Abstract
The canonical laboratory set-up to study two-dimensional turbulence is the electromagnetically driven shallow one- or two-layer fluid. Stereo-Particle-Image-Velocimetry measurements in such driven shallow flows revealed strong deviations from quasi–two-dimensionality, which are attributed to the inhomogeneity of the magnetic field and, in contrast to what has been believed so far, the impermeability condition at the bottom and top boundaries. These conjectures have been confirmed by numerical simulations of shallow flows without surface deformation, both in one- and two-layer fluids. The flow simulations reveal that the observed three-dimensional structures are in fact intrinsic to flows in shallow fluids because they do not result primarily from shear at a no-slip boundary: they are a direct consequence of the vertical confinement of the flow.