Causally significant structures in isotropic turbulence at low Reynolds number*

A large number of numerical experiments with 3D direct numerical simulations of homogeneous isotropic turbulence (HIT) are conducted at Reynolds number Re_λ≈50, to investigate the causally significant turbulent structures in HIT at low Reynolds number. We first determine whether significant flow regions exist, and then examine whether they share some common characteristics. To do this, 65 basic initial flows are created, and each of them is divided into 10³ cubical cells. The flow in each cell is modified, the effect of the modification on the flow is monitored, and casually significant cells are defined as those in which the modification leads to a large overall change in the flow at a later measurement time. In our study, the flow within each cell is modified in eight ways, including in both vorticity and velocity fields. The process is repeated for each cell and each kind of modification, for a total of 5.2×10⁵ simulations. Our results indicate that significant regions exist in HIT at low Reynolds number. The most sensitive cells in experiments involving vorticity are those with initial high enstrophy, while velocity experiments select regions with high initial kinetic energy. The structure and other characteristics of these significant regions are studied.