Osborne Reynolds (23 August 1842 – 21 February 1912) was an Irish engineer. He was born in Belfast, United Kingdom and died in Watchet in Somerset, England. He graduated from Cambridge University in 1867 after studying mathematics. In 1868 he became a professor of engineering at Owen's College in Manchester (a predecessor of the Victoria University of Manchester, merged with the UMIST in 2004 to become the University of Manchester), and was only the second to hold this role in England. He retired in 1905.
Reynolds famously studied the conditions in which the flow of fluid in pipes transitioned from laminar to turbulent. From these experiments came the dimensionless Reynolds number for dynamic similarity - the ratio of inertial forces to viscous forces. In 1877 he was elected a fellow of the Royal Society, and in 1888 he won the Royal Medal.
Reynolds also proposed what is now known as Reynolds averaging of turbulent flows, where quantities such as velocity are expressed as the sum of mean and fluctuating components. Such averaging allows for 'bulk' description of turbulent flow, for example using the Reynolds-averaged Navier-Stokes equations.
A crater on Mars is named in his honour.