Reproducing large scale (and not only) features in rotating turbulent flows: a laboratory study

Planetary circulations are characterized by the co-existence of turbulence and waves on all scales. In a rotating environment, the latitudinal variation of the Coriolis parameter, the so-called beta-effect, induces the anisotropization of the upward energy flux which in this case is preferentially directed to zonal mode leading to the formation and maintenance of jet-like zonal structures. The banded structure of alternating zonal jets is recognized as one of the most important features observed in the atmosphere of Giant gas planets as well as in the oceans, nevertheless the dynamics underlying jet formation is still unclear. We designed an experimental facility available at the Hydraulics Laboratory of the Sapienza University of Rome to deeply investigate these flows at the laboratory scale. We considered a rotating tank in which the paraboloid shape assumed by the free surface of the fluid induces the -effect and turbulence is generated by injecting energy as an electromagnetic forcing on a shallow layer of saline solution. Velocity fields are measured using image analysis. We considered a layer of 4cm of fluid rotating at 3rad/s, the magnets were arranged along three arcs in a 90° circular sector at increasing radial distance from the center of the tank, the polarity of magnets in each arc being the same. We performed an experimental campaign by changing the intensity of the electric current in the range 2A≤I≤6A. The space-time evolution of the flow was investigated via velocity/vorticity averaged and instantaneous maps, azimuth-time contour plots of the azimuthal mean flow and averaged radial profiles of the azimuthal velocity. The obtained results show a banded flow structure consisting in an alternation of eastward and westward zonal jets, as observed in natural environments. We were able to emphasize the effect of the forcing’s changing on the flow structure and to characterize the flow regime, in agreement with the literature