The 3D structure of a dipole in a shallow layers of fluid

The canonical laboratory set-up to study two-dimensional turbulence is the electromagnetically driven shallow one- or two-fluid layer. Stereo-PIV measurements in such driven shallow flows revealedstrong deviations from quasi-two-dimensionality, which are attributed to the inhomogeneity of the magneticfield 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 sur-face deformation, both in one- and two-fluid layers. 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 fromshear at a no-slip boundary: they are a direct consequence of the vertical confinement of the flow.