Excitation of inertial modes in a closed grid turbulence experiment under rotation

TL;DR

This study investigates how inertial modes are excited and can be controlled in rotating grid turbulence experiments, revealing that adding inner tanks reduces inertial mode energy and enables more freely decaying turbulence.

Contribution

It demonstrates the excitation of inertial modes in rotating turbulence and shows how to suppress them using inner tanks, enabling more ideal decay conditions.

Findings

Approximately 50% of kinetic energy is stored in inertial modes with a simple grid.

Adding inner tanks reduces inertial mode energy to about 15%.

Inertial modes' structure matches numerical predictions by Maas.

Abstract

We report an experimental study of the decay of grid-generated turbulence in a confined geometry submitted to a global rotation. Turbulence is generated by rapidly towing a grid in a parallelepipedic water tank. The velocity fields of a large number of independent decays are measured in a vertical plane parallel to the rotation axis using a corotating Particle Image Velocimetry system. We first show that, when a "simple" grid is used, a significant amount of the kinetic energy (typically 50%) is stored in a reproducible flow composed of resonant inertial modes. The spatial structure of those inertial modes, extracted by band-pass filtering, is found compatible with the numerical results of Maas [Fluid Dyn. Res. 33, 373 (2003)]. The possible coupling between these modes and turbulence suggests that turbulence cannot be considered as freely decaying in this configuration. Finally, we demonstrate that these inertial modes may be significantly reduced (down to 15% of the total energy) by adding a set of inner tanks attached to the grid. This suggests that it is possible to produce an effectively freely decaying rotating turbulence in a confined geometry.