The presence of the free surface adds an element of difficulty to the development of numerical and theoretical methods for the performance prediction of surfacepiercing hydrofoils. Existing methods of analysis for two-dimensional surface-piercing hydrofoils or blade sections of a surface-piercing propeller solve either a linear problem, assuming a thin section and ventilated surface along with linear free-surface boundary conditions, or a nonlinear problem in a self-similar setting. Both these approaches cannot be used when the effects of gravity are important, which is the case when a craft is operating at low speeds. A two-dimensional boundary-elementmethod-based numerical scheme is presented here that overcomes these drawbacks by solving the fully ventilated flow past a surface-piercing hydrofoil of finite dimensions and includes the whole gamut of nonlinear free-surface interactions. The unique aspect of the numerical scheme is that fully nonlinear boundary conditions are applied on the free surface which allows for the accurate modelling of the jet generated on the wetted boundary and the ventilated surface formed on the suction side as a result of the passage of the hydrofoil through the free surface. Moreover, the effects of gravity can be considered to take into account the influence of the Froude number. Ventilated-surface shapes predicted by the present scheme are compared with existing experimental results and are shown to be in good agreement.
The Paired-Column Semisubmersible (PC Semi) concept was selected as a dry tree alternative for ultra-deep water development by RPSEA in 2009. Since then, RPSEA continuously supported maturation of PC Semi. The PC Semi platform has relatively deeper draft and smaller columns compared to those of a generic Deep Draft Semisubmersible (DD Semi) platform. Due to its larger column slenderness ratio, it might be suspected to have a pronounced VIM response. VIM response characteristics of a floater are the key measures of its performance. It is an essential step to explore VIM response characteristics during concept maturation. An extensive VIM model test program funded by RPSEA was performed at MARIN in June, 2013. Key results are summarized and presented in this paper.Since the VIM response of a floater (Spar or Semi) is a controlling factor for mooring fatigue, VIM induced mooring fatigue analysis was followed by employing the measured test results. A typical loop current scatter diagram for the GoM was considered and the resultant fatigue damage was analyzed and highlighted. A parallel study for VIM induced mooring fatigue of a generic dry tree DD Semi was also carried out to establish a baseline for comparison of the PC Semi response features and mooring fatigue.
Vortex-Induced-Motion (VIM) is an important issue in offshore engineering as it impacts the integrity of the mooring system for floating structures such as oil platforms and wind turbine platforms. Understanding and predicting VIM is a challenging task because of the inherent complexity of vortex structure shedding and fluid-structure interaction (FSI) in high Reynolds number flows. Computational Fluid Dynamics (CFD) is one of the key tools in VIM studies and optimization of the offshore systems design. We report a CFD sensitivity study with focus on turbulence model, mesh refinement, and time-step selection. Experimental measurements in a tow-tank facility are used to validate the CFD results. Three types of tank tests are modeled numerically: current drag, oscillating free decay, and VIM. The effect of turbulence model is evaluated by comparing Delayed Detached Eddy Simulation (DDES) and Unsteady Reynolds-Averaged Navier-Stokes (URANS) models. The influence of mesh refinement and time step is investigated by using the grid convergence index (GCI). For present geometry and flow conditions (Re∼105), the DDES turbulence model demonstrates better agreement with experimental measurement in model scale VIM compared to the URANS model. In addition, DDES simulation captures the vortex structure more realistically, as evidenced by Q-criteria and turbulent eddy viscosity distribution. Finally, we show how the mesh refinement and time step selection affect simulation accuracy. Two viscous-flow commercial solvers are tested: the finite-volume solver ANSYS-Fluent™, and the finite-element solver Altair AcuSolve™. The results of this CFD Sensitivity study provide useful guidelines for CFD simulation of FSI and VIM problems for offshore engineering applications.
Malaysia plans to increase the total renewable energy mix to 30% by the year 2030 as part of the Green Technology Master Plan. Currently, the role of wind power is not included in the renewable energy mix of Malaysia and diversification of the renewable energy mix needs to be encouraged to include policy support for other sources of energy. The wind speed Malaysia is ranged from 3-7 m/s and most wind turbine requires 5 m/s as cut in speed. The low average wind speeds causes wind turbine to be the least cost-effective method to generate electricity at Malaysia especially at the west coast. Therefore, low-energy wind harvesting device is proposed as alternative at this climate. This device is also intended to generate power from vehicle-induced wind. Three potential configurations (electromagnetic, piezoelectric or electrostatic) for small-scale energy harvesting device were proposed by previous researchers and studied in this work. Electromagnetic configuration of energy harvesting by fluttering was selected after analysis of three configurations. A wind belt was designed to withstand environmental conditions such as fresh water (rain), UV radiation and acid/alkali conditions. Several important parameters such as belt width, location of the magnet etc for the design were evaluated experimentally. Taffeta silk was selected as the belt materials from potential materials. The optimum length and width of the belt in this study are 1 m and 12 mm. Neodymium N45 magnet was selected based on inductance and the optimum magnet position along the belt is 20cm from the edges of the main frame. Experimental results showed the peak power recorded in parallel connection are 81.02 mW @ 6 m/s with a belt tension of 0.816 N and 24.54 mW @ 4 m/s with belt tension of 0.612 N.
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