Two-dimensional flow developing between an inner solid rectangle and a hollow outer rectangle is investigated by the numerical approach. The rectangles are squares and they are placed coaxially and in the same phase. One lid side of the outer square or one lid side of the inner square is the boundary where the lid moves in the tangential direction. The unique geometrical parameter is the aspect ratio defined by the fraction of the side length of the outer square to that of the inner square. When the one lid of the outer square moves, the flow includes vortices at stationary boundary corners and the upstream stationary wall similarly to the single square cavity flow without the inner square. The inner square deactivates the development of the global vortex and small vortices appear on the side of the inner square. When the flow is driven by the inner side lid, four flow patterns appear: the steady tangential flow pattern, the steady deflected flow pattern, the one way flow pattern and the oscillating flow pattern. The tangential flow pattern appears at relatively smaller Reynolds numbers or the smaller aspect rations. The oscillating flow pattern emerges at higher Reynolds numbers. At the aspect ratio above 3.0, the flow tends to have one way flow pattern around the inner square regardless of the Reynolds numbers. In addition to the traditional single cavity flow, the dual cavity flow investigated in this study is expected to give a new benchmark system including steady and unsteady flows.
