Flow field in rearing tank is generated by aerators, however, few studies evaluating such flow field in detail had been conducted. In the present study, we attempted to clarify the relationship between the flow pattern and the topological character on a vertical cross section through the flow. The effect of aeration rate and aspect ratio (the ratio of water depth to tank radius) on overall flow pattern was investigated experimentally, by use of flow visualization techniques. The results showed that flow patterns were classified into 2 cases, namely 1 vortex pair and 2 vortex pairs systems, as the value of aspect ratio varies almost 1.0 to 2.0, respectively. On the other hand, the change of aeration rate weakly affected these change of vortex pair systems. In addition, corner vortex structures were observed in the both region between the free surface and the upper side wall, and between the bottom wall and the lower side wall. On the side wall, reattachment and separation points were found, which were closely related with these vortex structures.
