Buoyant Plumes and Internal Waves: Two Experiments in Turbulent Convection

A statically stable stratified water column that also contains horizontal property contrasts (either of passive tracer alone or of two dynamically active solutes) is generated and continuously maintained for a long period by releasing two turbulent buoyant plumes of equal buoyancy fluxes into opposite ends of a long channel of water. The bottom outflows from the plumes also continuously excite internal gravity waves that produce a series of counter-flowing quasi-horizontal shear layers which are quasi-stationary relative to the box but whose phase propagates downward through the upward-moving water column. We report that the flow further involves an oscillation associated with the internal waves that gives rise to a sequence of interleaving intrusions across the horizontal gradient region. The wave-driven intrusions are advected upward with the ‘filling-box’ circulation and have the appearance of a spatially growing instability. The intrusions are examined in cases having no horizontal property differences other than a passive tracer. In further experiments where one plume is salt solution and the other is sugar solution, there is vigorous double-diffusive convection on the interleaving intrusions, including salt fingering and diffusive density interfaces, but this convection has only a weak influence on the intrusion thicknesses and velocities. We conclude that under all conditions attained in these experiments, the interleaving is driven by internal waves and not by the property gradients, and we infer that the wave-generated intrusions enhance double-diffusive buoyancy fluxes.

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The energy flux spectrum of internal waves generated by turbulent convection

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.669 ◽

2018 ◽

Vol 854 ◽

Cited By ~ 18

Author(s):

Louis-Alexandre Couston ◽

Daniel Lecoanet ◽

Benjamin Favier ◽

Michael Le Bars

Keyword(s):

Internal Waves ◽

Energy Flux ◽

Wave Energy ◽

Three Dimensional ◽

Turbulent Convection ◽

Reynolds Stresses ◽

Wave Energy Flux ◽

Flux Spectrum ◽

The Waves ◽

Good Agreement

We present three-dimensional direct numerical simulations of internal waves excited by turbulent convection in a self-consistent, Boussinesq and Cartesian model of mixed convective and stably stratified fluids. We demonstrate that in the limit of large Rayleigh number ($Ra\in [4\times 10^{7},10^{9}]$) and large stratification (Brunt–Väisälä frequencies$f_{N}\gg f_{c}$, where$f_{c}$is the convective frequency), simulations are in good agreement with a theory that assumes waves are generated by Reynolds stresses due to eddies in the turbulent region as described in Lecoanet & Quataert (Mon. Not. R. Astron. Soc., vol. 430 (3), 2013, pp. 2363–2376). Specifically, we demonstrate that the wave energy flux spectrum scales like$k_{\bot }^{4}\,f^{-13/2}$for weakly damped waves (with$k_{\bot }$and$f$the waves’ horizontal wavenumbers and frequencies, respectively), and that the total wave energy flux decays with$z$, the distance from the convective region, like$z^{-13/8}$.

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Interpreting Patterns of Concentric Rings within Small Buoyant River Plumes

Remote Sensing ◽

10.3390/rs13071361 ◽

2021 ◽

Vol 13 (7) ◽

pp. 1361

Author(s):

George Marmorino ◽

Thomas Evans

Keyword(s):

Remote Sensing ◽

Internal Waves ◽

Hydraulic Jump ◽

River Mouth ◽

Field Studies ◽

Shear Instability ◽

The Black Sea ◽

River Plumes ◽

Buoyant Plumes ◽

High Resolution Imagery

High-resolution imagery of small buoyant plumes often reveals an extensive pattern of concentric rings spreading outward from near the discharge point. Recent remote sensing studies of plumes from rivers flowing into the Black Sea propose that such rings are internal waves, which form near a river mouth through an abrupt deceleration of the current, or hydraulic jump. The present study, using numerical simulations, presents an alternative viewpoint in which no hydraulic jump occurs and the rings are not internal waves, but derive instead through shear instability. These two differing dynamical views point to a clear need for additional field studies that combine in-water measurements and time-sequential remote sensing imagery.

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