Shinichiro Hirabayashi

Multi-scale modeling of CO2 dispersion leaked from seafloor off the Japanese coast.

A numerical simulation was conducted to predict the change of pCO(2) in the ocean caused by CO(2) leaked from an underground aquifer, in which CO(2) is purposefully stored. The target space of the present model was the ocean above the seafloor. The behavior of CO(2) bubbles, their dissolution, and the advection-diffusion of dissolved CO(2) were numerically simulated. Here, two cases for the leakage rate were studied: an extreme case, 94,600 t/y, which assumed that a large fault accidentally connects the CO(2) reservoir and the seafloor; and a reasonable case, 3800 t/y, based on the seepage rate of an existing EOR site. In the extreme case, the calculated increase in DeltapCO(2) experienced by floating organisms was less than 300 ppm, while that for immobile organisms directly over the fault surface periodically exceeded 1000 ppm, if momentarily. In the reasonable case, the calculated DeltapCO(2) and pH were within the range of natural fluctuation.

A Pore-Scale Numerical Simulation Method for Estimating the Permeability of Sand Sediment

A numerical method system to estimate the permeability of sand sediments, at a microscopic scale, was developed. Initially, 3D geometrical representations of the sand grains are reconstructed from a series of 2D X-ray CT scans of real sand grains. 2D cross-sectional slices of the grain outlines are combined together to produce 3D objects via spherical harmonics series expansions. Then, the reconstructed sand grains are packed randomly inside a cubic, microscopic, domain by a combination of a growth method and a simulated annealing method to achieve a predefined porosity. Finally, a single-phase water flow within the domain was simulated numerically, using the lattice Boltzmann method. The calculated permeability of these systems compares well with the values provided by conventional theoretical models. One of the contributions of this study is to show that it is possible to predict the permeability of sand sediments of variable porosities, using sand grains from CT images with changing size distributions and orientations.

Numerical simulation of dispersion of volcanic CO 2 seeped from seafloor by using multi-scale ocean model

Numerical simulations were conducted to predict the change of CO2 concentration in the ocean caused by volcanic CO2 seeped from the seafloor in Wakamiko Caldera in Kagoshima Bay. The behavior of CO2 bubbles, their dissolution, and the advection-diffusion of dissolved CO2 in the water column were numerically simulated. A multi-scale ocean model, in which hydrostatic meso-scale and non-hydrostatic small-scale models are combined, was used for this purpose. The calculated distribution of CO2 concentration was in good agreement with observed data and the validity of the present multi-scale ocean model was confirmed.

Development of an Analysis Code of Rotor-Floater Coupled Response of a Floating Offshore Wind Turbine

Coupled rotor-floater response analysis is essentially important for the design of Rotor Nacelle Assembly (RNA) and floating support structure of Floating Offshore Wind Turbine (FOWT). The authors have developed an analysis code UTWind for analysis of the coupled structural response. Blades and floater are modeled as frame structure with beam elements. Lumped mass model is use for mooring. Aerodynamic load on blade is calculated by Blade Element Momentum Theory (BEM), and hydrodynamic load is calculated by Hooft’s method, and Morison equation was modified to be applicable to cylindrical element with cross section with two axes of lines symmetry. The equations of motion of rotor, floater and mooring are solved in time domain by weak coupling algorithm. The numerical results by the code were compared with responses measured by experiment in wave and wind-and-wave coexistence field with/without blade pitch control and showed good agreement. Response by negative damping was reproduced by the code and showed good agreement with experiments.Copyright

Estimation of Risk of Progressive Drifts in a Wind Farm Caused by Collision of Drift Ship

In off-shore wind turbine, it is difficult to determine the risk of accident caused by the mooring destruction through experiment. In this paper, the authors discuss the risk, with the case of a drifting ship wanders into the wind farm. In the design of a floating offshore wind turbine (FOWT), drift of a FOWT is considered as a serious failure mode and the mooring system must be designed to avoid the failure. The failure of mooring line is not initiated just by extreme environmental load but can be initiated by collision with a drifting ship, which enters the wind farm. This phenomenon is difficult to investigate by a tank experiment. So far, little knowledge exists about the phenomenon. In this research, a simulator to reproduce the collision process of a FOWT and a drift ship and a progressive drift of FOWTs in a wind farm was developed. Using this simulator and statistics of drift incidents of a ship, a procedure to evaluate risk of progressive drifts in a wind farm was established. In that case, second accident that a wind turbine which has started drifting caused by the drifting ship collides with one another wind turbine is expected. As a result, the risk mainly depends on the risk of drifting caused by a large displaced ship. In addition, the risk partly depends on the arrangement of wind farm.Copyright

Numerical Simulation of Vortex-Induced Motion With Free Surface by Lattice Boltzmann Method

In this study, numerical simulation of the fluid flow by using lattice Boltzmann method is carried out and the vortex-induced motion (VIM) of a cylindrical floating structure is calculated. The way of calculate the fluid flow, fluid force and floating body’s movement is introduced.The fluid flow with free surface is also calculated. The height change of water surface exerts the effect to the evaluation of hydrostatic pressure and wave resistance. In this study, the method to express the movement of free surface is introduced.Copyright

Numerical Study on Vortex Induced Motion of Floating Body by Lattice Boltzmann Method

Flow around a solid cylinder is numerically simulated with allowance of the movement of the cylinder by the lattice Boltzmann method. The drag force of the fixed cylinder is in good comparison with the literature showing that the fluid force acting on the cylinder is accurately estimated in the computation. It is confirmed that as the body oscillates, the drag and lift forces increases and decreases, respectively. By changing the mooring tension, the locked-in phenomenon and the nonlinear interaction between the vortex-shedding and the mooring system are found.Copyright

Numerical simulation of tidal currents around Korea/Japan strait and application to speed trial

The speed performance of a vessel is a very important factor for shipbuilding companies and its owners, because the operating costs and time will increase, if the vessel could not qualify the contracted speed. Therefore, in the last phase of construction of a vessel, a shipbuilding company will perform a sea trial to measure the vessels speed performance. Korean shipbuilding companies sometimes have carried out the sea trials around the western channel of Korea/Japan strait, where the flow fields are very complicated due to the effect of various flows. Because the interference by these flows to a ship seems to be significantly high, the numerical reproduction of the flow-fields in the vicinity of the target sites can help better understanding the sea environments and performing the sea trials. In this study, we used the MEC ocean model to simulate the tidal currents around Tsushima Island and compared the simulated tidal amplitudes and currents with those of measurements by W.J. Teague. The tidal amplitudes of present simulation results agree well with the observations. Based on the numerical simulation, the optimal direction and proper sites for a speed trial were described.

Modeling of turbulent Prandtl number in stationary and homogeneous stratified turbulence

The dependency of turbulent Prandtl number on the stratification stability was numerically investigated by means of direct numerical simulation of homogeneous and stratified turbulence in which the gradient Richardson number ranged from 0.05 to 0.15. It was found that the mixing efficiency was almost linearly correlated with the gradient Richardson number. By using this relation, turbulent Prandtl number was expressed by a simple model of linear function of the Richardson number. The proposed model of turbulent Prandtl number was implemented to large eddy simulation to find a better representation of the subgrid-scale diffusion coefficient for heat than the conventional constant model.

Estimation of Small-Scale Vertical Diffusivity for CO2 Injected in the Deep Ocean

In this study, vertical diffusivity, the scale of which was O (10 m), at a particular site in the deep ocean was estimated by using numerical simulations with forcing low-wavenumber components, which had been reproduced from measurement data. Spatial information of velocity field was reproduced by spectral analysis of 4 sets of time-series measured simultaneously at different places in the real ocean. In order to estimate finer-scale structures, which are necessary to obtain statistical quantities such as energy dissipation rate, large eddy simulations were carried out with forcing low-wavenumber components of velocity reproduced in the spectral analysis. The low-wavenumber components generated by the nonlinear interaction of forced components and resolved components were successfully removed from the computational domain by introducing a partial spectral filter in place of the conventional FFT filter. Vertical diffusivity was estimated by using the energy dissipation rate of the reproduced flow field, which was 3.3×10−5 m2 s−1 on the time average.Copyright