強い安定密度成層乱流場でのスカラの逆こう配拡散に及ぼす分子拡散の影響 : RDTによる線形および非線形過程の検討(熱工学,内燃機関,動力など)

概要

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The linear Rapid Distortion Theory (RDT) is applied to stable thermally stratified air (Pr=0.7), thermally stratified water (Pr=6) and salt-stratified liquid (Sc〜600) flows without mean shear. Effects of diffusivity and viscosity are included in the analysis and turbulence quanitities such as turbulent scalar fluxes and their cospectra are obtained. The results are compared with previous laboratory measurements and direct numerical simulations (DNS). The results show that countergradient scalar transfer (CGST), which transports scalar counter to the mean gradient (i.e. negative eddy diffusivity), can be predicted by linear RDT as shown in the previous studies. However, the persistent downgradient scalar transfer (P-DGST) at small scales in air flows and the persistent CGST (P-CGST) at small scales in water flows can not be predicted by RDT. In a linear process, CGST occurs first at small scales and it spreads to all scales regardless of Pr or Sc when the initial length scale of turbulence is sufficiently large. The results suggest that the turbulent scalar transfer at small scales in a strongly stably stratified flow is dominated by nonlinear processes and only the large-scale wave-like motions are controlled by linear processes.
一般社団法人日本機械学会の論文
2004-10-25