On the origin of spin-up processes in decaying two-dimensional turbulence

A remarkable feature of two-dimensional turbulence in a square container with no-slip walls is the spontaneous production of angular momentum due to flow-wall interactions, also known as spontaneous spin-up of the flow. In this paper we address the statistics of spin-up and discuss its likely origin. A signature of the spontaneous production of angular momentum is the development of a large-scale circulation cell. It is found that the global turnover time of the flow guides the spin-up process, which can be considered as a relaxation process of the macroscopic flow to an angular momentum containing state. The high turnover rate of the small-scale vortical structures emerging from the no-slip walls apparently has no significant effect on the spin-up rate. The presented data on the spin-up process strongly suggest that spin-up is not the net result of isolated vortex-wall interactions, with its associated pressure fluctuations on the domain boundaries, alone. The rapid spin-up of the flow clearly suggests the attraction to an angular momentum containing state.