Aspect ratio effects in quasi-two-dimensional turbulence

The influence of the aspect ratio on the three-dimensionalization and subsequent saturation of freely decaying two-dimensional turbulence is studied numerically. We show that the aspect ratio strongly affects the saturation level of the perturbation and its feedback on the base flow. The saturation level scales linearly with the aspect ratio, as does the total damping exerted by the perturbation. In spectral space the feedback yields a positive eddy viscosity at large scales and a small eddy viscosity at small scales. A surprising result is the persistence of coherent vortices even after the saturation of the perturbation.