液中における二次元レイリー・ベナール対流に対する非オバーベック・ブジネスク影響(対流・拡散・渦・波動対流(1),一般講演)

概要

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Non-Oberbeck-Boussinesq (NOB) effects in the two-dimensional Rayleigh-Benard turbulence for water and for glycerol are numerically investigated by means of direct numerical simulations. The temperature dependences of the viscosity, the thermal diffusivity and the thermal expansion coefficient are fully considered. The center temperature T_c, and the deviation from the Oberbeck-Boussinesq (OB) Nusselt number Nu_<NOB>/Nu_<OB> are on the focus of the present study. The Rayleigh number Ra and the temperature difference Δ between top and bottom plates are varies up to Ra=10^8 and Δ=60K. Our numerical results of T_c and Nu_<NOB>/Nu_<OB> are consistent with the available experimental data. In addition, T_c is found to be well predicted by the Prandtl-Blasius Boundary layer theory and weakly dependent on Ra for fixed Δ. To quantify the physical mechanism to alter the Nusselt number from the OB to the NOB for fixed Ra and Δ, we decompose the Nusselt number ratio Nu_<NOB>/Nu_<OB> as the product of two effects: (1) the change of the thermal boundary layer thickness and (2) the changes of T_c and thermal diffusivity at top and bottom walls. This decomposition indicates that while for glycerol Nu_<NOB>/Nu_<OB> is governed by the first effect, for water it is by the second one.
2009-09-02