Meanwhile, an additional problem of the periodic particle locations redistribution in the present approach is still handled by MLS interpolants. With the proposed FPM technique, problems associated with the free surface flow around a ship are circumvented. A verification of numerical modeling is made using the Wigley hull and the validity of the proposed methodology is examined by comparing the detail of wave profile and wave-making resistance with Series 60 model. The results demonstrate that FPM is able to perform efficient and stable simulations of free surface flow around a ship.

고속 활주선의 선형에 따른 저항 성능 및 규칙파 중 운동 성능 고찰

Planing hull forms have significant influences on those hydrodynamic performances in calm water and in waves. Therefore, the hydrodynamic performance of a planing vessel should be predicted by model tests or theoretical calculations, and be confirmed whether it shows the performance requirements at the design stage. In this study, four planing hull forms are designed with the goal of the improvement of resistance and seakeeping performance, and 1/6.5 scale model tests are carried out in Seoul National University towing tank. The effects of design parameters such as length-to-beam ratio, deadrise angle and forebody shape on the hydrodynamic performance are investigated, based on model test results. Running attitude and resistance of model ships in calm water are also estimated by empirical formulae proposed by Savitsky (1964; 2007; 2012), and compared with the model test results. It is shown that calm water performance of non-prismatic planing hulls can be predicted well by Savitsky (2012)'s formula which improves the original Savitsky(1964/2007)'s formula by taking into account the variations of deadrise angles, and the actual angles between the hull bottom and the free surface.

Simulation on Model Performance of Ship’s Diesel Engine Propulsion in Calm Water

Ship performance is prevalently evaluated during calm water condition though contrarily the ships operated in actual rough sea environments. The paper elaborates a mathematical model of marine diesel engine propulsion system to estimate real performance of ships in calm water through the proposed computer-simulated model. A post Panamax container ship and LNG tanker are employed as target ships of this study. The analysis of the model includes propeller rotating speed and torque from which the required engine power is estimated to achieve the target speed. These input and output are real-time control system of propeller rotating speed reflecting the characteristics of marine diesel engine. The result has established ultimate limit of affecting features of a real-time control system to control ship speed, power and propeller torque. Further use of the model is advantageous to analyse different condition of ship speed for finding better design characteristics.

Hydrodynamic ship design for service conditions

An innovative design methodology considering the simulation of the operational life of a ship is presented, including weather factors and off-design floating conditions. Different methods and data are integrated within a simulation software to achieve this. Advanced numerical methods, mainly Computational Fluid Dynamics (CFD), are used for the analysis of the hydrodynamic performance of the ship in calm water and waves. An application example is shown and results are discussed.

Optimization of bow shape for a non ballast water ship

In this research, a commercial CFD code “Fluent” was applied to optimization of bulbous bow shape for a non ballast water ships (NBS). The ship was developed at the Laboratory of the authors in Osaka Prefecture University, Japan. At first, accuracy of the CFD code was validated by comparing the CFD results with experimental results at towing tank of Osaka Prefecture University. In the optimizing process, the resistances acting on ships in calm water and in regular head waves were defined as the object function. Following features of bulbous bow shapes were considered as design parameters: volume of bulbous bow, height of its volume center, angle of bow bottom, and length of bulbous bow. When referring to the computed results given by the CFD like resistance, pressure and wave pattern made by ships in calm water and in waves, an optimal bow shape for ships was discovered by comparing the results in the series of bow shapes. In the computation on waves, the ship is in fully captured condition because shorter waves, λ/L pp <0.6, are assumed.

The fluid mechanics of splashing bow waves on ships: A hybrid BEM–SPH analysis

The free surface bow flow around a fast and fine ship in calm water is studied with an emphasis on generation and evolution of the breaking and splashing bow wave using the 2D+t theory. A temporal domain-decomposition strategy is adopted which combines sequentially two Lagrangian methods: a potential-flow solution, given by a boundary element method (BEM), follows the jet evolution up to the breaking and then initiates a rotational solution, provided by a smoothed particle hydrodynamics (SPH) technique. The latter describes the splashing and the evolution of rotational flow structures generated during the evolution. The compound method is very efficient and proved to be able to capture the breaking features. A validation has been performed through comparison with experimental data and other numerical solutions available in the literature. The DDG51 vessel is used to carry on a parameter investigation in terms of the ship speed (Froude number) and surface tension. The analysis confirms a substantial influence of the Froude number on the patterned morphology and highlights the importance of surface tension for small-scale ships (e.g. small-scale models tested in towing tanks) in inhibiting the jet formation and evolution.