Mechanisms toward Kolmogorov's isotropy

Universality is a fundamental concept in turbulence which rests on the assumption of statistically isotropic motions at Kolmogorov scales. However, such mechanical equilibrium can be disrupted by different mechanisms such as shocks and shear. A pervasive and long-standing related question is on the specific dynamical processes involved in driving the system towards local isotropy. To study these turbulent processes, we present a theoretical analysis that unveils the mechanisms that drive the flows back to equilibrium at the small scales. Surprisingly, the mechanisms are found to be of dissipative nature. To study these findings, we use a database of direct numerical simulations of shock-turbulence interactions, a canonical well-studied flow configuration which provides a strong enough shear to induce non-equilibrium at Kolmogorov scales and study in detail the proposed mechanism. The database includes both the so-called wrinkled and broken regimes of the interactions which involve varying degrees of relative turbulence intensities. The DNS data show that the dissipative mechanisms peak at post-shock regions and decay monotonically as turbulence achieves equilibrium. In addition, the mechanisms are found to be more dominant in the wrinkled regime resulting in a stronger reduction of anisotropy. Despite their dissipative nature, the observations show that the mechanisms can be positive locally.