Structures and mechanism of heat transfer phenomena in turbulent boundary layer with separation and reattachment via DNS

This paper presents observations and investigations of the detailed structure and mechanism of turbulent heat transfer in the turbulent boundary layer with separation and reattachment by means of direct numerical simulation (DNS). In order to observe turbulent heat transfer in a boundary layer with reattachment and separation, a DNS of the boundary layer with heat transfer over a 2-dimensional block (2DB) is carried out, in which the effects of Reynolds number and block size are observed. The lengths of reattachment and maximum Stanton number points behind 2DB become longer with an increase in Reynolds number in the case of similar block size with one exception. On the other hand, these points become shorter with an increase in the width of the 2DB. Moreover, the counter gradient diffusion phenomenon (CDP) of the thermal field can be found on the 2DB. A quadrant analysis is carried out to investigate the turbulence motion which decides Reynolds shear stress and the wall-normal turbulent heat flux in the turbulent boundary layer with heat transfer over 2DB, in which it can be found that Q1 and Q3 events (i.e., interactions) affect the occurrence of CDP on 2DB. Also, the correlations among instantaneous fluctuating temperature, temperature gradient and vorticities to observe the turbulent structures of heat transfer around 2DB are clearly shown in the present DNS.