Layered Structure of Stably Stratified Turbulent Shear Flows

Data of a numerical simulation of a stably stratified turbulent Couette flow are analyzed for various values of the Richardson number. Two different methods are used: direct numerical simulation (DNS) and large-eddy simulation (LES). It is shown that the flow contains large organized structures, along with chaotic turbulence, regardless of the simulation method. These structures appear as inclined layers in the temperature field with weakly stable stratification, separated by very thin layers with large temperature gradients. The existence of such layered structures in nature is indirectly confirmed by the analysis of data from field measurements on the meteorological mast, where temperature gradient histograms are found to be far from the normal distribution and similar to temperature gradient probability distributions obtained in numerical model data. The simulations indicate an increase in the turbulent Prandtl number with an increase in the gradient Richardson number. It is likely that the identified structures serve as efficient barriers for vertical turbulent heat flux without blocking the momentum transfer. We propose a hypothesis that it is these structures which serve as a physical mechanism for maintaining turbulence under supercritically stable stratification.