**Turbulence at the edge of continuum**

M. A. Gallis, J. R. Torczynski, M. C. Krygier, N. P. Bitter, S. J. Plimpton, Phys Rev Fluids, 6, 013401 (2021).

In typical turbulent flows, noncontinuum effects are extremely small and hence can be ignored. However, for some high-Mach-number flows, the Kolmogorov length scale is of the same order as the molecular mean free path, which could introduce noncontinuum molecular-level effects into the turbulent energy cascade. To investigate this, the compressible Taylor-Green vortex flow is simulated for near-continuum conditions using both noncontinuum molecular gas dynamics (direct simulation Monte Carlo method) and continuum computational fluid dynamics (direct numerical simulation of the Navier-Stokes equations). Although the energy-decay histories are basically the same, molecular-level fluctuations are observed to break the symmetries of the initial conditions and thereby produce different but statistically similar routes (based on velocity spectra) from the initial nonturbulent flow to the long-time turbulent flow.

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