PrefaceThis issue contains a selection of papers based on the plenary and invited lectures presented at the meeting 'Theoretical fluid dynamics in the twenty-first century', which was held at Imperial College London on 15-16 December 2006. The principal purpose of the meeting was to celebrate the distinguished career of Prof. J. Trevor Stuart, FRS, as well as his 40 years of active service at Imperial College. More than 80 participants, comprising three generations of fluid dynamicists together with numerous research students, attended this successful meeting.Fluid mechanics, as a subject, has enjoyed huge success in the last century, with the spectacular advance in aeronautics and aerospace technology being a prime example. Theoretical fluid dynamics, being in the dominant position for most of this period, has played a crucial role in many technological breakthroughs. With the advent of powerful computers over the last two or three decades, the subject has arguably now come to crossroads. On the one hand, with computers able to tackle a variety of problems without making simplifications (as a mathematical theory usually does), it is easy to question the importance and relevance of theoretical fluid dynamics. On the other hand, long-standing fundamental problems remain unsolved while current technological developments are giving rise to many more of them. In this ever-changing landscape, the organizers decided that this occasion of celebration should also be an opportunity to reflect the role of theoretical fluid dynamics in the twenty-first century. The theme embodied in the title should be a fitting tribute to Prof. Stuart since his work continues to influence many aspects of current research and to inspire new generations of fluid dynamicists.The underlying themes of the meeting were expertly surveyed by the panoply of invited speakers. These included many of Trevor Stuart's contemporaries, who are themselves prominent fluid dynamicists, as well as leading researchers from younger generations. A total of 16 lectures were presented, which reported on recent (and ongoing) theoretical, experimental and computational progress in fluid dynamics. Naturally, there was a great deal of emphasis on the topics closely related to, or influenced by, Prof. Stuart's own work and interests. In all, a broad spectrum of topics were covered, including hydrodynamic instability, laminar-turbulent transition, vortex dynamics, mixing, aeroacoustics and the mathematical properties of the Navier-Stokes equations. Owing to page limitations and the limited time frame for the production of this issue, only nine papers are featured here; more extended contributions will be, or have been, published elsewhere.Each contributor has, in their own way, explored our underlying theme. The general tenor of this issue is aptly set in the paper by Prof. Hassan Aref (Aref 2008), which argues convincingly that theoretical fluid mechanics will remain a
