Increasing stratification in turbulent channel flow at Re_τ=1000

In this work we investigate the behavior of stably-stratified channel turbulence by running a series of  Direct Numerical Simulations (DNS) under the Oberbeck-Boussinesq (OB) approximation, at shear Reynolds number Re_τ=1000 and shear Richardson number in the range  0≤Ri_τ≤300. By increasing stratification, active turbulence is sustained only in the near-wall region, whereas intermittent turbulence, modulated by the presence of non-turbulent wavy structures (Internal Gravity Waves, IGW), is observed at the channel core.  In  such  conditions,  the wall-normal transport  of  momentum  and  heat  is  considerably   reduced  compared  to the  case  of  non-stratified  turbulence. By performing a cross power spectral density analysis of temperature and the wall-normal velocity fluctuation signals we show the presence of a ∼π/2 phase delay between these two signals.This constitutes a blockage effect to the wall-normal exchange of energy. In addition, we also show the scaling laws for friction factor and Nusselt number. These scaling laws, which seem robust over the explored range of parameters, are consistent with previous experimental and numerical data, and are expected to help the development of improved models and parametrizations of stratified flows at large Re_τ.