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CONTENTS
Volume 3, Number 3, September 2013
 

Abstract
Hydrodynamic characteristics of a bluff cylinder oscillating along transverse direction in steady flow were experimentally investigated at Reynolds number of 2X 105.The effects of non-dimensional frequency, oscillating amplitude and Reynolds number on drag force, lift force and phase angle are studied. Vortex shedding mechanics is applied to explain the experimental results. The results show that explicit similarities exist for hydrodynamic characteristics of an oscillating cylinder in high and low Reynolds number within subcritical regime. Consequently, it is reasonable to utilize the test data at low Reynolds number to predict vortex induced vibration of risers in real sea state when the Reynolds numbers are in the same regime.

Key Words
vortex induced vibration, forced oscillation experiment, subcritical regime, high Reynolds number

Address
Yuwang Xu, Shixiao Fu, Ying Chen, Qian Zhong and Dixia Fan : State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University,800 Dongchuan Road, Min Hang, Shanghai, China

Abstract
The main objective of this work is to investigate the sloshing behavior in a baffled and unbaffled three dimensional annular-sectored water pool (i.e., tank) which is located at dome region of the primary containment. Initially two case studies were performed for validation. In these case studies, the theoretical and experimental results were compared with numerical results and good agreement was found. After the validation of present numerical procedure, an annular-sectored water pool has been taken for numerical investigation. One sector is taken for analysis from the eight sectored water pool. The free surface is captured by Volume of Fluid (VOF) technique and the fluid portion is solved by finite volume method while the structure portions are solved by finite element approach. Baffled and un-baffled cases were compared to show the reduction in wave height under excitation. The complex mechanical interaction between the fluid and pool wall deformation is simulated using a partitioned strong fluid–structure coupling.

Key Words
liquid sloshing; free surface; numerical simulation; annular-sectored water pool; fluid-structure interaction

Address
M. Eswaran, P. Goyal, G.R. Reddy, R.K. Singh and K.K. Vaze : Bhabha Atomic Research Centre, Mumbai, 400085, India

Abstract
A PTO (power-take-off) mechanism by using relative heave motions between a floating buoy and its inner mass (magnet or amateur) is suggested. The inner power take-off system is characterized by a mass with linear stiffness and damping. A vertical truncated cylinder is selected as a buoy and a special station-keeping system is proposed to minimize pitch motions while not affecting heave motions. By numerical examples, it is seen that the maximum power can actually be obtained at the optimal spring and damper condition, as predicted by the developed WEC(wave energy converter) theory. Then, based on the developed theory, several design strategies are proposed to further enhance the maximum PTO, which includes the intentional mismatching among heave natural frequency of the buoy, natural frequency of the inner dynamic system, and peak frequency of input wave spectrum. By using the intentional mismatching strategy, the generated power is actually increased and the required damping value is significantly reduced, which is a big advantage in designing the proposed WEC with practical inner LEG (linear electric generator) system.

Key Words
WEC(wave energy converter); relative heave motion; inner dynamic system; LEG(linear electric generator); double resonance; maximum PTO(power take off); intentional mismatching; high performance band-width

Address
I.H. Cho : 1Department of Ocean System Engineering, Jeju National University, Jeju 690-756, Korea
M.H. Kim: Department of Civil Engineering, Texas A&M University, College Station, Texas, 77843, USA

Abstract
The accurate prediction of motion in waves of a marine vehicle is essential to assess the maximum sea state vs. operational requirements. This is particularly true for small crafts, such as Autonomous Surface Vessels (ASV). Two different numerical methods to predict motions of a SWATH-ASV are considered: an inviscid strip theory initially developed at MIT for catamarans and then adapted for SWATHs and new a hybrid strip theory, based on the numerical solution of the radiation forces by an unsteady viscous, non-linear free surface flow solver. Motion predictions obtained by the viscous flow method are critically discussed against those obtained by potential flow strip theory. Effects of viscosity are analyzed by comparison of sectional added mass and damping calculated at different frequencies and for different sections, RAOs and motions response in irregular waves at zero speed. Some relevant conclusions can be drawn from this study: influence of viscosity is definitely non negligible for SWATH vessels like the one presented: amplitude of the pitch and heave motions predicted at the resonance frequency differ of 20% respectively and 50%; in this respect, the hybrid method with fully non-linear, viscous free surface calculation of the radiation forces turns out to be a very valuable tool to improve the accuracy of traditional strip theories, without the burden of long computational times requested by fully viscous time domain three dimensional simulations.

Key Words
seakeeping; SWATH; viscous effects; viscous added mass and damping; Navier-Stokes equations solver for oscillating floating bodies; Autonomous Surface Vessels

Address
Stefano Brizzolara, Luca Bonfiglio and João Seixas de Medeiros : Massachusetts Institute of Technology, MIT Sea Grant and Innovative Ship Design Lab,77 Massachusetts Ave, 02139 Cambridge, MA, USA

Abstract
In many applications, Remotely Operated Vehicles (ROVs) are required to be capable of course keeping, depth keeping, and height keeping. The ROV must be able to resist time-variant external forces and moments or frequent manipulate changes in some specified circumstances, which require the control system meets high precision, fast response, and good robustness. This study introduces a Fuzzy-Incomplete Derivative Ahead-PID (FIDA-PID) control system for a 500-meter ROV with four degrees of freedom (DOFs) to achieve course, depth, and height keeping. In the FIDA-PID control system, a Fuzzy Gain Scheduling Controller (FGSC) is designed on the basis of the incomplete derivative ahead PID control system to make the controller suitable for various situations. The parameters in the fuzzy scheme are optimized via many cycles of trial-and-error in a 10-meter-deep water tank. Significant improvements have been observed through simulation and experimental results within 4-DOFs.

Key Words
ROV; fuzzy control; incomplete derivative ahead; FGSC

Address
Junliang Cao, Chunhu Liu and Lian Lian : State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University (SJTU) Shanghai 200240, China
Hanjun Yin: Offshore Oil Engineering Co,LTD, Tianjin, China


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