More about the Programme
It has become increasingly important in recent years to tackle the problem of turbulent flows because it holds the key to a number of important processes such as dispersion of chemical or radioactive components in the ocean and atmosphere. As well as having many significant applications, the turbulent flow problem is also important in its own right, leading to a better understanding of the complex phenomena associated with fluid transport in the atmosphere and oceans. This will bring great benefits for both short and long term weather forecasting.
An over-riding objective of the ESF programme on transport processes was to bring more harmony and homogeneity to the results obtained from other research projects in the field. Until now it has sometimes been difficult to compare results satisfactorily due to differences in the approaches taken, but hopefully having a joint community of closely interacting researchers will lead to a better overall understanding of transport in geophysical flows. Achieving a more coherent understanding would be of particular benefit both to applied scientists working in fields like chemical dispersal and to makers of decisions on environmental policy. The ESF programme therefore actively encouraged fruitful dialogue and exchange of results between the various research groups working in the area.
The scientific activity of the programme was focused on numerical simulations of the transport dynamics of large scale geophysical flows, with particular emphasis on supporting fundamental research on turbulent flows.
More than 200 scientists from all over Europe, Russia, Canada and the United States were involved in the programme. The emphasis was on collaboration between the various projects because it has already been found that considerable mutual benefits can be gained through comparisons between different approaches to transport problems. The overall problem was highly complex, and was best tackled by taking a variety of different approaches and seeking harmony between them.
The projects covered a variety of activities under three main headings:
1) Dynamical system approach to advection and transport
The key point here is that even very simple velocity fields such as 2D laminar time dependent or 3D steady flows lead to chaotic trajectories of the test particles. In order to gain a better understanding of the processes involved therefore, it is necessary to study the detailed properties of relevant geometrical objects such as particle trajectories, attractors and repellors. These objects often exhibit self-similar and fractal structures which can therefore be broken down and compared with experimental data using techniques that have already been developed in the fields of linear dynamics and chaos theory. Such a dynamical approach provides an understanding of remarkable phenomena that can occur such as super-fast mixing, diffusion, and trapping of particles.
A variety of topics are being covered under this heading, including anomalous transport, advection in flows with molecular and turbulent diffusivity, open flows, dynamics of non-neutrally buoyant passive particles with finite size, and transport of chemically and biologically active components. There is considerable overlap between these, with several requiring better understanding of how advection affects flows and the consequent impact on particles contained within the fluid system. Under the heading of open flows for example, advection in quasi periodic and chaotic open flows is being studied.
Another common factor shared by several of these topics is the need to develop new analytical techniques to study some of the flows involved. Very few relevant theoretical or experimental studies have been performed for example on the transport of chemically and biologically active components, a consequence of which is a dearth of suitable chemical or biological models.
2) Influence of coherent structures on turbulent transport
Turbulent transport has until now been studied largely on a statistical basis treating the whole fluid system as a homogenous whole, ignoring the impact of significant coherent structures and regions of concentrated vorticity. With this approach, model particles are assumed to disperse in a brownian fashion without being influenced by such structures.
Yet numerical experiments on 2D and quasi- geostrophic turbulence and Lagrangian observations in the ocean and atmosphere have shown that coherent vortices have a strong effect on tracer transport and dispersion. A strong impermeability is formed between the inside and outside of such coherent vortices, associated with long-term advection of trapped particles. In addition there is the possible presence of anomalous dispersion at intermediate times.
A number of issues are being studied under this topic: trapping/detrapping of passive tracers in coherent structures; dispersion in a turbulent flow characterised by the presence of vortices; extension of concepts developed for 2D transport to quasi-geostrophic baroclinic flows; study of the relationships between Eulerian and Lagrangian statistics in the presence of coherent structures; and analysis of laboratory experiments on transport in rotating and stratified flows.
These topics are being studied through a combination of in situ measurements, laboratory experiments, and numerical simulations. The former provides real data from large scale environments, but laboratory experiments have the advantage of being able to vary the control parameters. Numerical simulations on the other hand allow theories to be tested and parameters to be varied on larger scales, although with limitations on the complexity of the models.
3) Relevant geophysical issues that need further development
The main emphasis here is on developing and testing new data analysis methods for Lagrangian observations taken typically from neutrally buoyant floats or balloons, and also on identifying relevant statistical parameters for describing the Lagrangian transport process. Under this heading come five topic groups: dispersion properties and floats dynamics in the presence of trapping regions and recirculation zones; techniques for parameter estimation and statistical criteria for evaluating Eulerian flow characteristics from Lagrangian data; dispersion properties and floats dynamics in the presence of trapping regions and recirculation zones; large scale diffusion properties; and passive tracer climatology along cyclone tracks.
Some of these topics require new numerical models or statistical techniques, but in some cases existing models can be adapted.
Activities
1999 Study Centre / Reports on 1997 and 1998 Annual Workshops
Exchange Grants
Summer Schools
1997 Working Group Meetings
See also TAO brochure (PDF 102 KB)
Programme management
Steering Committee chaired by Dr. Peter Haynes, DAMTP, Cambridge, UK
8 contributing organisations
