Coupled convection and internal gravity waves excited in water around its density maximum at 4{\deg}C

TL;DR

This study experimentally and numerically investigates the interaction of convection and internal gravity waves in water near 4°C, revealing complex flow dynamics driven by water's density maximum.

Contribution

It introduces a novel experimental setup exploiting water's density maximum to study coupled convection and internal gravity waves, supported by numerical simulations.

Findings

Observation of large-scale convective circulation and rising plumes.

Detection of propagating internal gravity waves in the stratified layer.

Identification of a viscously driven horizontal reversing flow in the upper layer.

Abstract

Coupled mixed convective and stratified systems are common in natural flows. To study experimentally the associated dynamics, we use a singular property of water: its non-linear equation of state is characterised by a maximum density close to $4^{\circ}$C. By heating the top of a tank at $35^{\circ}$C and cooling the bottom at $0^{\circ}$C, a two-layer configuration spontaneously appears. The convective motion in the bottom layer consists mostly of a large-scale circulation and rising cold plumes. This turbulent flow generates propagating internal gravity waves in the upper stably-stratified layer. PIV measurements are performed and spectral characteristics of the convection and internal gravity waves are presented. An horizontal large-scale reversing flow in the stratified layer is observed which is viscously driven by a third, intermediate layer. This buffer layer is located between the convective and stratified layers and is thermally coupled with the convective one, hence sustaining a strong horizontal shear. Three dimensional direct numerical simulations with geometry and physical parameters close to the experimental ones corroborate our experimental results.