REYNOLDS NUMBER (RE OR R)

A dimensionless number used as an index of fluid flow characteristics in a pipe, duct, or around an obstacle. The expression for fluid flow in a pipe or duct is equal to: R = (V x d x p)/µ R = (V x d x p)/µ; where: V is the fluid velocity; d is the pipe or duct diameter; p is the fluid density; and µ is the fluid dynamic viscosity. V is the fluid velocity; d is the pipe or duct diameter; p is the fluid density; and µ is the fluid dynamic viscosity. For fluid flow around a particle it takes the form: R = (dp x vr x p)/µ R = (dp x vr x p)/µ; where: dp is the particle diameter; vr is the velocity of the particle relative to the fluid; p is the fluid density; and µ is the fluid viscosity. dp is the particle diameter; vr is the velocity of the particle relative to the fluid; p is the fluid density; and µ is the fluid viscosity. For fluid flow in a pipe or duct, a Reynolds number below about 2,100 is considered to be streamline, smooth, or Laminar Flow; above 4,000 the flow is turbulent; 2,100-4,000 is a transition zone. For the flow of fluid around a particle, a Reynolds number less than 1.0 is considered laminar flow and as the value increases above 1.0 turbulence increases. The difference between the conditions for laminar flow around particles and in pipes is explained by the impact of inertial forces as the fluid flows around a particle compared to the straight flow in a pipe or duct.