Sharif Digital Repository

Understanding and investigation of a high-speed craft dynamics, their longitudinal dynamic instabilities in calm water and behavior in waves are of a great importance. Determination of motion equation coefficients will help to analyze the dynamics of these kinds of vessels and the factors affecting their dynamic stabilities. Therefore, it could be useful in controlling the vessel instabilities. The main purpose of this research is to determine the coefficients of longitudinal motions of a planing catamaran using computational fluid dynamics (CFD) and evaluating the interceptor effect on hydrodynamic coefficients of that, which is widely utilized in controlling the motions of a high-speed... 

The effect of the asymmetric water entry over a submerged part of a ship on the hydrodynamic impact is investigated numerically. A wedge body is considered and the problem is assumed to be two-dimensional. The results of symmetric and asymmetric impacts are compared. The effect is found significant in the numerical simulation. The maximum hydro-dynamic pressure at a heel angle of 10 degrees becomes about 95% more than that of the symmetric entry. The result of the present work proves the importance of asymmetrical hydrodynamic impact loading for structural design of a ship. Besides, the numerical procedure is not limited to a wedge type cross section and it is possible to apply it for any... 

The effect of ihe asymmetric water entry over a submerged part of a ship on the hydrodynamic impact is investigated numerically. A wedge body is considered 10 study and the problem is assumed to be two-dimensional. The Results of symmetric and asymmetric impacts are compared together. The effect is found significant in the numerical simulation. The maximum hydrodynamic pressure at a heel angle of 10 degrees becomes about 95% more than that of the symmetric entry. The result of the present, work proves the importance of asymmetrical hydrodynamic impact leading for structural design of a ship. Besides, the numerical procedure is not limited to a wedge type cross section and it is possible to... 

This study is aimed to improve Zarnick’s (1978, A nonlinear mathematical model of motions of a planing boat in regular waves. David W. Taylor Naval Ship Research and Development Center, Bethesda, MD, USA) analytical model for motions of planing vessels. A nonlinear time domain mathematical model was developed for dynamic behaviour of longitudinal motion of high-speed planing crafts in regular waves. This model was based on two-dimensional strip theory method using expanding wedge theory and momentum. Due to the nonlinearity and complex dynamic behaviour of these vessels, the time-domain simulation was adopted. Total resistance, heave and pitch motions as well as acceleration were found very... 

This study is aimed to improve Zarnick’s (1978, A nonlinear mathematical model of motions of a planing boat in regular waves. David W. Taylor Naval Ship Research and Development Center, Bethesda, MD, USA) analytical model for motions of planing vessels. A nonlinear time domain mathematical model was developed for dynamic behaviour of longitudinal motion of high-speed planing crafts in regular waves. This model was based on two-dimensional strip theory method using expanding wedge theory and momentum. Due to the nonlinearity and complex dynamic behaviour of these vessels, the time-domain simulation was adopted. Total resistance, heave and pitch motions as well as acceleration were found very... 

Cavitation behavior of fully submerged propellers is an important issue in the marine engineering field. Cavitation tunnel is usually used for studying hydrodynamic performances and cavitation behavior of a propeller. In the present study, the cavitation behavior of the DTMB P4119 propeller was studied both in a uniform flow and in the wake field of a specific vessel. Considering the dimensions of the test section of K23 cavitation tunnel of Sharif University of Technology and related blockage effects, the propeller was scaled so that the model was 0.195 (m) in diameter. A wake screen was used to generate the desired wake field, which itself was primarily determined by CFD simulations around... 

Conventional propellers might undergo severe cavitation at high speeds and this phenomenon not only affects the efficiency of the propeller, but also may result in serious damages in propulsion system. Due to their special geometries, surface piercing propellers (SPPs) overcome this problem and achieve high efficiencies in high speeds. Therefore, SPPs are one of the popular propulsors for high-speed crafts. The present research is aimed to pursue SPP's performance in the off-design conditions. URANS method was used to study the performance of the 841-SPP (case with some available experimental results; Olofsson, 1996) in several immersion ratios (= 33%, 50%, 75% and 100%) and maneuvering... 

Properly adjusting the trim angle during the craft speed up will be extremely important in special cases as in sports competitions or military missions. In such applications, the goal of trim adjustment is to reach final speed in a minimum possible time which is an advantage to just passing the resistance hump. Present study tries to provide insight into how the angles of the drive system and trim tab of a planing craft should be changed during speed up so as to minimise the time to reach the final speed. This is a time-optimal control problem with the drive and trim tab angles as the control variables. Optimal control theory has been used previously for the motion control of marine vessels... 

Metal plates are essential parts of structures such as ship hulls and offshore oil platforms. These plates are typically under compressive axial forces. Hence, one of the main mechanisms for failure and collapse of such structures is buckling of plates. Thus, for safe and secure design, buckling strength of plates should be evaluated. Finite element analysis techniques are perfect tools for this purpose because of the accuracy and flexibility for performing simulations with different variables. In this study, the strength of aluminum plates has been studied using finite element analysis software and it was tried to study the influence of variables such as initial imperfections, plate... 

Aerodynamically alleviated marine vehicle (AAMV) is a high speed craft equipped with aerodynamic surfaces that operating in ground effect zone provides this craft with the ability to achieve much higher cruising speeds. Reducing the take-off mode of an AAMV is highly desirable. Additionally, it is seen where there is a considerable reserve thrust take-off can occur in the lower get-away speeds that shorten the take-off run and, therefore, is favourable. Accordingly, in this study an attempt has been made to develop a nonlinear mathematical model for an AAMV to simulate accelerations in take-off and landing phases, using semi-empirical equations mainly proposed for mono-hull high-speed craft,... 

The unbounded linearly increasing responses in galloping instability make sharp-edge oscillators a beneficial solution for bladeless energy harvesting. While the performance of single square oscillators has been the subject of several studies in the past, there is still little knowledge about their hydrokinetic energy performance in tandem configurations. An optimal oscillator should display high performance in a group when all bodies are free to harness energy. In this study, the energy harvesting performance of square oscillators is examined in multiple tandem configurations under free and fixed square wakes. The harmonic oscillator model is coupled with the electromagnetic transducer to... 

The aim of this work is to investigate the non-equilibrium effects of phase change in cavitating flows. For this purpose, the concept of phase change thermodynamic probability is used along with homogeneous model to simulate two-phase cavitating flows. For simulation of unsteady behaviors of cavitation, which have practical applications, unsteady PISO algorithm based on the non-conservative approach is utilized. For multi-phase simulation, single-fluid Navier-Stokes equations, along with the volume fraction transport equation, are employed. In this paper, phase change thermodynamics probabilities and cavitation model is briefly summarized. Thus, derivation of the cavitation model, starting... 

The present study focuses on the numerical simulation of cavitation around the NACA 0015. The unsteady behaviors of cavitation which have worthwhile applications are investigated. The cavitation patterns, velocity fields and frequency of the cavitating flow around hydrofoil is obtained. For multi phase simulation, single-fluid Navier-Stokes equations, along with a volume fraction transport equation, are employed. The bubble dynamics model is utilized to simulate phase change. SIMPLE algorithm is used for velocity and pressure computations. For discretization of equations the finite-volume approach written in body fitted curvilinear coordinates, on collocated grid, is used. In this study,... 

General purpose software is developed to simulate 6-DoF fluid-structure interaction in two-phase viscous flow. It is a VoF-fractional step solver based on the finite-volume discretization which uses a boundary-fitted body-attached hexahedral grid as the motion simulation strategy. As an application, a high-speed planing catamaran is simulated in steady forward motion as well as in turning maneuver. Results are compared with the available data and good qualitative and quantitative agreements are achieved. Numerical schemes and the solution algorithm of the software are consistent and show a good capability to model highly nonlinear ship motions. It can be further developed to represent a more... 

A numerical simulation algorithm based on the finite volume discretisation is presented for analyzing ship motions. The algorithm employs a fractional step method to deal with the coupling between the pressure and velocity fields. The free surface capturing is fulfilled by using a volume of fluid method in which the interface between the liquid (water) and gas (air) phases are computed by solving a scalar transport equation for the volume fraction of the liquid phase. The computed velocity field is employed to evaluate the acting forces and moments on the vessel. Using the strategy of boundary-fitted body-attached mesh and calculating all six degrees-of-freedom of motion in each time step,... 

Purpose - This study aims to present compatible computational fluid dynamics procedure for calculation of incompressible three-dimensional time-dependent flow with complicated free surface deformation. A computer software is developed and validated using a variety of academic test cases. Design/methodology/approach - Two fluids are modeled as a single continuum with a fluid property jump at the interface by solving a scalar transport equation for volume fraction. In conjunction, the conservation equations for mass and momentum are solved using fractional step method. Here, a finite volume discretisation and colocated arrangement are used. Findings - The developed code results in accurate... 

Numerical simulation of floating or submerged body motions is presented based on a Volume of Fluid (VoF)-fractional step coupling. Solving a scalar transport equation for volume fraction of two phases results in a single continuum with a fluid property jump at the interface. In addition, velocity and pressure fields are coupled using the fractional step method. Based on integration of stresses over a body, acting forces and moments are calculated. Using the strategy of non-orthogonal body-attached mesh and calculation of motions in each time step result in time history of hydrodynamic motions. To verify the accuracy of the numerical procedure in simulation of two-phase flow, sloshing and dam... 

In two phase flow investigation, there is a need for robust methods capable of predicting interfaces, in addition to treating the traditional governing equations of fluid mechanics (Navier-Stokes Eqs.). Such methods in a finite volume approach can be classified into two typical categories called interface tracking and interface capturing methods. According to their abilities, interface capturing methods are of more interest in free surface modeling, especially when complex interface topologies such as wave breaking are included. These methods solve a scalar transport equation in order to find the distribution of two phases all over the computational domain. That is, all properties of the... 

The tip-loaded propeller design is one of the new methods for improving propellers' hydrodynamic and acoustic performance. This study investigates the effects of different endplate geometries on the performance of a 5-blade B-series propeller with normal skew and rake using the RANSE numerical method. For each of the different geometries, hydrodynamic coefficients, cavitation volume, and propeller noise are calculated. Using an endplate, this research aims to reduce cavitation, tip vortex, and propeller noise. As a result of the research, propellers with a contraction of 0.95R and a flap angle of 60°, as well as propellers with a contraction of 0.9R and a flap angle of 60°, showed better... 

Dynamic behaviour of a trimaran vessel is investigated in this study. The body of the trimaran is composed of a centre hull with a quite slender wave piercing bow profile (a length-to-width ratio of 12.96) and two outriggers with Wigley mathematical body forms. Several seakeeping tests are conducted on the model of the trimaran vessel in a towing tank in order to study its heave and pitch motions at different Froude numbers of 0.2, 0.37, and 0.51. Generated waves in the towing tank are of regular type with the wave length changing from 0.6 m to 2.4 m by an increment of 0.3 m. Amplitude of the waves is equal to either 25 mm or 35 mm. A resonance peak can be detected on the curve of the heave...