Some remarks on the role of boundary layers in decaying 2D turbulence in containers with no-slip walls.

Direct numerical simulations of decaying two-dimensional (2D) turbulence inside a square container with no-slip boundaries have been carried out for Reynolds numbers up to 2000. The role of the boundary layers during the decay process has been illustrated with ensemble-averaged results for the power law behaviour of several characteristic properties of the coherent vortices which emerge during the decay of 2D turbulence. The evolution of the vortex density, the average vortex radius, the enstrophy and the vorticity extreme have been computed. An algebraic decay regime has been observed during the initial turbulent decay stage. The computed decay exponents disagree, however, with the exponents from the classical scaling theory for 2D decaying turbulence on an unbounded domain. This is attributed to the presence of no-slip boundaries. Additionally, the temporal evolution of the average boundary-layer thickness has been studied by computing the ensemble-averaged viscous sl ess and normal vorticity gradient near the no-slip boundaries. These computations reveal that delta(t) similar or equal to t(0.4) and that the average boundary-layer thickness is proportional with Re-0.5.