Osborne Reynolds

Osborne Reynolds was born in Belfast in 1842 into a family of clergymen and scholars. After early engineering work with his father and apprenticeship with a mechanical engineer in London, he took a mathematics degree at Cambridge, graduating as seventh wrangler in 1867. The following year, still in his twenties, he was appointed the first Professor of Engineering at Owens College in Manchester (later the University of Manchester), a chair he held for thirty-seven years.

Reynolds' most famous work came in 1883. He set up a glass tube with a bell-mouth inlet, ran water through it, and injected a thin filament of coloured dye at the entrance. At low flow speeds the dye drew a perfectly straight line down the tube; at high speeds the filament tore apart in a chaotic puff. Somewhere in between lay a transition, and Reynolds showed by careful dimensional argument that the transition was governed by a single dimensionless combination: ρvL/η, now called the Reynolds number. The critical value for pipe flow, approximately 2300, has been a bedrock number for engineers ever since.

Reynolds also introduced the averaging scheme that bears his name — the Reynolds decomposition of turbulent flow into mean and fluctuating parts, and the resulting Reynolds-averaged Navier-Stokes equations, which remain the workhorse of practical turbulence modelling a century and a half later. He wrote on lubrication theory, on the kinetic theory of gases, and on the mechanics of cricket balls.