Large-scale spiral structures in turbulent thermal convection between two vertical plates

By means of a three-dimensional numerical simulation of the Boussinesq equations, it is observed that the turbulent flow induced by thermal convection between two differentially heated vertical plates can generate strongly spiral structures on large scales. In this paper, the flow patterns and the temporal evolution of such large-scale spiral structures are manifested. It is shown that the length scale of the spiral structures is comparable to the width of the convection layer and their lifetime is about two or three orders of magnitude longer than that of small-scale structures of the turbulence. To understand the physical mechanism of the emergence of such structures, a topological interpretation of generating the structures and the role of helicity in preserving the structures are investigated and discussed.