Eulerian and Lagrangian velocity statistics in weakly forced two-dimensional turbulence

TL;DR

This paper investigates velocity fluctuation statistics in weakly forced two-dimensional turbulence, revealing differences between Eulerian and Lagrangian frames and showing that local exponent ratios align with Kolmogorov predictions in Eulerian but not in Lagrangian measurements.

Contribution

It introduces a novel analysis linking spectral ranges with real-space velocity difference moments, highlighting differences between Eulerian and Lagrangian statistics in 2D turbulence.

Findings

Eulerian local exponent ratios agree with Kolmogorov predictions.

Lagrangian local exponent ratios deviate from Kolmogorov expectations.

Spectral ranges correlate with ratios of real-space moment exponents.

Abstract

We present statistics of velocity fluctuations in both the Lagrangian and Eulerian frame for weakly driven two-dimensional turbulence. We find that simultaneous inverse energy and enstrophy ranges present in the Lagrangian and Eulerian Fourier spectra are not directly echoed in real-space moments of velocity difference. The spectral ranges, however, do line up very well with ratios of the real-space moments {\em local} exponents, indicating that though the real-space moments are not scaling ``nicely'', the relative behavior of the velocity difference probability distribution functions is changing over very short ranges of length scales. Utilizing this technique we show that the ratios of the local exponents for Eulerian moments in weak two-dimensional turbulence behave in agreement with Kolmogorov predictions over the spectrally identified ranges. The Lagrangian local exponent ratios, however, behave in a different manner compared to their Eulerian counterparts, and deviate significantly from what would be expected from Kolmogorov predictions.