Albert Alan Townsend. 22 January 1917 — 31 August 2010

Alan Townsend is renowned for his significant contributions to understanding turbulence. He began his research career studying nuclear physics but this was disrupted by the Second World War, after which his focus shifted to the study of turbulent fluid flow. Collaborating with George Batchelor (FRS 1957), Townsend conducted pioneering experiments that uncovered the spatially intermittent nature of small-scale turbulence, profoundly shaping subsequent theoretical advances. He further shaped the field by pioneering an approach highlighting the importance of eddies, localized regions of coherent or well-ordered motion within turbulent flows. His work distinguished the eddies underlying different classes of flow, particularly turbulence in open flows, free from solid boundaries, such as jets and wakes, and flows near such boundaries, such as flow over an aeroplane or ship or within a pipe or channel. A significant seminal contribution was his attached eddy hypothesis, which provides the basis of a model for how eddies in surface-bounded flows exert control over the transport of momentum, energy and other properties within the flow. The enduring relevance of this work resonates through contemporary research, challenging established paradigms and guiding ongoing research endeavours across the globe.