Disentangling inertial waves from eddy turbulence in a forced rotating turbulence experiment

TL;DR

This study analyzes rotating turbulence to distinguish inertial waves from eddy turbulence, revealing that large-scale anisotropy aligns with inertial wave dispersion, while smaller scales are dominated by sweeping effects, challenging existing wave turbulence theories.

Contribution

The paper introduces a novel space-time analysis method to identify inertial wave signatures in rotating turbulence experiments, highlighting the dominance of sweeping effects at smaller scales.

Findings

Large-scale anisotropy matches inertial wave dispersion relations.

Smaller scales are dominated by sweeping effects from quasi-2D flow.

Wave turbulence theory may not fully apply at moderate Rossby numbers.

Abstract

We present a spatio-temporal analysis of a statistically stationary rotating turbulence experiment, aiming to extract a signature of inertial waves, and to determine the scales and frequencies at which they can be detected. The analysis uses two-point spatial correlations of the temporal Fourier transform of velocity fields obtained from time-resolved stereoscopic particle image velocimetry measurements in the rotating frame. We quantify the degree of anisotropy of turbulence as a function of frequency and spatial scale. We show that this space-time-dependent anisotropy is well described by the dispersion relation of linear inertial waves at large scale, while smaller scales are dominated by the sweeping of the waves by fluid motion at larger scales. This sweeping effect is mostly due to the low-frequency quasi-two-dimensional component of the turbulent flow, a prominent feature of our experiment which is not accounted for by wave turbulence theory. These results question the relevance of this theory for rotating turbulence at the moderate Rossby numbers accessible in laboratory experiments, which are relevant to most geophysical and astrophysical flows.