Previously, Shiotani et al. (2005) investigated the flow field in a rearing tank as a single-phase flow and carried out two dimensional calculations by finite difference method. However, the result did not always agree with flow visualization results. The most likely causes were the assumption of single-phase flow and the boundary condition, in which tank center-line velocity was set equal to that on the free surface. Therefore, we focused on improving the precision of the calculation. In this study, we considered the flow field as a two-phase flow and re-examined boundary conditions to more closely match the actual flow. The calculation was also performed using finer grid resolution than the former study. We found that the vertical vortex pattern in a tank changed from a single-pair vortex system to two-pair vortex system as aspect ratio AR (the ratio of water depth to tank radius) varied between 1.0 and 2.0, consistent with previously reported results. Moreover, clear corner vortices were found in both the region between the free surface and upper side wall, and between the bottom and lower side wall. With increasing AR, the vorticity increased at the upper region, and it was larger than that of the lower region. Furthermore, at constant aeration rate in the same tank, the maximum vertical velocity on the center-line at the tank increased with increasing AR. Our results indicate that both H and AR of the rearing tank should be considered for optimizing flow field in larviculture.
