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Journal of Ocean and Ship Technology
Volume 2 Number 2

ISSN: 0975-301X
Publication Year: December 2011

On Prediction of Drag Characteristics over Multiple Bodies in Motion at Zero Incidence
- P. Jagadeesh

Abstract :This paper presents a comparative assessment of total drag, drag variation with Reynolds number and surface pressure coefficient distribution over four selected configurations of multiple axisymmetric bodies in motion at zero angle of incidence using Computational Fluid Dynamics (CFD) approach. For current study chosen motion configurations were, parallel, one leader-one follower under inline, one leader-one follower in staggered, and one leader-two followers in triangular formation. Two dimensional studies were carried out for a range of Reynolds number, 0.14 ? 106 to 1.397 x106 using k-? RNG turbulence model. Current study revealed that, maximum reduction of drag force on follower vehicle due to vehicular motion ranked first for one leader-one follower under inline and, together ranked second for one leader-one follower in staggered motion and one leader-two followers in triangular configuration. However, parallel motion showed that no influence by a vehicle motion on adjacent vehicles’ drag force. Over all study suggested that, one leader – one follower in staggered motion was an ideal choice among the selected configurations in collection of large spatial data and to correlate the measured values, and for an over all economy of the mission.

Hydrodynamic Performance of Semi-Submerged Propellers for Racing Boats
- Hassan Ghassemi and Mahmoud Ghiasi

Abstract :Among the various marine propulsors, semi-submerged propellers (hereafter SP) are exceptionally efficient for the racing boats. This paper presents the potential based boundary element method (BEM) for calculations of the hydrodynamic characteristics of the SP. An empirical formula for critical advance velocity ratio is developed using the Weber number, pitch ratio and number of blades in transition mode. Advance velocity ratio is an efficient parameter to distinguish the ventilated region. Two propellers SP-101 and SP-102 are selected for calculations of pressure distribution, thrust, torque and hydrodynamic characteristics during one cycle. The predicted numerical results show that the developed method is sufficiently accurate for computation of hydrodynamic characteristics of semi-submerged propellers.

A Numerical and Experimental Investigation into the Free Stream Hydrodynamics Characteristics of a Series of Flapped Rudders using Shipflow
- A. Bhattacharjee and O.P. Sha

Abstract :Manoeuvring of marine vehicles is defined as the controlled keeping and changing of course and speeds of the vessel, which is conventionally achieved by use of control surfaces, like rudders. In recent years, a number of high lift devices, like flap rudders have been developed primarily to increase lift at low speeds so as to achieve better low speed manoeuvring in ports and congested water ways. Flap rudders effectively work as a variable camber device and help in considerable increase of the lift force and also to delay stall. Such control surfaces in conjunction with both thrusters are able to achieve tighter turning circle diameter and better stopping ability at low speed. Kerwin et al., (1972) carried out water tunnel experiments to determine free stream characteristics of a series of twelve rudders with systematic variation in the amount of flap area of the fixed part and movable part for a rudder with skeg. Initially, to calculate the effectiveness and robustness of the computational model, Kerwin’s (1972) rudder is modelled and the lift, drag obtained from the computational analysis is compared with the water tunnel experiments of Kerwin (1972). The results are in good agreement and presented in the paper. Subsequently a series of flap rudder with varying percentage of flap areas (20%, 30%, 40%) and flap to rudder deflection are modelled and lift, drag and stall angle for varying angle of attack are numerically computed using RANS solver, SHIPFLOWR. The results of this systematic study are presented. Finally for a few of the above variation of flap rudders physical models are constructed and tested into wind tunnel experimental facility of Naval Science and Technology Laboratory, (NSTL) Visakhapatnam. The experimental results are presented along with the computational results for these cases. It is expected that the current investigation will assist the naval architect, at the early design stage, to select the flap rudder geometry that will enhance the manoeuvring characteristic of the vessel and overcome the stringent present day IMO

Effect of Welding Tacks and Sequences on Residual Stress in Stiffened Panel Fabrication
Pankaj Biswas, Sujoy Tikader, Rajesh Ranjan and N. R. Mandal
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Abstract :Stiffened panels are the basic building components of ship structures. The most commonly used fabrication process being fusion welding, it results in weld induced structural distortions and formation of residual stresses. These deformations and residual stress distribution are known to depend very much on sequencing of welding and scheme of tack welding. Though it is a well known fact in shipyards, however not much of systematic analysis have been carried out to develop a suitable computational method to predict and thereby control and mitigate these aspects. From the structural integrity point of view it is very much important to assess quantitatively the extent and distribution of these residual stresses.In the present work a numerical elasto-plastic thermomechanical model was developed for predicting the effect of welding sequences and welding tacks on formation of residual stresses in fabrication of T section. It was observed that with proper sequencing of welding, it resulted in decrease in residual stresses. It was also observed that tack welding leads to increase in residual stress levels. The results obtained showed a good agreement with available published results. The results are encouraging, however for further strengthening and validating the computational methodology, suitable laboratory experiments are to be carried out.Finite element analysis, moving distributed heat source, non-linear elasto-plastic analysis, residual stresses, welding sequences, welding tacks.

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