33 research outputs found
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Dynamics of Convectively Driven Banded Jets in the Laboratory
The banded organization of clouds and zonal winds in the atmospheres of the outer planets has long fascinated observers. Several recent studies in the theory and idealized modeling of geostrophic turbulence have suggested possible explanations for the emergence of such organized patterns, typically involving highly anisotropic exchanges of kinetic energy and vorticity within the dissipationless inertial ranges of turbulent flows dominated (at least at large scales) by ensembles of propagating Rossby waves. The results from an attempt to reproduce such conditions in the laboratory are presented here. Achievement of a distinct inertial range turns out to require an experiment on the largest feasible scale. Deep, rotating convection on small horizontal scales was induced by gently and continuously spraying dense, salty water onto the free surface of the 13-m-diameter cylindrical tank on the Coriolis platform in Grenoble, France. A âplanetary vorticity gradientâ or âÎČ effectâ was obtained by use of a conically sloping bottom and the whole tank rotated at angular speeds up to 0.15 rad sâ1. Over a period of several hours, a highly barotropic, zonally banded large-scale flow pattern was seen to emerge with up to 5â6 narrow, alternating, zonally aligned jets across the tank, indicating the development of an anisotropic field of geostrophic turbulence. Using particle image velocimetry (PIV) techniques, zonal jets are shown to have arisen from nonlinear interactions between barotropic eddies on a scale comparable to either a Rhines or âfrictionalâ wavelength, which scales roughly as (ÎČ/Urms)â1/2. This resulted in an anisotropic kinetic energy spectrum with a significantly steeper slope with wavenumber k for the zonal flow than for the nonzonal eddies, which largely follows the classical Kolmogorov kâ5/3 inertial range. Potential vorticity fields show evidence of Rossby wave breaking and the presence of a âhyperstaircaseâ with radius, indicating instantaneous flows that are supercritical with respect to the RayleighâKuo instability criterion and in a state of âbarotropic adjustment.â The implications of these results are discussed in light of zonal jets observed in planetary atmospheres and, most recently, in the terrestrial oceans.Engineering and Applied Science
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Dynamics of convectively driven banded jets in the laboratory
The banded organization of clouds and zonal winds in the atmospheres of the outer planets has long fascinated observers. Several recent studies in the theory and idealized modeling of geostrophic turbulence have suggested possible explanations for the emergence of such organized patterns, typically involving highly anisotropic exchanges of kinetic energy and vorticity within the dissipationless inertial ranges of turbulent flows dominated (at least at large scales) by ensembles of propagating Rossby waves. The results from an attempt to reproduce such conditions in the laboratory are presented here. Achievement of a distinct inertial range turns out to require an experiment on the largest feasible scale. Deep, rotating convection on small horizontal scales was induced by gently and continuously spraying dense, salty water onto the free surface of the 13-m-diameter cylindrical tank on the Coriolis platform in Grenoble, France. A âplanetary vorticity gradientâ or âÎČ effectâ was obtained by use of a conically sloping bottom and the whole tank rotated at angular speeds up to 0.15 rad sâ1. Over a period of several hours, a highly barotropic, zonally banded large-scale flow pattern was seen to emerge with up to 5â6 narrow, alternating, zonally aligned jets across the tank, indicating the development of an anisotropic field of geostrophic turbulence. Using particle image velocimetry (PIV) techniques, zonal jets are shown to have arisen from nonlinear interactions between barotropic eddies on a scale comparable to either a Rhines or âfrictionalâ wavelength, which scales roughly as (ÎČ/Urms)â1/2. This resulted in an anisotropic kinetic energy spectrum with a significantly steeper slope with wavenumber k for the zonal flow than for the nonzonal eddies, which largely follows the classical Kolmogorov kâ5/3 inertial range. Potential vorticity fields show evidence of Rossby wave breaking and the presence of a âhyperstaircaseâ with radius, indicating instantaneous flows that are supercritical with respect to the RayleighâKuo instability criterion and in a state of âbarotropic adjustment.â The implications of these results are discussed in light of zonal jets observed in planetary atmospheres and, most recently, in the terrestrial oceans
Experimental studies of lift and drag forces upon cylindrical obstacles in homogeneous, rapidly rotating fluids
Topographic Rossby normal modes simulated in the "Coriolis" rotating tank and by means of a mathematical model
Coherent vortices in rotating flows: A laboratory view
We discuss a series of laboratory experiments on the dynamics of long-lived coherent vortices in rapidly rotating flows. The experiments have been carried out on the "Coriolis" large-scale rotating platform (14 m of diameter) of the LEGI-IMG in Grenoble (France), with the aim of understanding a) how vortices evolve; and b) how to characterize the vortex ability to trap passive tracers for long times. In this work we describe the technical aspects of the different experiments and discuss some of the properties of vortex dynamics and of particle dispersion
Physical modelling of baroclinic development in the lee of the Alps
When baroclinic development is triggered by an obstacle, like an extended mountain range, the so-called lee, or secondary cyclogenesis can develop. The presence of the obstacle exerts a blocking effect on the lower layers of the impinging airflow, forcing them to go round its borders and reach the lee region with a delay. Blocking and delay are both responsible for the initial pressure decrease downwind of the mountain and for the subsequent proper downstream baroclinic development. According to this rather simple scheme, a cyclogenesis episode in the lee of the Alps was simulated in a hydraulic turntable. The results of these experiments showed a good agreement, both from a qualitative and quantitative point of view, with the analysis of an episode of lee cyclogenesis coupled to a cold outbreak in the Mediterranean, which actually occured in Southern Europe downstream of the Alps