2001 Annual Report
GEOPHYSICAL FLUID DYNAMICS INTRODUCTION
To Research Projects
Geophysical Fluid Dynamics is the study of fluid flows and their roles in transporting heat, mass and momentum in the Earths hydrosphere, crust and deep interior. In the Research School of Earth Sciences, the research in this field is focussed 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. The Geophysical Fluid Dynamics Group continues to emphasise the importance of dynamical modelling and a rigorous understanding of the underlying dynamical principles. Much of the research program is anchored strongly in experimental fluid dynamics and relies on laboratory facilities. A new purpose-built laboratory was occupied in 2000 and this year became fully operational. The research relies also on advanced computing facilities within the Research School and the Australian Partnership for Advanced Computing National Facility located at the University.
Work in the recently opened Geophysical Fluid Dynamics laboratory, showing apparatus being used to study cooling and solidifying channel flows and the dynamics of basaltic lava channels.
The research topics in Geophysical Fluid Dynamics this year include aspects of both the wind-driven upper ocean circulation and the buoyancy-driven thermohaline circulation of the oceans. Numerical modelling of circulation in a simple laboratory model driven by a surface wind stress continued, with the appointment of an Australian Postdoctoral Fellow, Dr A.E. Kiss, and an emphasis on the generalisation of an approximate mathematical formulation of the equations of motion suitable for numerical solution. A new PhD student, Ms J. Mullarney, joined the study of convective circulation driven by a horizontal gradient of surface heat flux and a freshwater (or salinity) input. The work has shown oscillatory behaviour for steady forcing, specifically for some values of the ratio of heat and freshwater buoyancy fluxes. Dr A.A. Bidokhti continued his nine month sabbatical leave from Tehran University and examined the structure of outflows from the Persian Gulf and Red Sea. He completed laboratory experiments with turbulent and double-diffusive (warm, salty) outflows, the results of which suggest that low-frequency internal gravity waves propagating downward in the water column may influence the observed vertical splitting of the outflows. Another visitor, Dr J.R. Taylor, spent five months in the Group on sabbatical leave from the University of New South Wales at the Australian Defence Force Academy, and carried out laboratory experiments modelling the formation and propagation of fronts in the atmospheric boundary layer resulting from differential surface heating over plateaux or escarpments. The results have been used to interpret data for inland propagating fronts over the highlands of south-eastern Australia.
In the area of mantle dynamics, Dr G. Davies has continued his modelling of the stirring of chemical heterogeneities in the convecting mantle driven by internal heating and the subduction of lithospheric plates. The modelling has predicted mean isotopic ages of mid-ocean ridge basalts of two billion years, which is much older than previous modelling had suggested and consistent with measured ages. If subducted oceanic crust is given a small mass anomaly in the new models, there is consequently a small degree of buoyant settling of this material and the models predict the depletion of mid-ocean ridge basalts, relative to ocean island basalts, in incompatible trace elements. Novel experiments, in this case in the laboratory, have also been carried out, by Professor R.W. Griffiths and Dr R.C. Kerr, to understand rapid basaltic lava flows in channels. These experiments form part of collaborative work with volcanologist Professor K.V. Cashman from the University of Oregon, and involve measurement of flows of fluid down a long sloping channel while it is solidifying at its surface (Figure 1). This work is at an early stage, but has already indicated a criterion for formation of lava tubes rather than open lava channels. It also demonstrates the usefulness of fluid dynamics experiments as a tool with which to learn more about the processes that govern cooling, solidification and flow front advance in lava flows.
The staff of the Group was complemented this year by Mr C.J. Morgan, who joined us as a Senior Technical Officer. Mr M. Wells submitted his PhD thesis, received his doctorate, and took up a research position in fluid mechanics at Eindhoven University of Technology, The Netherlands. Two long term visitors Dr J.R. Taylor and Dr A.A. Bidokhti have been mentioned above and the Group continued to enjoy the presence of Emeritus Professor J.S. Turner. The staff, students and visitors all acknowledge the vital contributions of our technical and administrative support staff, R. Wylde-Browne, A.R. Beasley, C.J. Morgan and F.A. Chivas, to our research program. Collaboration continued with Australian Scientific Instruments, who received a further order for the Geophysical Fluids Rotating Table.