Wenyang Duan

Incompressible SPH method based on Rankine source solution for violent water wave simulation

With wide applications, the smoothed particle hydrodynamics method (abbreviated as SPH) has become an important numerical tool for solving complex flows, in particular those with a rapidly moving free surface. For such problems, the incompressible Smoothed Particle Hydrodynamics (ISPH) has been shown to yield better and more stable pressure time histories than the traditional SPH by many papers in literature. However, the existing ISPH method directly approximates the second order derivatives of the functions to be solved by using the Poisson equation. The order of accuracy of the method becomes low, especially when particles are distributed in a disorderly manner, which generally happens for modelling violent water waves. This paper introduces a new formulation using the Rankine source solution. In the new approach to the ISPH, the Poisson equation is first transformed into another form that does not include any derivative of the functions to be solved, and as a result, does not need to numerically approximate derivatives. The advantage of the new approach without need of numerical approximation of derivatives is obvious, potentially leading to a more robust numerical method. The newly formulated method is tested by simulating various water waves, and its convergent behaviours are numerically studied in this paper. Its results are compared with experimental data in some cases and reasonably good agreement is achieved. More importantly, numerical results clearly show that the newly developed method does need less number of particles and so less computational costs to achieve the similar level of accuracy, or to produce more accurate results with the same number of particles compared with the traditional SPH and existing ISPH when it is applied to modelling water waves.

Corrected First-Order Derivative ISPH in Water Wave Simulations

The smoothed particle hydrodynamics (SPH) method is a meshless numerical modeling technique. It has been applied in many different research fields in coastal engineering. Due to the drawback of its kernel approximation, however, the accuracy of SPH simulation results still needs to be improved in the prediction of violent wave impact. This paper compares several different forms of correction on the first-order derivative of ISPH formulation aiming to find one optimum kernel approximation. Based on four benchmark case analysis, we explored different kernel corrections and compared their accuracies. Furthermore, we applied them to one solitary wave and two dam-break flows with violent wave impact. The recommended method has been found to achieve much more promising results as compared with experimental data and other numerical approaches.

A comparative study of diffraction of shallow-water waves by high-level IGN and GN equations

This work is on the nonlinear diffraction analysis of shallow-water waves, impinging on submerged obstacles, by two related theories, namely the classical Green-Naghdi (GN) equations and the Irrotational Green-Naghdi (IGN) equations, both sets of equations being at high levels and derived for incompressible and inviscid flows. Recently, the high-level Green-Naghdi equations have been applied to some wave transformation problems. The high-level IGN equations have also been used in the last decade to study certain wave propagation problems. However, past works on these theories used different numerical methods to solve these nonlinear and unsteady sets of differential equations and at different levels. Moreover, different physical problems have been solved in the past. Therefore, it has not been possible to understand the differences produced by these two sets of theories and their range of applicability so far. We are thus motivated to make a direct comparison of the results produced by these theories by use of the same numerical method to solve physically the same wave diffraction problems. We focus on comparing these two theories by using similar codes; only the equations used are different but other parts of the codes, such as the wave-maker, damping zone, discretion method, matrix solver, etc., are exactly the same. This way, we eliminate many potential sources of differences that could be produced by the solution of different equations. The physical problems include the presence of various submerged obstacles that can be used for example as breakwaters or to represent the continental shelf. A numerical wave tank is created by placing a wavemaker on one end and a wave absorbing beach on the other. The nonlinear and unsteady sets of differential equations are solved by the finite-difference method. The results are compared with different equations as well as with the available experimental data.

Ship Trim Optimization: Assessment of Influence of Trim on Resistance of MOERI Container Ship

Environmental issues and rising fuel prices necessitate better energy efficiency in all sectors. Shipping industry is a stakeholder in environmental issues. Shipping industry is responsible for approximately 3% of global CO2 emissions, 14-15% of global NOX emissions, and 16% of global SOX emissions. Ship trim optimization has gained enormous momentum in recent years being an effective operational measure for better energy efficiency to reduce emissions. Ship trim optimization analysis has traditionally been done through tow-tank testing for a specific hullform. Computational techniques are increasingly popular in ship hydrodynamics applications. The purpose of this study is to present MOERI container ship (KCS) hull trim optimization by employing computational methods. KCS hull total resistances and trim and sinkage computed values, in even keel condition, are compared with experimental values and found in reasonable agreement. The agreement validates that mesh, boundary conditions, and solution techniques are correct. The same mesh, boundary conditions, and solution techniques are used to obtain resistance values in different trim conditions at Fn = 0.2274. Based on attained results, optimum trim is suggested. This research serves as foundation for employing computational techniques for ship trim optimization.

Consistent formulation of ship motions in time-domain simulations by use of the results of the strip theory

Based on the three-dimensional time-domain impulse response function (IRF) and frequency-domain hydrodynamic coefficients, such as the infinite frequency added mass and damping, and radiation restoring coefficients, the IRF method is used in this work to obtain consistent formulations of linear radiation forces and ship motions. The new developments are twofold. First, the hydrodynamic added mass at infinite frequency and radiation damping in the time domain are theoretically derived using the strip theory. Second, via Kramers–Kronig (K–K) relationship for forward speed, it is observed that the added-mass and damping coefficients in the frequency domain and radiation restoring coefficients in the time domain should satisfy the K–K relationship to obtain the linear hydrodynamics in the time domain consistent with those from the frequency domain in strip theory. Following the K–K relationship and the frequency-domain added-mass and damping coefficients obtained by the strip theory, the new expressions of radiation restoring coefficients are derived. Finally, numerical validations are performed to confirm the consistent formulation proposed in this work.

On wave–current interaction by the Green–Naghdi equations in shallow water

This work is on the use of the Green–Naghdi (GN) nonlinear wave equations for simulating wave–current interaction in shallow water. The stream-function wave theory is used at the wave-maker boundary to generate nonlinear incident waves to consider the wave–current interaction. The nonlinear GN equations are solved in the time domain by use of the finite-difference method. The model is evaluated with data from three experimental studies. A strong opposing current over a submerged bar is investigated in the first test case. In the second test case, the interaction of waves with a uniform current over flat bottom is considered. In the third case, wave–current interaction over a variable bathymetry with the following and opposing currents is studied. The numerical results obtained by the GN equations are compared with the experimental data and results based on the Boussinesq equations. A good agreement is obtained for the three experimental studies considered for a wide range of wave and current conditions.

CFD Simulation of Flexible Ship in Regular Head Waves

The ship springing is continuous vibration of the hull girder due to encountered wave excitation, which is considered as a typical fluid-structure interaction (FSI) problem. In this study, a CFD approach is proposed for modeling ship springing induced by large amplitude regular waves. In the CFD model, nonlinear free surface flows are solved by a finite difference method based on CIP (Constraint Interpolation Profile) method. Flexible structure is calculated using one dimensional finite element method by idealizing the ship structure as a beam. A volume weighted method, which is based on IB (Immersed Boundary) method, is applied to couple the FDM and the FEM. The proposed numerical model is validated against an experiment on a flexible barge in regular waves. Discussions are made on the numerical results.Copyright

Propeller efficiency options for green ship

Environmental issues and rising fuel prices necessitate better energy-efficiency in all sectors. Shipping industry is one of the stakeholders in environmental issues. Shipping industry is responsible for 3 % of global CO2 emissions, 14-15 % of global NOX emissions and 16 % of global SOX emissions. Shipping industry also has critical role in global economy since 90 % of world trade goods are carried by ships. This trade has little or no alternate means of transportation other than ships at this point and foreseeable future. In addition, shipping is a better environmental option for transportation compared to other available means of transportation due to lowest gCO2/ton.km emissions. European Union (EU) and United Nations Framework Convention on Climate Change (UNFCCC) are pushing hard to regulate emissions in all sectors. International Maritime Organization (IMO) is working on regulating emissions from shipping with unprecedented attention and it seems clear that emissions from shipping will be regulated within few years. Furthermore, continuously rising fuel prices are also a reason to focus on new ways for better energy-effectiveness in addition to better environmental performance. Green ship concept requires exploring and implementing technologies/practices on ships to reduce emissions. Propeller efficiency is an important area to increase efficiency and reduce emissions since propellers are merely around 60 % efficient. This paper provides a comprehensive review of the propeller efficiency options to increase efficiency and reduce emissions. The paper will discuss basic concepts and principles, and popular technologies for enhancing propeller efficiency. The author will also comment on the core-issues and challenges, and scope for future development in propeller efficiency area.

Forced roll simulations and ship roll damping computations using URANS

Ship roll response and associated roll damping estimation have long been an area of research, due to their detrimental nature to ship safety and habitability, and owing to the challenging roll damping computations that remain vague because of difficulties arising from strong viscous effects typical of ship roll motion. This work presents an assessment of roll damping of S175 container ship by performing Unsteady RANS simulations on CFD tool FLUENT, hence inherently incorporating viscous effects. The simulations consist of forced roll tests similar to that performed in model tests. Bilge Keel, roll amplitude (particularly large amplitude rolling) and roll frequency effects on roll response are considered. Vortex shedding is captured through simulations; besides the manuscript delineates the effects of frequency, roll amplitude and bilge keel on vortex formation and roll damping. Numerical verification and validation is accomplished. Comparison with experimental results is overall satisfactory.