Flow past a sphere - Topological transitions of the vorticity field

On the basis of unsteady three-dimensional structures in a separated flowfield and of the visualization techniques for unsteady phenomena, the flowfield of a sphere placed in a uniform flow is analyzed numerically. In the initial flowfield, an axisymmetric flow structure characterizes this field. A topological transition suddenly takes place, and a periodical structure appears in the wake region. These structures are visualized and confirmed by analyzing the vector fields (velocity and vorticity fields). In these unsteady flowfields, the initial flowfield is defined by an axisymmetric structure but the developed field does not have the axisymmetry. A plane-symmetry, however, appears and is preserved.