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Abstract: We study experimentally and theoretically transition phenomena caused by theexternal forcing from Rayleigh-Benard convection with the large-scalecirculation LSC to the limiting regime of unstably stratified turbulent flowwithout LSC whereby the temperature field behaves like a passive scalar. In theexperiments we use the Rayleigh-B\-enard apparatus with an additional source ofturbulence produced by two oscillating grids located nearby the side walls ofthe chamber. When the frequency of the grid oscillations is larger than 2 Hz,the large-scale circulation LSC in turbulent convection is destroyed, and thedestruction of the LSC is accompanied by a strong change of the meantemperature distribution. However, in all regimes of the unstably stratifiedturbulent flow the ratio $\big\ell x abla x T^2 + \ell y abla y T^2 +\ell z abla z T^2\big - < \theta^2 >$ varies slightly even in the rangeof parameters whereby the behaviour of the temperature field is different fromthat of the passive scalar. Here $\ell i$ are the integral scales ofturbulence along x, y, z directions, T and \theta are the mean and fluctuatingparts of the fluid temperature. At all frequencies of the grid oscillations wehave detected the long-term nonlinear oscillations of the mean temperature. Thetheoretical predictions based on the budget equations for turbulent kineticenergy, turbulent temperature fluctuations and turbulent heat flux, are inagreement with the experimental results.

Autor: M. Bukai, A. Eidelman, T. Elperin, N. Kleeorin, I. Rogachevskii, I. Sapir-Katiraie



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