Abstract: By using the direct numerical simulation method, the characteristics of counter-gradient heat transport in homogeneous shear turbulence under stable thermal stratification were investigated. The cluster connectivity method was employed to thoroughly examine the statistical properties of counter-gradient heat transport structures, including their geometric features and spatial distribution. The Reynolds number was set at Re_x=5 000, and the Richardson numbers Ri were set to 0, 0.1, and 0.2, respectively. The simulation results reveal significant impacts of stable thermal stratification on turbulence structures. The number of counter-gradient transport regions significantly increases by introducing thermal stratification, and the relevant structures are elongated in the streamwise direction. Furthermore, the inclination angle between the structures and the streamwise direction is reduced by stable stratification. In the vertical direction, "ejection" and "sweep" heat fluxes exhibit a characteristic of paired distribution along the diagonal direction, and with increasing stratification intensity, "sweep" structures tend to appear below "ejection" structures. The flndings of this study deepen the understanding of counter-gradient heat transport in stratified shear turbulence and are useful for further exploration of the physical mechanisms underlying the counter-gradient transport phenomenon.

Key words: homogeneous shear turbulence, stable stratification, cluster connectivity method, counter-gradient transport