Comparisons of temperature structures with turbulent structures in thermally stratified channel flows by direct numerical simulation

Direct numerical simulation (DNS) in the thermally stratified turbulent channel flow with the Boussinesq approximation at Reynolds number Re-tau approximate to 395, Prandtl number Pr = 0.71 and Richardson number Ri(tau) = -10, 0 and 10 is performed in this study. Based on the DNS data, the three-dimensional structure of temperature fluctuations and the effect of stratification stability on temperature structure morphology, scale and volume are investigated. Results show that thermal buoyancy has a significant effect on the temperature structure under different stratifications. Specially, the three-dimensional structure of temperature fluctuations has similar morphological characteristics to that of streamwise velocity fluctuations only in the near-wall region under stable and neutral stratifications. The streamwise length of the temperature structure decreases with the height, but the structure volume increases linearly with the height. However, under the unstable stratification (Ri(tau) = -10), the thermal buoyancy makes the turbulence form a quasi-streamwise large-scale convective structure directly enhancing the large-scale temperature structure. So the streamwise length and spanwise width of the temperature structure increase with the height, and the spanwise width exhibits a discontinuous jump with the height. At the same time, the scale size of the temperature structure in the outer regions is almost constant, thus the structure volume increases nonlinearly with the height. In addition, the temperature fluctuations are also affected by thermal buoyancy. The thermal buoyancy enhances the large-scale temperature structure in the near wall region through the modulation of the large-scale turbulent structure in the outer regions on the temperature large-scale fluctuations near the wall. Which results in the similarity between the temperature structure and streamwise velocity structure in the near-wall region being destroyed.