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CONTENTS
Volume 11, Number 2, June 2021
 


Abstract
Motivated by many recent discoveries of marginal fields in deep water, this paper presents a novel and economical design concept of a minimal floating platform with around 10,000 cubic tons in displacement. The concept characterizes a simple hull geometry and an excellent seakeeping behavior. It incorporates a damping plate at the keel on the basis of a spar-like floater. The design procedure is explained and illustrated. The paper also describes a new design methodology that is capable of efficiently evaluating the seakeeping performance of the platforms with the viscous damping effect included. We integrate this methodology into an Evolutionary Algorithm (EA) to conduct a multi-objective optimization for our novel design. The hull shape is optimized by minimizing the heave motion in waves without sacrificing the cost in construction and installation. Several potential geometric configurations are considered. The optimization results provide a wealth of information that can be used to support practical design decisions.

Key Words
mini-platforms; marginal fields; damping plate; multi-objective optimization

Address
Sha Miao, Yuming Liu and Dick K.P. Yue: Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Abstract
Numerical simulation of a full-scale ship model, KVLCC2, has been conducted applying the Reynolds Averaged Navier-Stokes (RANS) approach using the STARCCM+ commercial computational fluid dynamics (CFD) software to calculate total resistance, seakeeping and Pitch Moments. Results are obtained for the speed of 15.5 Knots under different sea conditions (calm water, regular waves and irregular waves), The total resistance calculated for the KVLCC2 ship hull in calm water is in a good agreement with the results from experiments and the results for motion (heave and pitch) and added resistance in waves were compared to numerical and experimental findings from previous research with good agreement. In addition to wave excitations, the full-scale ship model was subjected to propeller excitations using the virtual disk model from the CFD software. The body force propeller method, which simplified the full propeller characteristic of the KVLCC2 into a resultant body force, is applied to the virtual disk model. Results are compared with results from the hull-only model. A comparison of the wake results with previous work is also presented.

Key Words
CFD; free surface flow; pitch moment; heave; pitch; total resistance

Address
Hassiba Ouargli and Benameur Hamoudi: Aero-hydrodynamic Laboratory (LAHN), Department of Maritime Engineering,
University of Science and Technology of Oran MB, Oran 31000, Algeria


Abstract
This paper analyzes the fish-ridge type wind turbine performance and characteristics of energy extraction applied in a low wave Oscillating Water Column (OWC) system. This article contributes to providing a better understanding of the application of OWC and VAWT in a low wave environment. The aerodynamic characteristics of the three-blade fish-ridge turbine in an OWC chamber have been successfully investigated. CFD simulation with Reynolds-Averaged-Stokes (RANS) equations was used to obtain airspeed and air pressure contours under compressed and decompressed conditions in the turbine blades. Experiments on laboratory scale test rig also obtained data. The blade torque and turbine power coefficient at different AoA were validated through the experimental test to obtain numerical equations for the relationship between airspeeds, torque, tip speed ratio, and turbine power. The turbine design was 0.2 m long and 0.1 m wide and with an overlap ratio of 15%. The maximum tested airspeed was 20m/s. We found that the fish-ridge turbine has a homogeneous air velocity distribution and pressure due to the 15% overlap area. The maximum efficiency of the fish-ridge turbine under compressed conditions was 30% at TSR 0.9, while under decompressed conditions, the maximum efficiency reached 28% at TSR 0.6.

Key Words
wind energy; wind turbine; oscillating water column; CFD; dynamic wind effects; pressure distribution; RANS equations; Navier-Stokes equations

Address
Nurul Hiron: Department of Electrical Engineering, University of Siliwangi, Jl Siliwangi 24, Tasikmalaya, Indonesia;
Department of Electrical Engineering, University of Udayana, PB. Sudirman, Bali, Indonesia
Ida A.D. Giriantari, Lie Jasa and I. Nyoman S. Kumara: Department of Electrical Engineering, University of Udayana, PB. Sudirman, Bali, Indonesia


Abstract
The nonlinear wave-uniform current interaction for a slender floating body is investigated by using the commercial CFD (computational fluid dynamics) tool STAR-CCM+ and author-developed simplified BEM (boundary element method) based on potential theory and perturbation approach. The STAR-CCM+ solves the fully non-linear Reynold Averaged Navier Stoke's (RANS) equation for real fluid in the finite volume framework. The viscous effect is accounted for by mesh refinement and the k-w turbulence closure model. Meanwhile, the fully non-linear body motion and free surface elevation are considered by the overset mesh and volume of fluid method, respectively. Two different input waves with different order of non-linearity are compared to see their effects on ship's motion and added resistance. A detailed step-by-step simulation setup is explained to ensure the reproducibility of the results. Several preliminary simulations such as static tank test, wave calibration, and towing tank case are also conducted for quality assurance. The CFD results show good agreements with both the BEM with Uniform Flow approximation (UF-BEM) and the experimental data by other researchers when the /L is large. The CFD simulation also shows that it can properly capture the second-order force (added resistance) and highly non-linear motion with breaking waves close to the pitch resonance frequency. However, the CFD simulation requires substantially higher computational cost than the UF-BEM. The comparison study shows that the UF-BEM can produce reasonably good results for practical applications with significantly less computational time and human effort. On the other hand, the CFD program can be used for proof computations for special cases.

Key Words
CFD; BEM; Navier-Stokes equation; fluid-structure interaction; fully non-linear; wave-current interaction; forward speed; added resistance

Address
Farid P. Bakti and MooHyun Kim: Department of Ocean Engineering, Texas A&M University, College Station, Texas, USA

Abstract
The aim of this paper is to verify the velocity profile and the pressure variation inside the fluid domain over one wavelength obtained from a numerically simulated Smoothed Particle Hydrodynamics model with some exact qualitative results (i.e., increasing/decreasing trend or constant value of a flow field) from a fully nonlinear Euler equation for water wave model. A numerical wave flume has been modeled and a regular wave train is created by the horizontal displacement of a wave paddle on one side of the flume. A passive beach is used to dissipate the energy of the wave on the other side. The extracted numerical results are compared with some recently available exact results from a nonlinear steady water wave model based on the Euler equations for irrotational flow. The flow properties under wave crests, wave troughs, and along the distance from the wave crest to the wave trough over one wavelength are investigated. The horizontal and vertical velocity components and the pressure in the fluid domain agree well with the analytical results.

Key Words
nonlinear water wave; smoothed particle hydrodynamics; velocity profile; pressure variation

Address
Hoa X. Nguyen: School of Engineering, Trinity College Dublin, Ireland;
Faculty of Civil Engineering, Vietnam Maritime University, Hai Phong, Vietnam
Van Nguyen Dinh: MaREI Centre for Energy, Climate and Marine, University College Cork, Ireland
Biswajit Basu: School of Engineering, Trinity College Dublin, Ireland


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