Initial evolution of 3D turbulence en route to the Kolmogorov state: emergence and transformations of coherent structures, self-similarity and instabilities

Enstrophy field in a decaying hyperviscous Navier-Stokes simulation. The emergence of vortex ribs and vortex threads as precursors of turbulence


In this work, we study numerically the temporal evolution of an initially random large-scale velocity field under governed by the hyperviscous incompressible Navier-Stoke equations. Three stages are clearly observed during the evolution. First, the initial condition development is characterized by a spectrum evolving in a self-similar way with a wave number front k∗(t) propagating toward high values exponentially in time. This evolution corresponds to the formation and shrinking of think vortex pancakes exponentially in time, as it has been previously reported in simulations of the incompressible Euler equations. At the second stage, pancakes become unstable, rolling up on the edges and breaking up in the middle, leading to the emergence of vortex ribs – quasi-periodic arrangements of vortex filaments. Those filaments then twist creating structures akin to ropes. At the last stage, a fully developed turbulent state is observed, characterized by a Kolmogorov energy spectrum and exhibiting a decay law compatible with standard turbulent predictions in the case of an integral scale saturated at the scale of the box.