Characteristics and structures in thermally-stratified turbulent boundary layer with counter diffusion gradient phenomenon

The objectives of this study are to investigate the counter diffusion phenomenon (CDP) in a stably thermally-stratified turbulent boundary layer by means of direct numerical simulation (DNS). In this study, four cases of stably thermally-stratified turbulent boundary layers are simulated to reproduce the CDP, in which two Reynolds numbers and four Richardson numbers are set. The CDP is discovered in both the velocity and thermal fields in three cases. DNS clearly shows the CDP, which indicates the negative sign of the Reynolds shear stress and the wall-normal turbulent heat flux with the positive sign of mean velocity and temperature gradients. The turbulent heat flux tensor is also shown in order to indicate the variation of the thermal field, in which the streamwise turbulent heat flux tensor maintains a high value even in the case of strong CDP occurrence. The relation between the vortex structure and the Reynolds shear stress fluctuation is shown, where the negative value of Reynolds shear stress fluctuation frequently appears around the vortex structure in the case of CDP occurrence.