Laboratory rotating tank experiments

At present there are two experiments under construction in the AOPP geophysical fluid dynamics laboratory.

The laboratory - atmosphere analogue

The thermally-driven rotating annulus is a rotating cylindrical annulus of fluid heated from the outside and cooled from the inside. By being forced this way, the annulus becomes a simple laboratory model of the Earth's (or another planet's) atmosphere as seen from directly above the poles, with the cool middle analogous to the pole, and the heated outer edge analogous to the equator. The experiments in the AOPP geophysical fluid dynamics laboratory are all variations on this theme. Most of our current experiments use a glycerol/water fluid mixture, but in the past we have also used air as the working fluid.

See our page on The Dynamics of Rotating Fluids for a set of video-illustrated presentations on the physics behind these experiments.

Thermally-driven rotating annulusAtmospheric analogue (NASA photo)

Current projects:

Bottom-heated annulus with topographic β-plane (Hélène Scolan, under construction)

This project investigates the nature of flows between a heat source and a heat sink that are displaced both vertically and horizontally relative to each other, in the presence of strong background rotation. In the absence of background rotation, if a heat source is located at a lower altitude than the sink, one would generally expect a strongly convective circulation to result, carrying heat directly and vigorously from the source to the sink. With background rotation, however, we expect that the resulting circulation may spontaneously partition itself into a convectively unstable/neutral region (where temperature becomes well mixed and does not vary much with height) that interacts with a statically stable, baroclinic region (where temperature increases with height and develops a thermal gradient from one side to the other). Wave-like instabilities may develop within this baroclinic zone that may play a crucial role in stabilising the vertical stratification and dominating the transfer of heat and momentum where they occur. Moreover, there is evidence to suggest that if the transport of heat by the instability acts more rapidly than other heat exchange processes, this stabilizing effect may act within a nonlinear feedback loop, somewhat like a thermostat, adjusting the flow back towards a weakly nonlinear/unstable 'critical' state - sometimes referred to as 'self-organized criticality'.

A laboratory analogue of the rotating, stratified flow configuration above is currently being designed and built. This new experimental configuration entails heating a body of fluid in a cylindrical container on a rotating platform along an annular ring at the bottom of the tank close to the outer radius, and cooling it through a circular disk near the centre of the tank at the upper surface. Such an experiment is a variant of the classic differentially wall-heated annulus experiment, but it removes constraints placed on background temperature gradients in the fluid and should allow the formation of a statically stable (though baroclinically unstable) zone between convectively unstable regions by the heated or cooled boundaries.

Schematic cross-section of the new experimental setup, with flow driven by heating at the bottom outer surface and cooling at the upper inner surface.

This configuration is relevant to a number of geophysical and astrophysical problems in which stably and unstably stratified flows interact in the presence of background rotation. These include the Earth's atmosphere and climate system and its response to variations in its radiative heating and cooling, other planetary atmospheres (notably Mars, Venus and the gas giant planets), and in stellar interiors (e.g. the tachocline region within the Sun). It may also be seen as an idealisation of a variety of industrial processes, such as in rotating semiconductor crystal growth melts, process mixing techniques in chemical engineering, and convective flows in turbo machinery.

Snapshot of the experimental design with an annular ring for the heat source and a cold plate at the center at the upper surface.Zoomed in image of the experimental design.

Rapidly-rotating annulus (under construction)

When built, this new experiment will be able to rotate at up to 10 revolutions per second, some 20 times more rapidly than the current experiments. This will allow us to probe interesting new flow regimes, particularly turbulent regimes relevant to the atmospheres of the giant planets.

Recent other projects:

Thermally-driven rotating annulus with topography (Sam Marshall)

A tank bottom with topography simulates the effects of mountains on the atmosphere. We study the effects these have on stationary and drifting waves that form in the fluid, and their effects on baroclinic instability.

Two layer annulus driven by differential rotation (Tom Jacoby)

Large baroclinic eddies can generate smaller-scale inertia-gravity waves (IGWs), which are not in geostrophic balance. IGWs cannot be observed easily in the atmosphere, and may be a significant source of uncertainty in operational weather forecasts. We study the mechanisms leading to IGW generation in the laboratory using a rotating, cylindrical tank filled with two immiscible fluids and a variety of optical visualisation and image velocimetry techniques.

Coupled annulus pair (Alfonso Castrejón-Pita)

We study the synchronization of periodic and chaotic oscillations between two coupled rotating baroclinic fluid systems both experimentally and numerically. The numerical part of the study involves a pair of coupled two-layer quasigeostrophic models, and the experimental part comprises two thermally coupled baroclinic fluid annuli, rotating one above the other on the same turntable. Various degrees of synchronization such as phase synchronization, imperfect synchronization (phase slips), and complete phase synchronization have been found in both model and experiments. The signatures (and implications) of phase synchronization in oscillatory atmospheric phenomena are also being investigated.

Two thermally coupled rotating annuliSchematic of the coupled experiments

Barotropic detached shear layer experiment (Ana Aguiar)

We have studied barotropic instability in a water-filled rotating tank with flat cylindrical geometry, where a detached shear layer occurs tangent to sections rotating at a different speed to the rest of the tank. For these sections we have used (1) discs at the top and bottom of the tank, (2) a single thick disc in the fluid itself, or (3) a ring (thin annular section) at the top, which produces a more jet-like zonal velocity profile. A range of vortex and wave structures develop depending on the applied parameters, and unstable modes can be stabilised when a sloping bottom is added to the tank. We have also applied this work to the long-lived North Polar Hexagon on Saturn, proposing that these structures correspond to wave modes caused by the non-linear equilibration of barotropically unstable zonal jets [ScienceNow media coverage].

Photograph of the experiment
Hexagonal laboratory flow patterns