Spontaneous spin-up during the decay of 2D turbulence in a square container with rigid boundaries

Direct numerical simulations of the decay of 2D turbulent Navier-Stokes flows inside a square container with no-slip boundaries are presented. Several numerical runs have been carried out for three different values of the Reynolds number. A surprising observation in a majority of these runs is the spontaneous spin-up of the flow during the initial stage of the self-organization of the flow, which is due to normal and shear stresses exerted on the fluid by the rigid boundaries.