Drops and bubbles in the environment

Bubbles and drops are ubiquitous in nature and play critical roles in many important environmental problems. Most familiar is the life-sustaining role of the water droplets that are rain. Less familiar are their roles in chemical and biological transport at the air-sea interface, the thermal budget of the atmosphere, volcanoes and exploding lakes, and disease transmission. The goal of this review is two-fold. First, we review the fundamental physics of the formation and dynamics of droplets and bubbles, thereby providing a framework for understanding their myriad roles in fluid transport and the sustenance of life within the aqueous and aerial environments. Second, we review the many environmental settings in which they arise, briefly reviewing well-studied problems while highlighting exciting new research directions.

See paper: Bourouiba & Bush (2013).

The drainage of glacial lakes and climate change

A cold event at around 8200 calendar years BP and the release, at around that time, of a huge freshwater outburst from ice-dammed glacial Lake Agassiz have lent support to the idea that the flood triggered the cold event. Some suggest that the freshwater addition caused a weakening of the North Atlantic meridional overturning circulation (MOC) thereby reducing the ocean transport of heat to high northern latitudes. Although several modeling efforts lend strength to this claim, the paleoceanographic record is equivocal. The authors’ aim is to use a coupled ocean–atmosphere model to examine the possibility that the two events are causally linked but that MOC reduction was not the main agent of change. It is found that the outburst flood and associated redirection of postflood meltwater drainage to the Labrador Sea, via Hudson Strait, can freshen the North Atlantic, leading to reduced salinity and sea surface temperature, and thus to increased sea ice production at high latitudes. The results point to the possibility that the preflood outflow to the St. Lawrence was extremely turbid and sufficiently dense to become hyperpycnal, whereas the postflood outflow through Hudson Strait had a lower load of suspended sediment and was buoyant.

See paper : Clarke, Bush & Bush (2009) .

The dynamics of the Tibetan Plateau

Dynamic stresses developed in the deep crust as a consequence of flow of weak lower crust may explain anomalously high topography and extensional structures localized along orogenic plateau margins. With lubrication equations commonly used to describe viscous flow in a thin-gap geometry, we model dynamic stresses associated with the obstruction of lower crustal channel flow due to rheological heterogeneity. Dynamic stresses that depend on the mean velocity, viscosity and channel thickness are then applied to the base of an elastic upper crust, and the deflection of the elastic layer is computed to yield the predicted dynamic topography. We compare model calculations with observed topography of the eastern Tibetan Plateau margin where we interpret channel flow of the deep crust to be inhibited by the rigid Sichuan Basin. Model results suggest that as much 1500 m of dynamic topography across a region of several tens to a hundred kilometres wide may be produced for lower crustal material with a viscosity of 2 × 10^18 Pa s flowing in a 15 km thick channel around a rigid cylindrical block at an average rate of 80 mm/yr.

See paper:  Clark, Bush and Royden (2005)

Roll waves on flowing cornstarch suspensions

Roll waves on flowing cornstarch suspensions.

 

 

We report a traveling wave instability in low Reynolds number flows of aqueous concentrated suspensions of corn starch. The experimental observations are difficult to reconcile with theoretical predictions based on simple rheological models which indicate that flows are stable at low Reynolds number.

 

See paper here: Balmforth, Bush & Craster (2005)

The dynamics of splash-form tektites

Splash-form tektites are generally acknowledged to have the form of bodies of revolution. However, no detailed fluid dynamical investigation of their form and stability has yet been undertaken. Here, we review the dynamics and stability of spinning, translating fluid drops with a view to making inferences concerning the dynamic history of tektites. We conclude that, unless the differential speed between the molten tektite and ambient is substantially less than the terminal velocity, molten tektites can exist as equilibrium bodies of revolution only up to sizes of 3 mm. Larger tektites are necessarily non-equilibrium forms and so indicate the importance of cooling and solidification during flight. An examination of the shapes of rotating, translating drops indicates that rotating silicate drops in air will assume the shapes of bodies of rotation if their rotational speed is 1% or more of their translational speed. This requirement of only a very small rotational component explains why most splash-form tektites correspond to bodies of revolution. A laboratory model that consists of rolling or tumbling molten metallic drops reproduces all of the known forms of splash- form tektites, including spheres, oblate ellipsoids, dumbbells, teardrops, and tori.

See paper:  Elkins-Tanton, Aussillous, Bico, Quere & Bush (2003).

Sedimentation from particle clouds

We examine the settling of monodisperse heavy particles released into a fluid when the resulting motion is sufficiently vigorous that the particle cloud initially assumes the form of a turbulent thermal. A laboratory study is complemented by numerical simulations of particle cloud dynamics in both homogeneous and stratified ambients. In the homogeneous ambient, the cloud evolves in a manner consistent with a classical fluid thermal. The cloud grows through turbulent entrainment and decelerates until its speed is exceeded by that of the individual particles, at which point the particles rain out as individuals. Following fallout, the particles sink at their individual settling speeds in the form of a bowl-shaped swarm. In a stratified environment, the mode of fallout depends explicitly on the degree of stratification. nevertheless, following particle fallout, the fluid entrained by the thermal ascends and intrudes at a rebound height given to leading order by 3/4 times the fallout height. Criteria for three distinct modes of particle deposition in a stratified ambient (left) are developed.

See paper:  Bush, Blanchette & Thurber, JFM (2003).

Sedimentation from riverine outflows

The different modes of convection observed beneath a riverine outflow.

A series of laboratory experiments were conducted in order to elucidate the sediment-induced mixing processes accompanying riverine outflows, specifically, the discharge of a warm, fresh, particle-laden fluid over a relatively dense, cool brine. In a parameter regime analogous to recently acquired field measurements, hypopycnal (surface) plumes were subject to a convective instability driven by some combination of heat diffusing out of the warm, fresh, sediment-laden plume and particle settling within it. Convection was robust in the presence or absence of intense turbulence, at sediment concentrations as low as 1 kg/cubic meter, and took the form of millimetric sediment-laden fingers descending from the base of the surface plume. A consequence of the convective instability of the original hypopycnal plume is the generation of a hyperpycnal (bottom-riding) flow. The experiments presented here indicate that natural river outflows may thus generate bottom-riding plumes when sediment concentrations are 40 times less than those required to render the outflow heavy relative to the oceanic ambient. The resulting hyperpycnal plumes may play an important role in transporting substantial quantities of sediment to the continential slope and beyond.

See paper here:  Parsons, Syvitski & Bush, Sedimentology (2001)

Line plumes in rotation: leads and megaplumes

The development of coherent vortices from the discharge of a 40 cm long line plume into a rotating stratified fluid, as viewed from above. The plume rises to its level of neutral buoyancy, where it spreads as a neutral cloud. The spreading is influenced by the system rotation: the Coriolis force generates a strong shear across the neutral cloud, which goes unstable and breaks into six anticyclonic lens-shaped vortices. The dependence of the scale of the structures on the source conditions is developed on the basis of scaling arguments, and validated experimentally in  Bush & Woods, JFM (1999) . The relevance of this study to two oceanographic problems has been considered.

The discharge of hydrothermal fluid from active mid-ocean ridge spreading centers is discussed in  Woods & Bush (1999), and the generation of arctic eddies by lead-induced thermohaline convection in Bush & Woods (2000) .

Drop motion in rapidly rotating flows

 

Motivated by an interest in vigorous convection in the Earth’s molten outer core, I devoted my doctoral research to the motion of buoyant fluid drops rising through a rapidly rotating fluid.

The figures illustrate silicone drops rising slowly along the rotation axis of a rapidly rotating tank of water rotating at 60 rpm. Note the blocked regions, or `Taylor columns’ accompanying the drop motion. A theoretical and experimental study of axial drop motion in rapidly rotating fluids is presented in a pair of papers: Bush, Stone and Bloxham (1992) and Bush, Bloxham and Stone (1993).

A review of this and related subsequent work is presented in Bush, Stone & Tanzosh (1994) .