This volume is dedicated to Professor Stephen H. Davis on the occasion of his seventieth birthday. Steve, as he is known to his many friends and colleagues, has served the fluid mechanics community for a good part of those seventy years. He has been distinguished by election to the National Academy of Engineering and the National Academy of Sciences of the USA. A variety of professional societies have honoured Steve, including the American Physical Society, with the Fluid Dynamics Prize in 1994, and the Society of Engineering Sciences, with the G. I. Taylor Medal in 2001. Among a number of named lectureships, he was selected as the G. K. Batchelor Lecturer in 2003 at the University of Cambridge; this is an honour that befits the third scholar to enjoy a dedicated volume of Journal of Fluid Mechanics (JFM), following those dedicated to George Batchelor himself in 1990 (volume 212) and to Philip Saffman in 2000 (volume 409). Steve's editorial services have benefitted the Annual Review of Fluid Mechanics, the Proceedings of the Royal Society of London, SIAM Monographs on Mathematical Modeling and Computation, and the Cambridge University Press Monographs in Mechanics. However, the community to benefit most from Steve's editorial leadership has surely been the authors and readers of JFM. Steve served as an assistant editor and then associate editor for twenty years, from 1969 to 1989, and then returned as editor of the Journal in 2000.