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Abstract: The effect of a background rotation on the decay of homogeneous turbulenceproduced by a grid is experimentally investigated. Experiments have beenperformed in a channel mounted in the large-scale -Coriolis- rotating platform,and measurements have been carried out in the planes normal and parallel to therotation axis using particle image velocimetry. After a short period of about0.4 tank rotation where the energy decays as $t^{-6-5}$, as in classicalisotropic turbulence, the energy follows a shallower decay law compatible with$t^{-3-5}$, as dimensionally expected for energy transfers governed by thelinear timescale $\Omega^{-1}$. The crossover occurs at a Rossby number $Ro\simeq 0.25$, without noticeable dependence with the grid Rossby number. Afterthis transition, anisotropy develops in the form of vertical layers where theinitial vertical velocity remains trapped. These layers of nearly constantvertical velocity become thinner as they are advected and stretched by thelarge-scale horizontal flow, producing significant horizontal gradient ofvertical velocity which eventually become unstable. After the $Ro \simeq 0.25$transition, the vertical vorticity field first develops a cyclone-anticycloneasymmetry, reproducing the growth law of the vorticity skewness, $S \omegat\simeq \Omega t^{0.7}$, reported by Morize, Moisy and Rabaud {\it Phys.Fluids} {\bf 17} 9, 095105 2005. At larger time, however, the vorticityskewness decreases and eventually returns to zero. The present results indicatethat the shear instability of the vertical layers contribute significantly tothe re-symmetrisation of the vertical vorticity at large time, by re-injectingvorticity fluctuations of random sign at small scales. These results emphasizethe importance of the initial conditions in the decay of rotating turbulence.



Author: F. Moisy, C. Morize, M. Rabaud, J. Sommeria

Source: https://arxiv.org/







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