by Norman Riley

© Oxford University Press 2004 All rights reserved

**Stewartson, Keith** (1925-1983), mathematician, was born on 20 September 1925 in Barnsley, Yorkshire, the youngest of three children (two boys and a girl) of George Chalmers Stewartson, master baker, and his wife, Mabel. Following the family's move to Billingham, co. Durham, he was educated at Stockton secondary school (1936-42), from where he won a state scholarship and Kitchener memorial scholarship to St Catharine's College, Cambridge. In two years (1942-4) he completed the requirements for a first-class degree in mathematics and was awarded the Drury Jones prize. War service with the Ministry of Aircraft Production took him to first the gunnery range at Orfordness, Suffolk, and then the Guided Projectiles Establishment at Westcott; there his interest in applied mathematics was confirmed, and he received his first introduction to the problems of compressible fluid flow.

In 1946 Stewartson returned to Cambridge, and in a three-year period of postgraduate work completed part three of the mathematical tripos, with distinction, and a PhD (1949) under the supervision of Professor Leslie Howarth, an authority on fluid dynamics. During this period Stewartson was awarded the Mayhew prize and a Rayleigh prize. In 1949 he was appointed as lecturer at Bristol University, with promotion to reader in 1954. While at Bristol he transformed the fortunes of the university boat club as coach. On 5 September 1953 he married (Elizabeth) Jean Forrester, a schoolteacher; they had three children, two boys and a girl. In 1958 Stewartson accepted the invitation of the University of Durham to take its newly created chair of applied mathematics. During his time there he played a major role in the creation, in 1964, of the Institute of Mathematics and its Applications, and was one of its first vice-presidents. In that year he was appointed as Goldsmid professor of applied mathematics at University College, London, in succession to W. R. Dean. He was elected FRS in 1965.

Stewartson's early research work was concerned with the theory of boundary layers, a topic in fluid dynamics which relates to the layers of a fluid that are immediately adjacent to a solid boundary, though this description is less applicable to his own contributions, some of which involved the extension of boundary-layer theory to the layers which may appear in the midst of a (rotating) fluid without the presence of a solid boundary. His first research paper (1949) concerned the correlation between incompressible and compressible boundary-layer flows, and boundary-layer theory can be traced as a continuous, but not exclusive, thread in his research. Boundary layers are thin regions in which viscous effects, negligible elsewhere in the flow when viscosity is small, are important. Aeronautics in the Second World War stimulated their study, and in this aerodynamic context Stewartson made seminal contributions to the understanding of flow separation, in both the incompressible and in the high-speed compressible situations that can prove so catastrophic. The concept of multi-structured boundary layers which emerged from his studies was to breathe new life into the subject.

In work which he carried out from the 1950s onward Stewartson made important contributions to the study of rotating fluids and to magnetohydrodynamics, extending his analysis from the motion of bodies in a rotating fluid to the motion of bodies in a conducting fluid subject to a magnetic field. His analysis helped explain the nature of the 'upstream wakes' that form ahead of an obstacle in a flow parallel to the rotation vector, or to the applied magnetic field. In both areas his major contributions were associated with boundary layers; in the case of rotating fluids the new types of boundary layer that he introduced became known as 'Stewartson layers'. These two areas of research merged in a geophysical context, where he contributed to an understanding of the effect of lunar-solar precession on fluid motions in the earth's liquid core, of the westward drift of the earth's magnetic field, and of the types of motion in the core that can contribute to the dynamo effect responsible for the earth's magnetic field. An area distinct from those above, which occupied much of his last decade, is stability theory. Laminar flows lose stability to the little-understood turbulent state. Stewartson demonstrated that in plane parallel flow the evolution of a small disturbance may terminate in a 'burst' at a particular location. The same methods were employed to study the stability of stratified shear flows.

Over a period of more than three decades Stewartson established himself as one of the most mathematically profound, and original, applied mathematicians of his time. His development of the influential research school at University College, London, brought more collaborative ventures than previously, as did his frequent visits overseas, most often to the United States. To work with him was a challenging and rewarding experience, and he was never less than generous to his collaborators. Following a heart attack Stewartson died in University College Hospital on 7 May 1983; he was cremated at Golders Green.

NORMAN RILEY

**Sources **

J. Lighthill, *Memoirs FRS,* 31 (1985), 545-69

N. Riley, *Bulletin of the London Mathematical Society,* 17 (1985), 63-76

personal knowledge (2004)

**Likenesses **

W. Bird, photograph, *c.1966,* RS *[see illus.]*

**Wealth at death **

£10,600: probate, 17 Nov 1983, *CGPLA Eng. & Wales*

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