Numerical Study on CO_2 Ocean Sequestration in Middle Scale Ocean by Using Moving/Nesting Grid Methods

摘要

The direct injection of CO_2 into the deep ocean is one of the feasible ways for the mitigation of the global warming. In the assessment process of it, the estimation of environmental impacts is strongly required, such as the change of CO_2 concentration and its spreading area. It is necessary to make CO_2 disperse as fast as possible and it is thought that injection with a pipe by a moving ship is effective for this purpose. To numerically investigate the time change of CO_2 concentration by turbulence diffusion in both small and meso-scales in the ocean, a moving and nesting grid technique was developed. The sizes of the mesoscale and small domains are approximately 111 km × 111 km × 5 km and 6.9 km × 6.9 km × 0.85 km, respectively. The small domain moves at the velocity of 2 m/sec along the trajectory of the moving ship with the CO_2 injection rate of 150 kg/s. To numerically generate a proper fluctuating flow field, which is very important for the dispersion process of CO_2 concentration, a low-wavenumber forcing technique was adopted in the mesoscale domain. Here we reconstructed the directional spectra calculated from the time-sequential data obtained at seven measurement points by using the Maximum Likelihood Method. To generate high-wavenumber components in the small domain, velocity and temperature of mesoscale domain were extracted and forced to the small domain in every time step. The developed method was tested and validated by the numerical simulation of the ocean turbulent field. From the result of CO_2 dispersion simulation, it was found that the moving ship method reduces the biological impacts in a large extent, compared with fixed-point release.