Group seminar on 15. December, 14:15 CET
Energy transfers in turbulent flows: a coarse-graining approach
Atmospheric flows are highly turbulent and are associated with non-trivial energy transfers between scales. Depending on the degree of thermal stratification and of rotation, the turbulent energy cascade can exhibit different characteristics, and the co-existence of direct and inverse energy cascades has been highlighted based on spectral methods. Spectral approaches, however, rely on assumptions of statistical homogeneity that are not necessarily fulfilled in complex atmospheric flows. Instead, coarse-graining approaches allow to quantify energy transfers locally in space and time. I will show two examples of applications of such coarse-graining approaches to quantify the direct and inverse energy cascades. One example will be a strongly stratified near-surface flow over a glacier, where the Reynolds stress tensor often highlights very anisotropic turbulence. We will explore the presence of an inverse energy cascade as a way to detect the possible presence of two-dimensional turbulence. The second example will be a strongly rotating flow; we will map the energy cascade around a hurricane during its dynamical evolution. Such coarse-graining approaches diagnose the scale-by-scale dynamics and thereby help understand the self-organization of turbulence for different degrees of rotation and stratification.