Professor Stewart Turner, geophysicist

Professor Stewart Turner

Professor Stewart Turner is a geophysicist with a distinguished career spanning three continents. He began his research in theoretical nuclear physics before changing to cloud physics and finally to the physical processes in the ocean. After spending many years at Cambridge University and the Woods Hole Oceanographic Institution in Massachusetts, he returned to Australia as Foundation Professor of Geophysical Fluid Dynamics in the Research School of Earth Sciences at the Australian National University.

He established the connection between the physical processes in the ocean and liquid rocks (lava and magma), and wrote the influential book, Buoyancy Effects in Fluids. His research also discounted the suggestion of towing icebergs from Antarctica to arid coastlines as a source of freshwater. He has received numerous awards and continues to be active in research as Emeritus Professor and Visiting Fellow at the Australian National University.

Teachers' notes to accompany this transcript.
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Interviewed by Dr Trevor McDougall in 2004.


Early years in Sydney, London and New York

Stewart, would you tell us where you were born, and something about your family background?

I was born in Sydney in 1930, the only child of loving and very supportive parents. My father, being the son of a teacher in small, one-teacher schools, had grown up in little country towns around New South Wales. He eventually went to Sydney University on a Teachers' College scholarship and did a BSc and MSc. As a result, he was awarded an Overseas Scholarship and went to Cambridge to do another undergraduate degree, a BA in mathematics. He was the first member of the family ever to go to university.

My father returned to Australia to marry my mother, who was also trained as a teacher but (as was the custom) had to give up her job on marriage. He taught for a very short time in schools and then joined the Sydney Teachers' College staff, becoming head of the Mathematics Department and eventually the College's highly regarded Principal.

As I grew up, my father always made it clear that he wished he could have continued with more study and research in his first love, mathematics. He regarded an administrative job as second best, yet he did have a significant effect on the teaching of mathematics in Australia. Through his interest in comparative education he also affected the organisation of schools. A great advocate of decentralisation, he believed that it was ridiculous for teachers to be moved in and out of country areas as they got promotion: it made more sense for them to be able to make a career within a given country area. I believe he had a significant effect on the ACT getting its own education system, independent from New South Wales.

I guess that through all of these achievements my father remained a frustrated mathematician. But his background gave me an appetite for science and for travel.

Where did you go to school?

For the first two years I walked from our home in Mosman to Middle Harbour Public School. But when I was seven my father got a grant to study in Britain and the United States, so I went next to a state school in an outer London suburb. I have vivid memories of going to the Science Museum in London with my mother while my father was studying. After just under a year we moved on to New York, where my father did a PhD at Columbia University, and I went to a very different, very progressive school attached to the Columbia Teachers' College – where I did my first experiments!

We arrived back in Australia just two weeks after war had broken out, after we had spent 11 weeks driving across the United States and then travelled across the Pacific by ship. There is a family story that soon after I returned I got terribly upset because I failed an exam in geography (I didn't know the rivers in New South Wales. They didn't teach me that in New York!).

Tell us about your secondary school time.

I went to North Sydney Boys' High School, which was a short tram ride away from home – a selective school and very good, with a strict but excellent headmaster. The school was concentrated on academic studies and sporting activities, giving a much more narrowly based schooling, with far fewer options, than the schools that our children went to in Canberra 30 years later.

My sporting activities at high school were pretty limited. A recurring childhood illness took me away from school for quite a while, but I did well academically and finished the Leaving Certificate with honours in mathematics. My extracurricular activities included a lot of reading and also doing odd jobs about the house. I was good with my hands and enjoyed fixing things, and an old next-door neighbour taught me carpentry, which I have been very grateful for ever since. Also I belonged to the Scouts, enjoyed camping and quite often went swimming at the beach in Manly, where both my sets of grandparents lived.

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From engineering to physics

You went to Sydney University as an engineering undergraduate but you graduated in physics. What caused you to change direction?

Well, I got a University Exhibition in engineering, which covered fees in those days. (At first I was living at home.) Like a number of my contemporaries, I chose engineering because it seemed to leave many more options open than going initially to a science degree. I had three science subjects at the advanced level plus four engineering subjects in the early years, including things like workshop practice and mechanical drawing, which have been useful background ever since.

In third year it was common for engineering students to spend a year doing their science and get a science degree on the way to engineering, so I did third year physics and mathematics. I then went on, however, to an Honours degree in theoretical physics, graduating with first class honours and sharing the University Medal and also the prize for general proficiency over the science course.

The general proficiency prize was shared with David Buckingham, a theoretical chemist, but the result in physics was a very curious one. The University Medal was split four ways, among Neville Fletcher and Brian Robinson – now also Fellows of the Australian Academy of Science – Bill Smith and me, and there were also four other first class honours graduates that year. It was a very good year. Professor Bailey, the Professor of Physics, was regarded as completely crazy for having given so many Medals to a class, but perhaps he was justified by the subsequent performance of those graduates.

Can you tell us something about the influences you felt as an undergraduate?

One very strong influence on me, then and since, was Owen Phillips, whom I met soon after going to the university. We became practical physics partners in our first year and actually won the prize for practical physics, and we decided to stay working as partners in our second and third years as well. (Owen did mathematics Honours, however, not physics.)

The two people who ran the laboratory courses – Phyllis Nichol in first year and Phillip Guest in third year – were very good. Phyllis Nichol gave me good habits of recording in experimental notebooks: you always write it down in a book, not on a piece of paper. And Phillip Guest set up a really good series of experiments for the third year class, to give a thorough general training. Of course, although I am throwing these Christian names around, in those days there was no way that anybody would call their lecturers by Christian names, nor did they call us by anything but surnames.

Another important thing was that after two years living at home I became a resident of Wesley College, in the University of Sydney. So for my final two Science years plus my Masters year I was living in college on the university campus.

Your Masters degree from the University of Sydney was also in physics rather than engineering. How did you choose the topic of that degree?

I had done so well in physics that it seemed worth continuing with that, and not to return to engineering, so I got a grant to do a Masters degree in the physics department. By reputation Dick Makinson was the brightest member of the physics staff, so I did an MSc with him on a theoretical nuclear scattering problem. It seems a rather odd choice, looking back, but I chose the supervisor rather than the subject. I published three papers, two with him and one on my own, which I am sure played a big part in my later getting an Overseas Scholarship.

Why didn't you continue with similar research after your Masters?

Towards the end of my Masters year I was invited by a new young lecturer in the theoretical physics department at the ANU to come and give a seminar. In the audience were both [Sir Ernest] Titterton, who was rather kind, actually – he asked a gentle question – and [Sir Mark] Oliphant, who was very sceptical of the whole problem I was working on. He said right out that he didn't think that was the way to solve problems in nuclear physics. This reinforced my own notion that although I got a good training and background from Makinson, it wasn't my thing. It was just too hard even to explain to my friends what I did! Much later, after I had been elected to the Academy, I told Mark Oliphant about this incident, and the negative but important effect it had.

You then moved into the Cloud Physics group at CSIRO. How did that come about?

The Chief of the Division of Radiophysics, Dr E G (Taffy) Bowen, was interested after the war in the use of radar in radioastronomy – things they had seen during wartime radar studies – and also to track clouds, to look at rainfall. By the time I was graduating, he had broadened the program to a general cloud physics group and he wrote to various members of the graduating Masters class asking if we would be interested to come and talk with him about joining the Division. It came at a very good time for me, when I thinking about changing subject anyway, so I went into his Cloud Physics group. (Brian Robinson went into Radioastronomy – and, later, Neville Fletcher came back to the Radiophysics Division after doing his PhD in Harvard.)

Your research project involved quite a lot of fieldwork, I believe.

Yes. I must say that Bowen was a marvellous first boss for a young researcher to have: he set me on a good problem and he let me loose. The problem was trying to understand how raindrops grow on salt nuclei, and this involved measuring the salt concentration in drops as a function of drop size. At first we set up two wind tunnels, a sort of raindrop spectrograph, to separate out the drops according to size. One of these was near the coast in the Radiophysics Division and the other up in the Blue Mountains. I spent the best part of a year collecting rain and measuring it.

But it turned out that rather little of the rain was of the kind we really wanted to study. We were interested in 'warm rain', which forms on salt nuclei, rather than the 'freezing rain' which forms higher up from the melting of ice particles. So Bowen decided that the place to do this study was in Hawaii, where the trade winds regularly come in from the sea in the right season and where, with several mountain stations, you could do what we had been trying so unsuccessfully to do in Sydney.

I went there on a Rockefeller Grant that Bowen had obtained, but in the first month I was there it didn't rain at all! Here was I, a young researcher with my first project, my whole career hanging in the balance, and the conditions were untypical. In the second and third months it did actually rain and I got some good results which turned into part of my PhD thesis in Cambridge, but the whole experience had another important, negative effect: it put me off fieldwork forever. I was driven back into the laboratory, where I have stayed ever since.

How long did you stay with CSIRO at that time?

Altogether it was about 18 months, during the latter part of which the award of an 1851 Exhibition Overseas Scholarship was announced. They were extremely flexible. I had applied on the basis of nuclear physics but I was able to persuade them to accept my change of field to cloud physics, and also to defer the tenure of the scholarship for a year until I had finished the work in Hawaii.

One other memorable thing for me was that I got a personal letter from the Chairman of CSIRO, Sir Ian Clunies Ross, congratulating me on the awarding of the scholarship. I wonder how many heads of CSIRO would still do that.

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A move to Cambridge

When did you travel to Cambridge to begin your PhD studies?

I went by sea in 1954, in time for the October term. The shipping lines had a scheme whereby people who had got scholarships to support them in Britain were given free first class passages – in the off season, so it wasn't quite as high minded as it might have been! But we were required to dress for dinner, so I needed to buy a dinner jacket and attend a formal dinner every night with a group of people who were very different from the poor student travellers.

The custom in Cambridge is that you enrol in a college, not in a department, and it seemed very curious to enrol in a university where no department knows until term begins how many students it has got. Anyway, I enrolled in Trinity College – mainly for sentimental reasons, because it had been my father's college, although in fact it has the top reputation for science and mathematics. At that time even research students were under strict rules appropriate to undergraduates: they wore gowns after dark and had to be in with a curfew of 10 o'clock. It was a very different scene to the way students behave now!

Which group were you attached to in Cambridge?

Because of my connections with CSIRO Cloud Physics I had corresponded in advance with people in the Meteorological Physics group. I hadn't been there very long, however, when Owen Phillips – who had gone straight on to Cambridge two years ahead of me, without doing a Masters degree – persuaded me that the Fluid Mechanics group, where he was working, was much more distinguished and exciting.

George Batchelor and Alan Townsend had gone there to work with Sir Geoffrey Taylor (G I Taylor), and they had begun to build up a group of, particularly, overseas students around them. The group was a very loose kind of connection between Batchelor's students in mathematics and Alan Townsend's in the Cavendish Laboratory, which is in physics. But they all shared offices in the Cavendish space. There was no doubt; I was easily persuaded that that was the way to go.

Taylor had an enormous influence on that group and indeed everybody in Cambridge – a grand old man of fluid mechanics in so many different fields. Incidentally, his friends and senior colleagues called him 'GI' and I will probably refer to him that way too, but students were very much more formal: it was always 'Sir Geoffrey'.

Who supervised you, Stewart, and what projects did you work on?

Well, Batchelor, Townsend and GI all had a strong influence on me, but I was initially supervised by Alan Townsend, who was the experimenter. I started off on an experimental study of drop collisions, suggested by some of the things I had done in Sydney. After about a term, Alan came back to Australia on leave and GI became my temporary supervisor. It also seemed to be a good time at which to change direction.

George Batchelor had a theoretical student, Bruce Morton, working on a study of convection. (This was really following up a line that GI had pursued during the war and, without Batchelor's knowledge, was just coming back to.) George saw that some experimental tests of Bruce's theory would be useful, so I, by that time having a label 'experimenter', did the laboratory experiments. They turned out well, and there was a good correspondence between the theory and the experiment. It led to a paper – much quoted since – by Morton, Taylor and Turner; GI added his name to it at the last minute but made it very clear, in a very generous footnote, that he attributed the main work to Bruce and me.

After that, GI found it hard to suggest another problem for me to work on, but he showed me some correspondence with my boss in Sydney, Taffy Bowen, in which Taffy had sent photographs of explosions which formed vortex rings and had asked GI whether this was a possible way of seeding clouds with silver iodide. GI suggested that perhaps the cloud buoyancy played a part in sharp rings being formed.

So, using water and methylated spirits in a laboratory tank, I started an experimental study of buoyant vortex rings. I also came up with a theory which explained my results. GI, however, was not really very convinced that students could produce useful results. When I showed him the connection between my experiments and the theory, he looked at it for a while and said, 'But this is a definite result!'

This also had applications to the nuclear explosion clouds. I came across a photograph in Picture Post, an English magazine, which showed a very sharp red-hot core poking out the edge of an explosion cloud from an American test. Just at that time the Maralinga tests were being carried out in Australia, and – presumably spurred by GI – somebody from the Atomic Weapons Research Establishment came to see me and started asking about these vortex rings. It appeared that their predictions of the height to which the clouds had gone in the South Australian explosions were wrong by a factor of 2½. They had gone much higher than expected. My experiments and the theory showed why that could be so.

I narrowly avoided having my PhD thesis 'classified' as a result, but because I had been working on it from a totally different point of view it did see the light of day, being published as an internal report of AWRE.

What was the overall theme of your PhD thesis?

I worked on a number of problems, all vaguely related to cloud physics though not in name. After two years I had four pieces of work which were just about ready for publication: one with a fellow student, Philip Saffman, which then appeared as the second paper in the very first issue of the Journal of Fluid Mechanics; the two things that GI had started me off on; and my work in Hawaii. George Batchelor suggested that it might be time to write a thesis applying for a Fellowship at Trinity College, and we decided on the title that covered all the things I had done, 'Dynamical aspects of cloud physics' – very specific! I missed out on the Fellowship, but George suggested that I had done enough work to merit a PhD so I put it in again, as a PhD thesis, and the degree was awarded a few months later.

Would you like to say something about the friends you made in Cambridge, and your recreations there?

In addition to Owen and Merle Phillips and Bruce and Alison Morton, I became friendly with Harold Grant, a Canadian fellow experimental student, and his wife Teddy (they have remained our friends in Canada since then) and also particularly Tom Ellison. He was in the Meteorological Physics group when I arrived, and I kept going to their seminars although I had technically moved to the Fluid Mechanics group. Tom remained a good friend for many years after that, and later we worked together in Manchester.

Singing was a very important recreation for me. I was untrained but while I was at the University of Sydney I had enjoyed singing with the University Musical Society, and throughout my Cambridge years I was in the Cambridge University Musical Society – and later, at the Australian National University, I belonged to the choir for a while.

Walking in the Scottish hills was also a considerable recreation, with friends I had made through a Presbyterian youth group. Their families asked me to their holiday houses in Scotland during the vacations.

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New environments: marriage, and methane in coal mines

You were married in England at that time. Tell us how you met Sheila, and where you went to after your PhD.

We met in London, where Sheila was a trainee nurse. We have both said since that we are glad we didn't meet until after I had finished my PhD; I have got too much of a one-track mind and I wouldn't have been a very good companion during those years! We became engaged after I had begun a post-doc at the University of Manchester, and we were married in the second year of that post-doc, in her home town of Formby, in Lancashire. Our honeymoon included walking in the Scottish hills; hill walking – or later, in Australia, bushwalking – has been a very important recreation for us both.

What project were you involved with at the University of Manchester, and how did it work out?

Before I left Cambridge after my PhD, I had a bit of extra time on the 1851 Scholarship so I started on another project, on mixing in stratified flows. This research then led the Safety in Mines Research Establishment to offer me a contract to work in the Department of the Mechanics of Fluids at the University of Manchester. The problem was to understand the mixing of methane in coal mines; the methane forms layers which still result in explosive mixtures and cause disasters in mines around the world. Tom Ellison joined me there. Being what was then called a 'gentleman of independent means', having inherited family money and not really having to work, he came and worked on the project purely through friendship to me, interest in the problem and an enjoyment of doing research, at no cost to the grant.

We did experiments, using salt water and fresh water, on the mixing of wall layers into a surrounding flow. The established practice was to try and ventilate mines in accordance with gravity: you ventilated uphill because the light methane layers were flowing uphill anyway. We came to the opposite conclusion – the most effective way of getting rid of the layers was to ventilate downhill; they mixed much more vigorously and went below explosive concentrations very rapidly. The monitoring laboratory in the Research Establishment repeated our experiments on a larger scale to check that we were talking sense, and within a year our recommendations were in the mining regulations in Britain. It was one of the most satisfying transfers of pure research into action, and certainly the most rapid, of anything I have ever done.

What led you to return to Australia when you did?

Well, during the whole time we were living in Manchester I had the notion that I wanted to come back to Australia soon. So, when Taffy Bowen happened to be in Manchester for discussions with the engineers there about the building of the Parkes telescope, I had a conversation with him about the possibility of returning to his Radiophysics Division – from which, incidentally, I had been on leave during the whole of my PhD. I also had some correspondence with Bill Priestley, the Chief of the Division of Atmospheric Physics at the time, who invited me to go there as an alternative. But for largely family reasons we decided to go back to Sydney, and I took up my job again in the Cloud Physics group.

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A return to CSIRO leads to a US opportunity

What kinds of problems did you work on in Taffy Bowen's group when you returned?

There were various laboratory-based convection problems – mixing processes, with experiments and associated theory, and following up some of the PhD themes that I had begun in Cambridge. I also did my first numerical experiment, using the then very new computer, SILLIAC, in the Physics Department of the University of Sydney. (That involved detailed programming and punching of tapes, feeding them through, waiting for results and so on.) I worked on that problem with Pat Squires, another member of the Division. It was interesting and we got some good results out of it, but again it was not really to my taste and I have ever since been much more interested in analytical models than numerical ones.

I think you also did some lecturing at the university. How did that come about?

That too happened somewhat by chance. Tom Fink, who had been in the aeronautics department at the University of Sydney, was appointed to the chair of Mechanical Engineering, and within a very short time he had transformed the department, made the course much more attractive, and got very good students and so on. He also had the idea that he would bring in people to give specialised courses in various subjects related to mechanical engineering, and I gave a course to the final year on turbulence. In fact, two of those students turned up in Cambridge later, as students (but not mine) in the Department of Applied Mathematics and Theoretical Physics.

After only a couple of years in Sydney you were again given an opportunity to travel, this time to the USA. I would be interested to hear about that.

In September 1961, my second year back in Sydney, I gave my first talk in the Academy Dome – a review paper on theoretical and laboratory models of convective clouds – at an international cloud physics conference which brought rain and cloud physicists here from around the world. One of those people was Eric Kraus, from the Woods Hole Oceanographic Institution in the United States, who had previously worked in the Sydney group. He must have been impressed by my paper because a couple of months later, out of the blue, I got a letter from the Director at Woods Hole offering me a Rossby Fellowship, which was support for a year and return fares for myself and family. Sheila and I were very happy in Sydney at that time with our first son, Bruce, who was a year old. But an opportunity to be away for a year in an interesting place seemed just too good to miss.

We must have had an inkling, though, that it would turn out to be more than a year, because we decided to go in a manner that allowed us to stay away. We sold our house, which was already rather too small for us, and got immigrant visas – actually, on the basis of the British quota through Sheila, because at that time the Australian quota was tiny. And in fact it was another 13 years before we got back permanently.

What research did you get involved with at Woods Hole after you arrived?

Soon after I arrived I met a very significant influence on my career, Henry Stommel. He was then a Professor at Harvard, although he had been on the Woods Hole staff and still lived near Woods Hole. He had written a notable paper on convective mixing into cumulus clouds, which of course I knew about from the cloud physics days. But when I arrived in Woods Hole he was clearly much more excited about oceanographic problems he had been looking at recently, and in particular he told me about an experiment that he and two colleagues had just done on the 'perpetual salt fountain', which he called an oceanographic curiosity. It led to a paper by Melvin Stern a few years later which was the basis of the whole field of 'salt fingers' and 'double-diffusive convection' – which has formed a large part of what I have done ever since.

Henry was interested in another aspect of the work. In a situation where hot salty water lies on top of cold fresh water, you get long narrow convection cells which are called salt fingers. In the opposite case, where you have hot salty water below cold fresh water, you get a series of layers forming. This case had been predicted by Stern but nobody had actually looked at it. Henry was just about to do some experiments when I arrived, so we did them together, first in a very elementary way like putting a cylinder of stratified water on a radiator overnight – we decided the next morning that we had got layers but for the wrong reason, because of the heating from the side – and then properly with the heating just from below. We published a paper together.

I went on to do a study of the transport of heat and salt through a sharp interface called a 'diffusive interface'. But on that paper Henry declined to be a co-author. It was typical of him to dip into a subject and get people excited about it, and then for us to discover after a little while that he wasn't himself actually going to get involved. Having begun the problem he was content to let other people do it.

Tell us some more about your colleagues at Woods Hole at that time, and how you interacted with them then and in later years.

During that time I worked on a number of other problems: I followed up some of the cloud physics problems that I had done before, with models of evaporation and condensation, and did a laboratory model of a tornado vortex which I think was quite convincing, one of the few rotating experiments I have ever done in my career. At the time it caused quite a lot of interest.

I also had a fruitful collaboration with Eric Kraus, who had got me to Woods Hole in the first place. We did a theoretical and experimental model of the seasonal thermocline, which I think has stood the test of time. It has been the basis of a number of more detailed models.

A very stimulating group of theoretical meteorologists and oceanographers was there at the time: Henry Stommel, Melvin Stern, Duncan Blanchard, Eric Kraus and others including George Veronis, who has become a long-term collaborator. Shortly after I left, a number of these people moved to senior positions in US universities, but they kept coming back for summers. Many of them had houses in the area, and they formed a very stimulating group with a long-running geophysical fluid dynamics (GFD) summer school. So it was always a stimulating place to go back to.

Sheila and I soon decided that a year in Woods Hole would really not be long enough so I applied for, and was granted, a second year on the Rossby Fellowship – during which our daughter Sandra was born in the local hospital. Then for another 18 months I went on the staff at the Oceanographic Institution as an Associate Scientist.

Your next move was back to Cambridge but you had a continuing connection with the Oceanographic Institution, didn't you?

Yes. I accepted an invitation from George Batchelor to go back to Cambridge. When we left, there was no assurance that I would ever return to Woods Hole. But it turned out that for many of the years while I was in Cambridge I was offered my old job again for two or three months of the summer. I was supported by a small part of the large Office of Naval Research grant, and I wrote a one-page proposal about what I wanted to do and I did it. The stimulation of the place was that there were such productive interactions between the seagoing oceanographers, experimenters like me and the theoreticians. We would all be interested in different aspects of similar problems, particularly observations of layering and salt fingers in the ocean and so on.

Sometimes one person initiated the project – an observation triggered the experimenters to do something, or I would do an experiment and somebody would go out and look for it in the ocean. It was extremely important for me when I moved to a much more theoretical environment to have this continued contact with the variety of people who knew about the real ocean and kept me on track with real problems.

Another important collaboration during one of the summers was with Melvin Stern, when we did the first experiments by means of an analogue system to model temperature and salinity. We used instead two solutes with different diffusivities, salt and sugar. They have been used for many experiments since because you don't have any problems with heat losses from side walls. I also, incidentally, had one dive in the submersible Alvin, which was run by Claes Rooth, another summer visitor. But I was not tempted to keep going with the observational side of oceanography.

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Back to Cambridge

What post did you return to in Cambridge, and how did your role develop?

When I left Woods Hole, Henry Stommel was very clear that double diffusion was exciting and I should continue working on it – which I certainly did, both in Cambridge and on visits back to Woods Hole. The post in Cambridge was initially a rather ill-defined one which George called Research Associate. It then became Assistant Director of Research, at first supported by a grant from the British Admiralty but later, after a few years, as a proper university post. By the time I left I had a personal Readership.

The contract had the broad title 'The Dynamics of the Upper Ocean', within which you could do practically anything, and my intended research certainly fitted in with it. The grant supported many distinguished people during about 14 years. George Batchelor was the first Principal Investigator, and then Owen Phillips, who wrote a notable book with the same title as the contract, Dynamics of the Upper Ocean. John Elder and I came next, and later the grant supported Adrian Gill and Herbert Huppert and Paul Linden, plus our technical staff. The output was increased by the students we had, although they were never supported directly under the grant.

My main administrative tasks were to run the laboratory and to be the overall administrator of the Admiralty grant. During my years away from Cambridge, the loose connection between Batchelor's mathematicians and Alan Townsend's experimental students had become formalised when George succeeded in establishing the Department of Applied Mathematics and Theoretical Physics (DAMTP).

The fact that it had a laboratory was most unusual for a department of that name. In order to get it at all, George had to fight very hard against opposition from the physics and engineering departments, who asked, 'Why would a theoretical department try and do things that we are already doing?' But he won through in the belief that applied mathematics, particularly in fluid mechanics, was best carried out with a detailed knowledge of the phenomena and, particularly, close to experiments happening in the same laboratory. This attitude has certainly been justified by the continuing success, both in that laboratory and in others (including mine) started by people who have been in that environment.

What were your most significant achievements in Cambridge at that time?

I was able to spend a very large fraction of my time doing research, with freedom to choose any problems in the broad context of ocean dynamics. So I followed up new ideas and earlier ones, some of them generated by the summer visits to Woods Hole. I worked on differential transports across interfaces due to both mechanical mixing and double diffusion; the structure of salt fingers; convection from sources in closed regions; and a theory of spilling breakers – a one-off, sideways thought. I collaborated and published with various colleagues in DAMTP: Adrian Gill, Francis Bretherton, Herbert Huppert and Michael Longuet-Higgins. I also particularly enjoyed working with a number of long-term visitors: Doug Baines, from Toronto, Tony Chen, then from Rutgers, but later at the Unviersity of Arizona, and Tim Shirtcliffe, from Wellington.

Would you like to comment on the students you supervised in Cambridge?

I have never had a large number of research students during my career, but many of them in Cambridge were from Australia and New Zealand. I will comment particularly on the ones who have stayed in fields close to mine.

Paul Linden was an early student who stayed on in Cambridge for many years, and in fact became the head of the laboratory after I had left. Peter Baines and Peter Manins returned to CSIRO Atmospheric Research, in senior positions, and stayed there for a long time. And you, Trevor, were my last student in Cambridge – I ran out on you to come back! But I think you would agree now that it was better to finish your PhD with Paul Linden and to come back later on a QEII Fellowship and Research Fellowship to the ANU, having the best of both worlds.

On the whole, I never actively worked with my research students in the laboratory. I set them what I hoped was a good problem and let them get on with it, and encouraged them to publish on their own. (Most of my own published collaborative research, both in Cambridge and at the ANU, has been with senior colleagues and visitors.)

Would you like to comment in more detail on the general style of research at DAMTP in Cambridge, and how it affected your research?

The style of research in fluid mechanics, both when I was a student and later when I went back to the new department set up by George Batchelor, developed directly as a consequence of George's contact with and his admiration of G I Taylor and his way of approaching research problems. GI had a unique style of investigating significant problems directly and simply, identifying the underlying physical processes and then studying them in as direct a way as possible.

Within his context he was a powerful but simple mathematician who didn't ever seek to learn new techniques. But he was a unique experimenter: he did lovely experiments, with simple apparatus. His approach is summed up in the title of a symposium which was held in Cambridge to celebrate the 100th anniversary of his birth, 'Fluid Mechanics in the Spirit of G I Taylor'. I learnt very early not to try to compete directly with the extraordinarily well-qualified mathematicians in Cambridge, but to complement their work by doing my own thing as an experimenter.

I strongly believe in a statement that GI made when he was asked to comment on how he would plan a strategy for research: ' general it seems to me that it is through particular problems which can be subjected to experimental verification or compared with natural phenomena that most advances are made'.

How would you summarise your own experimental techniques, your own style?

I believe that laboratory experiments can usefully be carried out at three levels. The first is the qualitative, exploratory experiment, where you identify the important parameters underlying a phenomenon that you see in nature and, if you are lucky, even identify some new phenomenon.

The second stage is doing a careful series of quantitative experiments, with a small number of parameters controlled and varied, so that you gain real understanding in a quantitative way, in a way that can be extrapolated to the real world by getting your dimensions and parameters right. And I believe in simple, direct measurements – for example, temperature, salinity, conductivity and refractive index, density. I have used a lot of films, both still and time-lapse, not only to measure velocities but also for presenting the work at meetings and as a simple way of telling people about what I have done. It is important, too, to interpret such measurements in terms of theory, to relate the experiments to a simple theory.

But the third stage is one that I have rarely got into myself: to set up a much larger scale model, perhaps on engineering scale, with many more parameters operating at the same time, and to use much more sophisticated techniques to measure everything you can and compare the experimental results with numerical experiments. A number of colleagues in Cambridge and in Canberra have done that, but I haven't personally.

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An influential monograph

While you were in Cambridge, you wrote a monograph which has become quite influential. Tell us about that.

It arose from an invitation to write a review article which appeared in the first volume of the Annual Reviews of Fluid Mechanics, in 1969, as 'Buoyant plumes and thermals'. That was well received by the editors and the readers, and shortly afterwards Owen Phillips – who was by then an editor of a series of monographs at Johns Hopkins University – asked if I would contribute to the series that he was editing for Johns Hopkins Press. The review gave me confidence that I could actually write that kind of thing, and in fact my Annual Reviews article became the basis for a chapter in my book. Over about three years I wrote three drafts, with long pauses while I was spending time in Woods Hole, and I also gave a series of lectures based on the material as the introductory lectures to the GFD summer school at Woods Hole in 1972.

The book was eventually published as a research monograph by the Cambridge University Press, in the series that George Batchelor edited, not by Johns Hopkins after all. That was in 1973. It has gone to a paperback edition since then, because after five years or so it was regarded as a graduate textbook. So in paperback form it has survived; it is still in print after 30 years. It was also published as a Russian translation – unauthorised, so I didn't get any royalties, but nice to have.

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New opportunities in Australia

What led to your decision to come back to Australia, to the ANU?

Well, living in Cambridge was a good experience for Sheila and me and our family. Our second son, Ian, was born while we were there. And summer visits to Woods Hole were particularly good for the family – they had a subsidised summer holiday while I was allowed to talk to colleagues and have a great time doing science! By 1974 I had a Readership, I had just been elected a Fellow of Darwin College and I was feeling that I had specialised myself out of a possible job in Australia.

But then the Australian National University's Research School of Earth Sciences was set up, by the breakaway of the Department of Geophysics and Geochemistry from the Research School of Physical Sciences to become an independent research school, and the new Director, Anton Hales, came to Cambridge during an around the world recruiting trip. He was looking for people to lead a couple of new groups that were being set up, including geophysical fluid dynamics.

The advertisement was for large-scale GFD research but I knew that my interests were in small-scale processes so I didn't apply for the job. Some time later, Anton wrote to me, saying that he had not filled the post and inviting me to come to Canberra 'for discussion with the academic staff on the development of GFD within the School'. It was actually a sneaky way of getting me out there, because although I had taken the precaution of preparing a seminar, I discovered that a full-scale electoral committee meeting had been set up for the time I was there, and my 'advice' was really to answer the question, 'Do you want to come here?'

During that interview they had decided that small-scale experimental work was acceptable for the post, so by the time I left Canberra to go back to Cambridge I had been offered the job.

Sheila recognised, probably sooner than I did, that returning to Australia was such a long-held ambition of mine that this was an opportunity we really shouldn't miss. And the children were at a good stage for moving without disrupting their education.

After negotiations with Anton, the ANU allowed us to translate our customary airfares into passages by sea, on the Oriana. To travel that way on a cruise ship was really a marvellous experience for us all, and it certainly made it clear that we were making a big change in our lives – very different from getting on a plane and 24 hours later having to sort out everything at the other end.

Tell us about your early years in Canberra and your first appointments in the group.

I was appointed as Foundation Professor of Geophysical Fluid Dynamics – in fact, some months before I left Cambridge. Anton asked me to plan the new laboratory in an intended new building which no longer 'existed' by the time I arrived. (The funding priorities had changed.) One PhD student, Ross Griffiths, accepted before I had left England, so he was ready and waiting to start. And soon after my arrival I appointed two technical officers, Ross Wylde-Browne and Derek Corrigan, who have (in 2004) just both received their 25-year long service awards from the ANU: an indication of how important long-term technical staff have been to research schools in the ANU. In Earth Sciences, particularly, they have been the backbone of how we do our research.

The laboratory made do with smaller space in the old building, and gradually we expanded into other rooms along the corridor. But a purpose-built laboratory only appeared after I had retired, when a review of the group said, 'The work's great, but the laboratory space is hopelessly inadequate.' Now there is a marvellous laboratory designed largely by Ross Griffiths and his architect wife Candida, the best GFD laboratory that I know of anywhere.

What directions did your research take in those first few years at the Research School of Earth Sciences at ANU?

Because we weren't able to make any tenured junior staff appointments – which I was promised and which again didn't happen for many years – we relied a great deal on senior visiting fellows. Anton was able to give the group perhaps larger than our normal share of visitors' funds in order to make that happen. I continued with research on double-diffusive processes and Ross Griffiths worked in that area too for his PhD, as did a new Research Fellow Barry Ruddick and Tim Shirtcliffe, who had already been a visitor in Cambridge and came again to the ANU.

A more surprising appointment at that time was of David Stevenson, who had just finished his degree at Cornell. He applied but was quite frank about the fact that, being a theoretical planetary scientist, he didn't at all meet the criteria for an experimental oceanographer. But his referees' reports were outstanding, he was clearly the best applicant, and I persuaded Anton that he ought to be appointed in spite of the selection criteria. I am quite sure that couldn't happen now.

David has gone on to a meteoric career in UCLA and Caltech, so again something has been justified by subsequent events. And perhaps, in a way, in selecting David for appointment I was making use of the sort of flexibility that I had benefited from when I received the 1851 Scholarship and, later, when the ANU decided that research on small-scale processes was acceptable and relevant!

Stewart, you opened up several new areas of research during this time at ANU. Did you have those areas in mind when you arrived from Cambridge?

No, I didn't. Another significant but unforeseen change of direction came about through conversations with Lew Gustafson, who had been appointed as Professor of Economic Geology at the same time as I was appointed in GFD. He told me about various processes of ore formation on the sea floor, which involved hot saline solutions coming through the cracks in the sea floor and crystallising there, and I saw the strong connections with the double-diffusive processes that I had studied with the ocean in mind. So we wrote a review paper together, pointing out these analogies and suggesting that experiments ought to be done with that geological problem in mind. Also, we speculated at the end that similar processes might be important in liquid rocks – in magmas and lavas – because they too have significant temperature and concentration differences.

We then did some laboratory experiments explicitly exploring that idea and showing that you could make crystal columns and grow crystals at side walls in a way that differentiated the different components in a mixed fluid, just like the behaviour of magma chambers. I guess these experiments led to a new field that is now called Geological Fluid Mechanics, and we have broadened the scope of that to study other processes such as solidification of lavas as they flow.

One significant extension was to add crystallisation to the processes studied in the oceanographic context. Ross Kerr worked with me on a problem of crystallisation for six months before he went off to do his PhD in Cambridge, and after several other moves he is now back on the staff of GFD.

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Achievements at the Research School of Earth Sciences

What would you regard as the most important research achievements of the GFD group that you led at RSES?

Most important to me is the interdisciplinary research, establishing the connection between the processes in the ocean and in liquid rocks. The experiments with Lew Gustafson demonstrated that you can produce density differences near boundaries due to crystallisation, and that can stratify the surroundings.

Also there were earlier experiments that I did with Herbert Huppert, a former colleague in Cambridge who came for many periods of sabbatical leave in Canberra. During his first sabbatical we did experiments on melting ice in stratified surroundings – spurred, actually, by a suggestion that you might tow icebergs from the Antarctic to arid coastlines. We showed that in fact the meltwater would not just rise to the surface where it could be scooped or pumped off; there would instead be considerable mixing with the surroundings, and in a stratified ocean none of the water would get to the surface at all. It would spread out in layers in the interior.

We later realised that the crystallisation or melting were essentially the same dynamical process. If you looked outside the boundary at the environment, then in each case you were producing a boundary layer of different composition: in one case with the solid boundary extending by crystallisation, in the other case with the boundary receding, but in both cases with dynamics of the flow outside that were exactly the same. That and some later experiments prompted Herbert and me to write a review article comparing the different processes in geological contexts and oceanographic ones in which double-diffusive convection was important. It appeared in an issue of the Journal of Fluid Mechanics which was the Editors' Volume – after 25 years of the journal, all editors were asked to write unrefereed papers.

But at the end of this review we asked ourselves: is it possible to deliberately organise crossover of information from one field to another? Is it likely that a geologist would read a paper on melting icebergs and draw the right conclusions about the importance in his field? We had to concede that, unfortunately, you can't organise such crossover. Really, that depends on individuals, on people with different perspectives on a common physical problem getting together. It helps if people work in multidisciplinary institutions like the Research School of Earth Sciences and others which have interests across the boundaries of the two fields, but I believe that you really can't organise such interactions from the top down. The boss can't say, 'Let's do this together,' and put people to do it. It has got to come from individuals.

What other interdisciplinary or multidisciplinary collaborations would you say were important to you?

One of the most rewarding collaborations I have had was again a very multidisciplinary one. Herbert Huppert, a mathematician, and Stephen Sparks, a perceptive field geologist, came for six months to Canberra and we worked in the laboratory together on problems of mixing in magma chambers. Each of us brought to the subject something that the others didn't have. We couldn't expect to become an expert in anybody else's field but we needed to understand enough of what they were talking about to be plausible collaborators.

Another important collaborator, Ian Campbell, at first was a visitor from Toronto and then came on the RSES staff. He was interested in replenished magma chambers – magma of different composition coming in to the bottom of the chamber and mixing with the surroundings in a way that caused the precipitation of ore. Particularly, platinum ores can be formed if you have enough mixing between the incoming magma and the resident magma.

A very unlikely interdisciplinary project arose out of that. At the same time Doug Baines – another person who had been a visitor to Cambridge and was on sabbatical in Canberra – had come with an interesting problem of heating large buildings such as aircraft hangars from the top, where you pump in hot air and form a layer which extends down towards the floor without obstructing the floor. We realised that these two problems are physically and dynamically exactly the same, except one is turned upside down. The mixing in magma chambers with a 'fountain' of dense fluid coming in from the bottom and the aircraft hangar with hot air coming from the top could both be studied by doing experiments in salt and fresh water. So we published a paper together which was applicable to both these rather unlikely different problems. (Ian Campbell later left the GFD group and went on to be head of the Ore Genesis group in RSES.)

Also, Ross Griffiths and I collaborated briefly on a study of viscous plumes impinging on a density interface, and you and I had common interests, Trevor, in both oceanographic and geological problems.

What were some areas of research that the GFD group developed without your personal involvement?

When Ross Griffiths first came back to the GFD group as a Research Fellow, he had spent time in Grenoble and Cambridge working on rotating flows and so he was responsible for building a rotating turntable. There are now two of them being used in the new laboratory to model the rotating Earth when people are looking at large-scale motions in the ocean. Incidentally, the design has been commercialised and is being sold around the world to other laboratories.

Ross has now become interested also in different geological applications like solidifying lava flows and so on. Another problem that Ross has worked on is large-scale convection in the Earth's mantle, plumes coming right through from the core–mantle boundary to the surface. And Geoff Davies has introduced another new element into the group with his numerical studies of mantle convection.

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Finding a balance between research and administration

How do you react to administrative and committee work?

Well, I was never enthusiastic about committee work, though I think I served on my share of university committees of various kinds. I recall Anton Hales's advice to me just before he retired: 'It is important for people like you who are reluctant to go on committees to agree to do it, in order to keep those off who enjoy it'! I did quite enjoy my involvement in broader reviews of departments and research schools, because they were related to my scientific expertise, and I did this both at the ANU and in other institutions.

You also served on several national and international scientific committees. Did you feel differently about that kind of committee?

Yes, I did. I felt that on that kind of committee I could contribute more directly to the support of my area of science. A particularly satisfying one was the Australian Marine Science and Technology Advisory Committee (AMSTAC), which I was invited to join by its first chairman, Sir Rutherford (Bob) Robertson – a former President of the Academy. Government did translate a lot of the advice of that committee into action, and an associated funding committee gave some important support to oceanography.

I also served on the Academy's National Committee for Oceanic Research and was a member of the International Scientific Committee on Oceanic Research, and I was Vice-President of the International Association for the Physical Sciences of the Ocean (IAPSO) in the four years leading up to its meeting in Canberra in 1979. I chaired a review of the outcomes of grants in Fluid Mechanics for the Australian Research Council, and following my retirement I have also chaired an evaluation of the fields of Glaciology, Geosciences and Oceanography in the Antarctic Science Program.

I know you have declined several senior administrative positions in various research organisations. I'd be glad to hear what your current view is on doing those jobs.

I am sure that in preferring to avoid administrative positions I have been very strongly influenced by my father's experience and attitude. I have never sought to run any organisation larger than a group of scientists close to my own interests. When it was suggested that I might take on the Directorship of the Research School of Earth Sciences, I knew I could not comfortably handle that in addition to the two things that were important to me: doing my own research and spending enough time with my family. I also turned down an invitation (delivered in person by Bob Robertson and Sir Geoffrey Badger, the then President of the Academy – a high-powered committee) to become chairman of AMSTAC. I said no for, again, similar reasons. I did stay on the committee for another couple of years, but not as chairman.

At the Canberra meeting of IAPSO I declined the invitation of the nominations committee to become President for four years following my Vice-Presidency. That would have taken me into a commitment to overseas travel and to high-level committee work that was just not for me. You were sitting next to me at that meeting when Sir George Deacon, the chairman of the nominations committee, sent me a hand-written message saying, 'Congratulations on being nominated President of IAPSO,' and I think you were surprised at how rapidly I made up my mind to say no!

You did take on a big load of administrative work as Associate Editor of the Journal of Fluid Mechanics, after you arrived in Australia. Tell us about that.

George Batchelor was the founder and chief editor of the Journal of Fluid Mechanics. As I left Cambridge, he asked if I would be an Associate Editor to cover the Australian region and Asia and so on, as they had the policy that authors should send their papers to the nearest editor, one in their own country if they could. I had no doubt that I would agree, because it is arguably the prime journal in our field and has remained so. I stayed as an editor for 20 years until I retired, keeping in touch during that time with engineering fluid mechanics and a whole range of other things I wouldn't have otherwise known about – the broad spread of fluid mechanics in Australia.

The journal has a very unusual system of editing. Instead of an editorial board making joint decisions, each Associate Editor (after appointment by Batchelor and the senior people in Cambridge) had complete freedom about sending out papers, choosing referees, making decisions on whether to publish or not. Only twice in my 20 years as editor did I call on George Batchelor for some ruling on a matter of broader principle. The standards were kept consistent by the comparison of the acceptance rates of all the editors – a carefully guarded secret, which was revealed in six-monthly reports. If one editor was too lenient, he was told so. And if someone was being too tough, he was told to let a few more papers through.

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Scientific recognition and replenishment

You have received many academic honours during your career. Which of those have given you the most satisfaction?

I must begin by mentioning again the 1851 Exhibition Overseas Scholarship, not only because of its long history and the distinguished people who have held it but because of the opportunities it opened up for my PhD studies. Also, the Rossby Fellowship taking me to the Woods Hole Oceanographic Institution was very important in starting me off on a new line of research which has been significant ever since.

Election to the Academy in 1979 and the Royal Society in 1982 were of course very important recognitions of the influence of the research that my colleagues and students and I had carried out over many years.

You have been invited to give several prestigious lectures in Australia and overseas, and you have made many trips overseas for conferences and also on sabbatical. Perhaps you could tell us, first, about some of the highlights of those trips.

Professors at the ANU received extremely generous support for travel and leave arrangements. I managed something like one major overseas trip a year during my ANU time, and I didn't use up all my sabbatical leave. Most of my periods of leave were taken in Cambridge, collaborating in particular with Herbert Huppert in my old department of Applied Mathematics and Theoretical Physics.

A particular highlight was a long period of leave in 1985, when I gave a couple of series of lectures as I travelled across the United States to take up an Overseas Fellowship at Churchill College, in Cambridge. I can say now that, of the three Cambridge colleges that I have been associated with, Churchill has been the most generous. I am still eligible for two weeks of free accommodation any year I care to go there – and I have taken it up several times since the longer period there as an Overseas Fellow.

I was also very pleased to be appointed a 'Sherman Fairchild Distinguished Scholar' at Caltech in 1993, another institution with high standards and great freedom for people to do what they wanted to do. I was sponsored by the Division of Geological and Planetary Sciences, but in fact I also managed to do an experiment involving people in two engineering departments.

What were your most memorable lectures?

The most memorable one was a 'Friday Evening Discourse' in 1985 at the Royal Institution of Great Britain. This is a private foundation, established in 1799 in a building in central London, which has had various long-term Directors, including Michael Faraday for something like 50 years. The Royal Institution supports research – some of Faraday's research, where he discovered electromagnetic induction, was done in the basement laboratory there – and also has a long tradition of public lectures such as a series of Christmas lectures and the Friday Evening Discourse series.

The Friday lectures are very formal occasions, both the speaker and the audience being in evening dress. The speaker is expected to do experiments in the hour of the lecture, which is given at the same bench where many important discoveries were announced by well-known names. It is the most daunting after-dinner speech that I have ever given. After dinner in the Director's suite, the speaker and the Director go to a room where they are locked away for 10 minutes, a tradition going back to the time when Wheatstone – of Wheatstone Bridge fame – disappeared out into the street before his lecture and wasn't seen again that night!

After this period of enforced chat with the Director, you process through the hall into the lecture room, arriving and starting precisely as the clock strikes nine. There is no introduction, only some literature as background to what you are supposed to be doing, and you just plunge straight in to your subject. I spoke on 'Models of Oceans and Volcanoes' and did experiments which were only possible because the people in my old department helped to put things together in Cambridge before I started. You are also expected to stop precisely at 10 o'clock. My demonstrations worked all right, I was pleased about that, but the pressure to get my timing right was rather extreme.

I was also very pleased to be asked to give the Matthew Flinders Lecture at the Australian Academy of Science. I chose the theme 'Models of Mixing in the Ocean', where the word 'mixing' meant different things and 'models' also meant different things to many people – numerical models, laboratory models or conceptual models of the mixing process.

A notable occasion also was the inaugural G K Batchelor lecture at the 1995 Australasian Fluid Mechanics meeting in Sydney. I talked about 'The Influence of Laboratory Experiments on the Development of Geophysical Fluid Dynamics'. During that conference my colleagues in RSES arranged a dinner marking my retirement, which happened a couple of months later.

Another memorable event was in Vancouver at the 5th International Symposium on Stratified Flows, in 2000. Friends and colleagues arranged a special session of talks to mark my 70th birthday, which occurred earlier that year, and introductory lectures were given on various themes in my research by old friends Owen Phillips, Herbert Huppert, Paul Linden and you, Trevor. A great occasion.

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Mechanisms for the funding of research

Stewart, I'd like to ask a more general question about your research career. Would you care to comment on the way your research has been funded in the various institutions where you have worked?

In each organisation in which I have worked – the CSIRO Division under Bowen, Woods Hole, the University of Cambridge and finally the ANU – I have had extraordinary freedom to pursue my own research interests. In each place my collaborators and I have been funded by part of a large block grant, with a broadly defined area of interest but few detailed requirements to produce stated 'outcomes' at specified intervals.

Outstanding leaders and mentors have been terribly important in their positive approach to this mode of support. Their attitude has always been that they have appointed me because of my record and interests, and it has been up to me to decide exactly how my goals should be achieved. My performance has always been judged by the results and their significance, not by comparison with a defined goal. It was particularly important at the ANU, where I arrived with the label of oceanographer and within a few years Anton Hales had agreed to my moving sideways and doing geological problems. One thing led to another in a logical but totally unplanned way. And the Directors of RSES who followed Anton have also been very good about allowing the GFD group to change direction.

How has research funding changed over the years? In particular, do you think it would still be possible to conduct research in the way you did?

I was never comfortable with grand strategic plans, or individual detailed grant proposals, and luckily I retired before these became the norm. During visits to the USA I became familiar with the huge amount of effort spent by colleagues on writing proposals for the Office of Naval Research and the National Science Foundation, and how envious they were of the way in which my research at the ANU had been funded by a direct grant. All this has changed here too, and younger colleagues are forced to spend far too much of their time writing proposals and administering grants. I certainly couldn't behave now as I did throughout my research career.

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Life in retirement: the excitement of doing something new

What have you have been doing in the years since your retirement?

Well, I have become an Emeritus Professor at the ANU and have been fortunate to be able to keep a room and access to the facilities, including the laboratory. (I spend most days in RSES when we are in Canberra.) It was particularly pleasing to be awarded a University Fellowship for a couple of years, which meant that during 1999–2000 I had some support to travel, to go to international meetings, without turning it into a very expensive retirement hobby.

During the other years I have been a Visiting Fellow in RSES and have continued to write papers and review articles, and to do experiments – a couple of them being done with George Veronis during his periods here on leave. Most of the problems I have worked on since 1995 have arisen from past work: unfinished business, things that had been left lying around without my really having the time to follow them up.

Sheila and I have also travelled more, both within Australia and overseas, especially to visit family. We have also added on extra trips, usually with sightseeing and a walking component, and we are fortunately still fit enough to enjoy these activities together.

I hope to continue interacting with colleagues and students both here and elsewhere, exploring interesting problems, as long as I am still able to do so. I have also begun to sort out my scientific papers and, particularly, my films, and to make some hard decisions about what should be kept and where. This process has been slow – it always seems more exciting to do something new than to spend time sorting through old things!

What has motivated you in your research career, particularly in moving from one place to another or in choosing a topic of research?

What I have enjoyed most is identifying and understanding various novel physical processes. The hands-on experience of making an experiment work in the laboratory, once I have devised it, is great fun.

It is satisfying to feel that I have had an effect on my field of science and have been able to influence individuals within that field. It has not been important to me to actually carry my work through to direct application. I have been content to produce applicable results, things of lasting value which people can pick up later and use, and I really enjoy having my research remembered and used by other people.

I am glad to have contributed to the understanding of various basic physical processes in fluid mechanics. Some of them now have generic names which summarise what the field is about, like entrainment into turbulent plumes; turbulent gravity currents; filling box behaviour (which I won't try and explain!); double-diffusive convection. Each of my moves, to new institutions or into new fields, has been made in order to follow up interesting research opportunities. I think it is fair to say that it has not been through any conscious desire to 'rise in my profession'. In fact, as I look back I realise I have never actually applied for a job; I have just said yes when attractive opportunities have come up!

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Some advice for younger scientists

Do you have any advice for younger scientists?

As I've said earlier, I have been fortunate to have had freedom to pursue the most promising ideas as they have arisen – largely as a result of favourable and far-sighted funding structures and support from Directors and Chiefs. This may no longer be as easy in the present funding climate, but I would still offer young researchers some advice.

You should strive to:

  • concentrate on the most difficult and fundamental problems in your field, but always using your particular training and expertise;
  • collaborate with colleagues as your research develops, and seek people out who have complementary skills, if you see the need to apply different methods which you don't have expertise in; and
  • follow through to publication as soon as possible at each stage of the project. Don't wait till you think you understand everything to your complete satisfaction.

Stewart, thanks for so generously sharing your career with us, and for your thoughts about the process of scientific discovery.

Thank you, Trevor, for helping me to prepare for the interview and guiding me through it this afternoon.

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