Adopting the perspective of statistical state dynamics (SSD) has recently resulted in progress in understanding planetary scale atmospheric turbulence as well as turbulence in wall-bounded shear flows at laboratory scale. In the SSD approach, rather than adopting the traditional method of extracting statistical quantities from long simulations of a single realization of a turbulent flow, the statistical quantities themselves are taken as variables and the dynamics expressed in these variables so that the statistical quantities are solved for directly. However, the primary advantage of the SSD approach is not facilitating calculation of statistical quantities but is instead conceptual: fundamental aspects of the dynamics of turbulence that have clear reflection in observation and simulation have analytical expression only in SSD. It follows that adopting the SSD framework makes available a new theoretical approach to understanding a wide range of phenomena in turbulent flows. Examples from recent work on turbulence in Couette flow will be presented.
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