Flowing in pipe 2. The distinguish criterion of laminar flow and turbulent flow Definition As the pipe diameter d and fluid motion viscosity v are constant the average velocity of laminar flow becoming turbulent flow iS DC constant too. This velocity is called supercritical velocity which is expressed by D. The average velocity of turbulent flow becoming laminar flow if constant too and this velocity is called lower critical velocity which is expressed by U>U The flow state has relations with not only velocity of flow U but also pipe diameter d, fluid density and motion viscosity. According to the dimension analytic method in chapter 4 we can build up the above 4 parameters into a dimensionless number which is called reynolds number do ud R11 2. The distinguish criterion of Laminar Flow and Turbulent Flow The flow state has relations with not only velocity of flow but also pipe diameter , fluid density and motion viscosity. According to the dimension analytic method in chapter 4 we can build up the above 4 parameters into a dimensionless number which is called Reynolds number. d  v d d    Re = = Definition： c  c As the pipe diameter and fluid motion viscosity are constant d v the average velocity of laminar flow becoming turbulent flow is constant too . This velocity is called supercritical velocity which is expressed by . The average velocity of turbulent flow becoming laminar flow if constant too and this velocity is called lower critical velocity which is expressed by  c  > c