Propagating internal waves generated by an impulsive plume

Significance 

In hydrodynamics, a plume is a column of one fluid moving through another. Several effects control the motion of the fluid, including momentum (inertia), diffusion, and buoyancy (density differences). Outflows of plumes and jets result from natural and anthropomorphic sources including volcanoes, hydrothermal vents, gaseous eruptions, room ventilation, and sewage outflows – among others. As witnessed in the atmosphere and ocean environments, short duration impulsive or intermittent plumes, as opposed to continuous emissions often occur. In situations where the background environment is stratified, both plumes and jets will generate internal waves that propagate away from the source.

A plethora of literature exists on turbulent plumes, jets, and fountains mainly focusing on steady outflows. In particular, most studies on turbulent plumes in stratified environments have focused on continuous releases and measurements, usually on a sharp pycnocline interface. In contrast, very few studies consider the internal wavefield generated when the plumes evolve in stratified environments. Worse off, there are limited studies focusing on intermittent plumes in stratified environments.

Toward this end, Johns Hopkins University Applied Physics Laboratory researchers, Dr. Alan Brandt and Kara Shipley recently presented a study that investigated the generation of internal waves by intermittent turbulent forced plumes and estimated the fraction of energy in the plume that is transferred to the internal wave field. They performed laboratory experiments that focused on releasing an impulsive forced plume into a finite thickness stratified layer in order to obtain measurements of the total internal wave (IW) energy and compare that energy level to the energy of the forcing plume. Their work is currently published in the research journal, Physical Review Fluids.

The scaling relationships for the finite time plume release and the IW energy were obtained by conducting a series of experiments examining the dynamics of a downward release of an impulsive plume of homogeneous heavy fluid into a fluid with a density-stratified layer above a region of constant density. Their experiments varied the duration of the impulsive plume release, the density of the plume, and the thickness of the stratified layer in order to study the relationships defining the properties of buoyant and forced plumes.

The authors observed that after the plume descended to a maximum depth it rebounded to an equilibrium level where the ensuing oscillation resulted in the generation of a propagating internal wavefield. It was also found that the coupling of the plume energy to the internal wave field depended primarily on the density of the plume in relation to the ambient stratification. The degree of coupling was found to be on the order of 8%–10%, which is in line with other investigations of internal wave forcing from continuous sources.

In summary, the Brandt-Shipley study presented a series of laboratory experiments with a focus on short duration impulsive, forced plumes and the generation of internal waves within a finite thickness stratified layer. The extent of energy transfer to the internal wave field by an impulsive plume was measured. In contrast to prior studies, the experiments presented considered turbulent-IW coupling resulting from an impulsive source. In all cases considered, the forcing was turbulent and incoherent.

Turbulent plume descending and rebounding in a stratified fluid (With permission from Physical Review Fluids, volume 4, 014803 (2019))

Reference

Alan Brandt, Kara R. Shipley. Internal gravity waves generated by an impulsive plume. Physical Review Fluids, volume 4, 014803 (2019)

Go To Physical Review Fluids

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