Travel between 2D ad 3D turbulent flows
We investigate two-dimensional (2D) incompressible turbulence with three velocity components. Such systems can be called 2D3C flows. Two main classes are considered, Cartesian 2D3C flows and axisymmetric 2D3C flows.
For the first class of flows we show that helicity is associated with a correlation between the vertical vorticity and a scalar field, here representing the vertical velocity. Statistical mechanics allows to predict the possible persistence of large-scale helical structures. This prediction is verified in numerical simulations and its implication for mixing applications is discussed. Indeed, the vorticity-scalar correlation will importantly affect the mixing-efficiency in 2D flows.
We then investigate, in axisymmetric turbulence, a critical transition between a purely poloidal 2D flow and a 2D3C flow. We transpose the ideas to a toroidal geometry and argue that this transition can play a major role in tokamaks, where a critical transition leads to enhanced plasma confinement.
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