Yuji Hattori (Institute of Fluid Science, Tohoku University, Japan). Evolution of mean flow and disturbances in strained vortices.

Formation of concentrated vortices like tornadoes and tropical cyclones in rotating fluids is of much interest in atmospheric flows. It is shown by direct numerical simulation that selective decay of inviscid invariants leads to concentration of vorticity in a destabilized vortex. Here by selective decay we mean that circulation of the mean flow decays much faster than the angular momentum or energy. Initially localized disturbances are super-imposed onto the two-dimensional flattened Taylor-Green vortices to trigger the elliptic instability. In the later stage of nonlinear evolution of the disturbance circulation decays much faster than angular momentum and energy, giving rise to a sharp peak in the vorticity distribution of the mean flow. The selective decay is due to reconnection and the subsequent annihilation of vortex pairs at the cell boundaries as shown by a simple model. Relevance of the mechanism to previous experiments and general cases is also discussed.