ARTICLE:

• LINEAR AND NONLINEAR PROCESSES IN SMALL-SCALE SCALAR TRANSFER IN STRONGLY STABLE DENSITY STRATIFIED FLOWS  download article

K. Nagata
Nagoya University, Japan

S. Komori
Department of Mechanical Engineering, Kyoto University, Yoshidahonmachi, Sakyo-ku, Kyoto 606-8501, Japan

Y. Sakai
Nagoya University, Japan

ABSTRACT

Linear and nonlinear processes in small-scale scalar transfer in strongly stable stratification are investigated by using a three-dimensional direct numerical simulation (DNS) and a linear rapid distortion theory (RDT). The RDT is applied to unsteady, unsheared, stable thermally stratified air (Pr=0.7), thermally stratified water (Pr=5) and salt-stratified liquid (Sc=600) flows. To investigate the nonlinear effects, the DNS based on the finite difference method is also conducted for stable thermally stratified water (Pr=5) and air (Pr=0.7) flows. The results show that persistent countergradient scalar transfer (P-CGST) at small-scales in water flows and persistent downgradient scalar transfer (P-DGST) at small-scales in air flows cannot be predicted by the linear RDT. On the other hand, the results of the DNS reveal the difference in the scalar transfer at small-scales between water and air flows, which is consistent with the previous measurements. The results from the RDT and DNS suggest that the turbulent scalar transfer at small-scales in a strongly stably stratified flow is dominated mainly by the nonlinear process and only the large-scale wave-like motions are controlled by the linear process.

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