Two-dimensional turbulence on a bounded domain

Several features of decaying and forced two-dimensional turbulent flows confined between no-slip walls are addressed, with emphasis put on the crucial role played by the solid walls. Such walls are essential in that they act as sources of vorticity filaments and in that they provide shear and normal stresses that exert torques on the fluid, hence possibly changing its net angular momentum. In the case of decaying 2D turbulence on a square domain this may result in an increase of the fluid's absolute angular momentum. Numerical simulations of forced 2D flow have revealed that sign reversal of the total angular momentum may occur, owing to breakdown of the organized central cell as a result of erosion by wall-induced vorticity filaments and the subsequent re-establishment of a cell (of either sign).