Untitled Document

2003 Annual Report


GEOPHYSICAL FLUID DYNAMICS

INTRODUCTION


Geophysical Fluid Dynamics is the study of fluid flows and their roles in transporting heat, mass and momentum in the oceans, atmosphere and Earth’s deep interior. In RSES, the research in this field 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
• convection of the solid silicate mantle, with its implications for plate tectonics.

Geophysical Fluid Dynamics (GFD) emphasises the importance of dynamical modelling. At the ANU much of the research program in this area is anchored strongly in experimental fluid dynamics and relies on the excellent facilities of a recently constructed 400 sq. m laboratory and workshop area. This is the premier GFD research group and experimental facility in Australia and is well known around the world for its contributions across fluid dynamics, oceanography and ‘solid earth’ geophysics. The research relies also on advanced computing facilities within the Research School and the Australian Partnership for Advanced Computing (APAC) located at ANU.

 

 

Figure 1: Work in progress in the GFD laboratory

The research topics in GFD this year included two key aspects of ocean circulation driven by the surface winds and the global thermohaline circulation of the oceans. Numerical modelling of the wind-driven circulation by A.E. Kiss (an Australian Postdoctoral Fellow, ARC) continued to focus on the nature of instabilities that cause western boundary currents to produce large eddies and separate from the continental boundary.

The dynamics of the global thermohaline circulation of the oceans was addressed by Prof. R.W. Griffiths, Dr G.O. Hughes and PhD student J.C. Mullarney. In this study, attention turned to the purely thermally-driven case, using laboratory experiments with convection forced by a horizontal gradient of surface heat flux (Figures 2 and 8), along with numerical solutions developed on the APAC supercomputing facility. The most important result is a demonstration that heating and cooling, with zero net heat input, at the same horizontal boundary (such as the ocean surface) can drive a vigorous and turbulent overturning circulation. The resulting temperature distribution involves a stably stratified boundary layer (or thermocline) and a very small vertical gradient through much of the interior. Exciting new theoretical ideas on the dynamics and forcing of the ocean overturning circulation were developed based on insights obtained from the experiments.

 

 


Figure 2: A movie clip of passive dye tracer in “horizontal convection” in a long box of water. The box is cooled through the right hand half of the base and heated through the left hand half of the base, and has reached thermal equilibrium in which there is zero net heat input. The box is 1.2 m long and 0.2 m high, but only the left 1/3 of the length is shown. Features include small-scale convection in a mixed layer along the heated base of the tank, a narrow turbulent plume ascending on the left hand wall, and a turbulent outflow at the top. (see also Figure 9)

In mantle dynamics, Dr G.F. Davies began work with a three-dimensional code for simulation of mantle convection, in which he has implemented passive tracers in order to follow material parcels as they move around, disperse or congregate in the flow. This well-developed parallelised code represents a major expansion of our capacity to model the dynamics and chemical evolution of the mantle. He also began a major overhaul of his existing two-dimensional code.

Dr R.C. Kerr and Dr C. Meriaux completed a series of experiments that explored the stirring of tracers by sheared mantle plumes when they draw on chemical heterogeneities in their source region (Figures 3 and 4).

Figure 3: Front and overhead views of a strongly sheared thermal plume, with a high viscosity ratio.

The results of laboratory experiments with the subduction of lithospheric slabs into the mantle, carried out by Prof. C. Kincaid (Visiting Fellow from the University of Rhode Island) and Prof R.W. Griffiths in 2002, were analysed and published in Nature. They showed marked differences in the temperature-depth trajectories for the model slab surface (implying different melting histories for mantle slabs), depending on the relative rates of slab sinking and slab rollback (or trench migration) and are related to the differing patterns of three-dimensional flow in the surrounding mantle.

In volcanology, modelling of lava flow dynamics continued, in collaboration with volcanologist Prof. K.V. Cashman from the University of Oregon. An experimental study of the patterns of solidification of (basaltic lava) flows in simple uniform, sloping channels was completed and the data interpreted to reveal the combined (and somewhat opposing) effects of downstream flow and thermal convection. The flow surface solidified to form either a lava tube containing well-insulated internal flow or only a relatively small amount of mobile solid crust on its surface. Thus a criterion for lava tube formation was established. In other experiments a PhD student, A.W. Lyman, used rapid releases of solidifying viscous and Bingham (yield-strength) fluids to determine the stopping time and distance and the conditions under which these are controlled by yield strength or solidification.

The group continued to benefit from the presence of Emeritus Professor J.S. Turner and enjoyed a 6-month stay by Ms C. Menesquen (a Masters student from ENS, Paris, France). Mr T. Prastowo, from Indonesia, commenced his PhD program in GFD and is working on mixing in exchange flows through straits. Ms M.A. Coman was enrolled in the inaugural year of the Physics of the Earth Honours program at RSES and carried out her thesis project on convection flows in two connected chambers.

Mr D.I. Osmond completed and graduated from the PhD program. PhD student Ms J.C. Mullarney was awarded a Geophysical Fluid Dynamics Summer Fellowship by the Woods Hole Oceanographic Institution, USA, to study at the Woods Hole GFD summer program. Prof. Griffiths and Drs Hughes and Kiss taught the “Dynamics of Ocean Circulation” unit in the new RSES Physics of the Earth Honours program. Prof. Griffiths again taught an undergraduate course on fluid dynamics and ocean-atmosphere dynamics in the Department of Physics and Theoretical Physics, ANU.

The staff, students and visitors all acknowledge the vital contributions of our technical support staff, A.R. Beasley and C.J. Morgan, to our research program. Collaboration continued with Australian Scientific Instruments in commercialisation and sales of the ‘Geophysical Flows Rotating Table’, and we were awarded funds by the ANU Major Equipment Committee to purchase and equip a new rotating table in 2004. Three new ARC Discovery grants, including two Australian Postdoctoral Fellowships to commence in 2004, were awarded to members of the group.