2002 Annual Report
GEOPHYSICAL FLUID DYNAMICS INTRODUCTION
Geophysical Fluid Dynamics (GFD) is the study of fluid flows and their roles in transporting heat, mass and momentum in the oceans, atmosphere and the earth's deep interior. It examines the dynamical processes underlying the climate system, climate change and the evolution of the solid earth. In RSES, the research in GFD is focused on the exploration of physical processes of importance in three different areas:
- convection, mixing and circulation in the oceans,
- magmatic and volcanic processes,
- and the convection of the solid silicate mantle, with its implications for plate tectonics.
GFD emphasises the importance of dynamical modelling, and much of the research program at RSES is anchored strongly in experimental fluid dynamics. The research relies on the excellent facilities of a new 400 sq. m laboratory and workshop area, opened in March 2000, with services designed to cater specifically for a range of geophysical scale experiments. The program relies also on advanced computing facilities, both within the Research School and in the National Facility of the Australian Partnership for Advanced Computing (APAC) located at ANU.
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Figure 1: Work in progress in the GFD laboratory. Claire Menesguin (left) is a MSc student visiting from ENS, France, for a 6 month research project, and is working with Dr Andrew Kiss (centre). They are using Particle Image Velocimetry software to analyse the vorticity dynamics and stability of a model Western Boundary Current similar to the East Australia Current. The flow is generated in the experimental apparatus on the rotating table (seen in the background and cloaked in a black curtain for photographic purposes). |
The ocean dynamics projects in GFD this year included basin-scale circulation driven by the surface wind-stress and mechanisms important to the buoyancy-driven thermohaline circulation. Numerical and laboratory modelling of wind-driven circulation, and the instabilities of intense Western Boundary Currents generated by this forcing, was continued by A. E. Kiss, an Australian Postdoctoral Fellow (ARC). He used a laboratory model flow driven by a horizontal surface stress and placed on the rotating table (figures 1 and 2), along with a numerical scheme designed to simulate the laboratory experiment.
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Figure2. Velocity vectors (arrows) and vorticity (colours) obtained from laboratory experiments, showing eddy shedding from a separated western boundary current.
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A study of processes relevant to the global thermohaline circulation of the oceans and its climatically important fluctuations was continued by Prof. R. W. Griffiths, Dr G. O. Hughes and PhD student J. C. Mullarney. In this study a convective circulation was driven by a horizontal gradient of surface heat flux and a simultaneous input of relatively fresh water at one end, the latter mimicking greater freshwater inflow to the oceans at high latitudes. The system was found to have oscillatory behaviour for steady forcing under some values of the ratio of heat and freshwater buoyancy fluxes. Numerical solutions were obtained for the purely thermally-forced case at very large Rayleigh numbers and for a basin width much wider than the water depth.
The laboratory was host to an undergraduate research project, on buoyancy-driven exchange flows, of a student from the Department of Physics and Theoretical Physics. A Visiting Fellow, Prof. G. Veronis of the Department of Geology & Geophysics, Yale University, spent 5 months carrying out experiments with Prof. J. S. Turner on horizontal and vertical heat transport and rates of surface ice melting in the presence of a density stratification. The latter was motivated by observations of a large thermal anomaly at depth in the Arctic Ocean and provided measurements of heat transport with which theoretical analyses will be compared.
In the area of mantle dynamics Dr Davies continued modelling of the stirring of chemical heterogeneities in the convecting mantle, where motion is driven by internal heating and the subduction of lithospheric plates. One conclusion is that the concentrations of radioactive heat sources in much of the "depleted" mantle are still quite uncertain, and in particular that they could be substantially higher than has usually been inferred. It is also possible that residual primitive helium (helium-3) could persist in subducted remnants of early mafic crust. In examining the dynamics of the very early mantle the stirring models have shown that a low viscosity in the upper mantle can permit relatively rapid settling of the heavier basaltic component, which sinks into the lower mantle. The result is that two compositional zones develop.
Figure 3. Images from a numerical model of the Hadean mantle. Upper panel: temperature and streamlines. Lower panel: tracers plotted over the viscosity structure. Viscosity is temperature-dependent with a superimposed increase by a factor of 30 entering the lower mantle. Tracers simulate basaltic component (orange), which melts to form oceanic crust (green) before being subducted (black). Blue tracers have passed through the lower 20 km of the model, which is a potential plume-feeding zone. Tracers have a density excess of 70 kg/m3, which causes them to settle out of the low-viscosity upper mantle, but not out of the higher-viscosity lower mantle. Two "fluids" have developed, a cooler depleted fluid over a hotter, enriched fluid.
In another project examining stirring in the mantle, Dr C. Meriaux and Dr R. C. Kerr carried out experiments in which they determined the pattern of tracers expected at the Earth's surface due to hot up-welling plumes, if these draw on compositional heterogeneities at their source.
Novel laboratory experiments in mantle dynamics were also carried out by a Visiting Fellow, Prof, C. Kincaid, who spent 12 months at ANU while on sabbatical from the Graduate School of Oceanography, University of Rhode Island. These experiments explored the thermal evolution of slab surface temperature and the mantle circulation as an oceanic plate subducts into the hotter mantle. Effects of slab sinking speed, slab dip and roll-back subduction were studied by forcing a rigid model plate into a tank of very viscous syrup with various components of motion. The results showed marked differences in the temperature-depth trajectories for the slab surface (implying different melting histories), and these are can be related to the differing patterns of three-dimensional flow in the surrounding mantle.
The modelling of physical processes in volcanology was again a highlight in the laboratory activity. In collaborative work with volcanologist Professor K.V. Cashman from the University of Oregon, Prof. R. W. Griffiths and Dr R. C. Kerr carried out experiments to study the solidification of channellized basaltic lava flows. In the experiments polyethylene glycol flowed down a long sloping channel under cold water. The flow surface solidified in different patterns depending on the thermal conditions, slope and flow rate, and could form either a well-insulated flow through a lava tube or flow with only a relatively small amount of mobile solid crust on its surface. This work provided a criterion that distinguishes between conditions giving lava tubes and those giving open lava channels. More generally, it demonstrates the usefulness of fluid dynamics experiments as a tool for learning about the complicated processes that govern solidifying lava flows. In related experiments A. Lyman, a PhD student who commenced this year, is using rapid dam releases of yield-strength slurries, with and without surface cooling, to investigate the dynamics of rapid emplacement of large andesite flows.
The group hosted a number of visitors. Two long-stay visitors, Prof G. Veronis, Yale University, and Prof. C. Kincaid, University of Rhode Island, have been mentioned above. Dr N. Stevens, from the Institute of Geological and Nuclear Sciences, New Zealand, visited for two weeks to discuss large andesite lava flows, and the group continued to enjoy the presence of Emeritus Professor J.S. Turner. Mr A. Lyman, previously of Arizona State University, commenced his PhD program and is working on the dynamics of lava flows. Prof. Griffiths again taught an undergraduate course on fluid dynamics and ocean-atmosphere dynamics in the Department of Physics and Theoretical Physics, ANU, and the laboratory hosted an undergraduate student project for the unit 'Research Projects in Physics'. Dr C. Meriaux left the Group and returned to Paris at the end of the year. The staff, students and visitors all acknowledge the vital contributions of our technical support staff, R. Wylde-Browne (who retired this year and moved to the coast to follow his sailing interests), A.R. Beasley and C.J. Morgan to our research program. Collaboration continued with Australian Scientific Instruments in sales of the 'Geophysical Flows Rotating Table'.
Research Projects
Sources of heat and helium in the dynamic mantle G. F. Davies
Dynamics of the Hadean Mantle G. F. Davies
The structure of mantle plumes sheared by mantle flow C. Meriaux and R.C. Kerr
Patterns of Solidification in Long Channelized Lava Flows R.W. Griffiths, R.C. Kerr and K.V. Cashman
Modelling processes that drive the ocean thermohaline circulation J.C. Mullarney, R.W. Griffiths and G.O. Hughes
Localised mixing due to a turbulent patch G.O. Hughes
The influence of double-diffusive processes on the melting of ice in the Arctic Ocean J.S. Turner and G. Veronis
Internal waves as a source of finestructure at ocean fronts. A.A. Bidokhti and R.W. Griffiths
Dynamics of ocean circulation driven by surface wind stress A. E. Kiss