Scaling of space-time modes with Reynolds number in two-dimensional turbulence

It has been estimated that the number of spatial modes (or nodal values) required to uniquely determine a two-dimensional turbulent flow at a specific time is finite, and is bounded by Re-4/3 for forced turbulence and Re for decaying turbulence. The usual computational estimate of the number of space-time modes required to calculated decaying two-dimensional turbulence is N similar to Re-3/2. These bounds neglect intermittency, and it is not known how sharp they are. In this paper we use an adaptive multi-scale wavelet collocation method to estimate for the first time the number of space-time computational modes N necessary to represent two-dimensional decaying turbulence as a function of Reynolds number. We find that N similar to Re-0.9 for 1260 <= Re <= 40400 over many eddy turn-over times, and that temporal intermittency is stronger than spatial intermittency. The spatial modes alone scale like Re-0.7. The beta-model then implies that the spatial fractal dimension of the active regions is 1.2, and the temporal fractal dimension is 0.3. These results suggest that the usual estimates are not sharp for adaptive numerical simulations. The relatively high compression confirms the importance of intermittency and encourages the search for reduced mathematical models of two-dimensional turbulence (e.g. in terms of coherent vortices).