Direct numerical simulation of stably stratified turbulence under Oberbeck-Boussinesq hypotheses at high Reynolds and Richardson numbers

Abstract

In this thesis, we focus on stably stratified turbulent channel flow at high shear Reynolds number Re . We performed an extensive campaign of pseudo-spectral direct numerical simulations (DNS) of the governing equations (written under OB approximation) in the shear Richardson number space Ri= Gr/Re 2, where Gr is the Grashof number. Specifically, we fix the Reynolds number Re=1000 and we change Gr so to cover abroad range of Ri values. Our results of stratified turbulence indicate that the average and turbulent fields undergo significant variations compared to the case of forced convection, in which temperature is a passive scalar (Ri= 0). In particular, we observe that turbulence is actively sustained only near the boundaries, whereas intermittent turbulence, also flavored by the presence of non-turbulent wavy structures (Internal Gravity Waves, IGW) is observed at the core of the channel. Naturally, the interaction between turbulence and stratification alters also the overall transfer rates of momentum an heat. We believe that the present results may give an important contribution to future turbulence modeling in this field.