Final Report Summary - MULTIFLOW (Multiscale dynamics of turbulent flows)
Turbulence in fluids is a ubiquitous phenomenon responsible for a substantial fraction of the energy expenditure worldwide, but which also has beneficial effects in many industrial mixing and combustion, and in equalising the world climate. All those processes are known to proceed by multiscale stages that, for example, span in the atmosphere from continental scales to millimetres. The details of those multiscale (‘cascade’) processes are not well understood, and the goal of this project has been to elucidate them. It uses as test cases turbulent shear flows, including boundary layers in vehicles, pipes, and channels, which are by themselves the main contributors to energy expenditure in transportation. The techniques used are mainly large-scale numerical simulations and very-large-scale mining of the data, and the results suggest that the simplest models used up to now for the flow of causality in turbulence were incomplete. Rather than the one-directional flow of information and energy assumed by most physical and computational models, typically from the largest to the smallest scales and from the wall to the outer layers, the causality flow is more complicated and two-directional, with the overall flux appearing as a residue between two large quantities. Also, the effect of the wall on wall-bounded flows has been shown to be mainly indirect. It causes the shear that powers turbulence, but the turbulent structures are created throughout flow, and do not necessarily originate from the wall. Although this was suspected in a global sense, it had never been observed for individual structures, and has profound consequences on the development of computational models and control strategies. In fact, the results show that a substantial part of the turbulent energy production can be described by fairly simple linearized models.