Direct numerical simulations (DNS) of boundary layers with various thermal stratifications are carried out to investigate the turbulent structures of these flows. The present DNSs quantitatively provide the characteristics of thermally stratified turbulent boundary layers. In particular, the counter gradient diffusion phenomenon is found in a strong, stable stratified boundary layer. On the other hand, in order to adequately predict turbulent boundary layers with various thermal stratifications, an appropriate turbulence model should be employed in the calculation. Thus, using a database obtained by DNS, the strict assessment of turbulent heat transfer model is made so as to construct a reliable advanced turbulence model. The results of in-depth turbulent model evaluation are indicated, in which we have explored the prediction potential of the proposed nonlinear eddy diffusivity models for momentum and heat in both stable and unstable stratified boundary layers.

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