The coexistence of turbulence and fluid structures on all scales characterizes planetary circulations. In a rotating fluid, the latitudinal variation of the Coriolis parameter, the so-called β-effect, induces the anisotropization of the upward energy flux, which in this case is directed preferentially to zonal modes and leads to the formation and maintenance of jet-like zonal structures. The banded structure of alternating zonal jets represents one of the most relevant features observed in the atmosphere of giant gas planets as well as in the oceans. Nevertheless, the dynamics of the underlying jets formation are still unclear. At the Hydraulics Laboratory of the Sapienza University of Rome, we designed an experimental facility to investigate these flow patterns in a controlled environment. We considered a rotating tank in which the β-effect is induced by the paraboloid shape assumed by the free surface of the fluid under rotation and the injection of an electromagnetic forcing on a shallow layer of saline solution generates a turbulent flow. In particular 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 the magnets in each arc being the same. We performed an experimental campaign considering a fluid layer of 4 cm rotating at Ω = 3 rad/s and by changing the intensity of the electric current in the range \({\text{2A}} \le I \le {\text{6A}}\) ; velocity fields were measured using image analysis. We investigated the space–time evolution of the flow 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 clearly show a banded flow structure consisting of an alternation of eastward and westward zonal jets, as observed in nature at the planetary scale. We were able to emphasize the effect of the forcing on the flow structure and to characterize the flow regime in agreement with the literature.

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Reproducing Large-Scale (and Not Only) Features in Rotating Turbulent Flows: A Laboratory Study

  • Stefania Espa,
  • Guglielmo Lacorata

摘要

The coexistence of turbulence and fluid structures on all scales characterizes planetary circulations. In a rotating fluid, the latitudinal variation of the Coriolis parameter, the so-called β-effect, induces the anisotropization of the upward energy flux, which in this case is directed preferentially to zonal modes and leads to the formation and maintenance of jet-like zonal structures. The banded structure of alternating zonal jets represents one of the most relevant features observed in the atmosphere of giant gas planets as well as in the oceans. Nevertheless, the dynamics of the underlying jets formation are still unclear. At the Hydraulics Laboratory of the Sapienza University of Rome, we designed an experimental facility to investigate these flow patterns in a controlled environment. We considered a rotating tank in which the β-effect is induced by the paraboloid shape assumed by the free surface of the fluid under rotation and the injection of an electromagnetic forcing on a shallow layer of saline solution generates a turbulent flow. In particular 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 the magnets in each arc being the same. We performed an experimental campaign considering a fluid layer of 4 cm rotating at Ω = 3 rad/s and by changing the intensity of the electric current in the range \({\text{2A}} \le I \le {\text{6A}}\) ; velocity fields were measured using image analysis. We investigated the space–time evolution of the flow 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 clearly show a banded flow structure consisting of an alternation of eastward and westward zonal jets, as observed in nature at the planetary scale. We were able to emphasize the effect of the forcing on the flow structure and to characterize the flow regime in agreement with the literature.