No-slip walls as vorticity sources in two-dimensional bounded turbulence

This paper addresses the role of no-slip boundaries as vorticity sources in two-dimensional turbulence on bounded domains. In recent years, numerical evidence from decaying two-dimensional turbulence simulations in circular and square domains has been provided showing that vorticity produced at solid boundaries enters the flow at scales comparable with the average boundary-layer thickness. The thin detached boundary layers contain high-amplitude vorticity, and these boundary layers roll up to form small strong vortices which are advected by the background flow into the interior of the flow domain. As a result, the vortex statistics of the flow are modified: while the merging process active in the interior leads to a decrease of the number of vortices, the solid walls act as sources of vortices. We will briefly review numerically obtained results for decaying two-dimensional turbulence, and provide supporting experimental evidence from forced quasi-two-dimensional turbulence in rotating containers. Additionally, numerical simulations of forced two-dimensional turbulence are reported illustrating the dynamical impact of the presence of solid boundaries: a large-scale vortex is gradually built up and later on suddenly destroyed by the destabilizing effect of strong small-scale vortices produced near the solid walls. After the collapse, the self-organization process may start anew, and most strikingly, the circulation of the central vortex may even show sign reversal.