Reynoldsluvut

Reynoldsluvut, or Reynolds numbers, are dimensionless quantities in fluid dynamics that characterize the relative importance of inertial and viscous forces in a flow. They are denoted Re and defined as Re = rho v L / mu = v L / nu, where rho is fluid density, v is characteristic velocity, L is a characteristic length scale of the problem, mu is dynamic viscosity, and nu is kinematic viscosity.

In practice, Reynolds numbers help predict whether a flow will be laminar or turbulent. Low Reynolds numbers indicate viscous forces dominate and flow tends to be smooth; high Reynolds numbers indicate inertial forces dominate and turbulence is more likely. The exact transition depends on geometry and surface roughness, but practical ranges are well established for common configurations. For example, in pipe flow, laminar flow generally occurs at Re below about 2100, while turbulent flow occurs above about 4000, with a transitional region in between. In external flows, critical Re varies with shape and flow conditions.

Reynolds numbers are used in engineering and physics to scale model experiments to real systems and to design components such as pipes, pumps, aircraft wings, automobile bodies, and biomedical devices. The concept originated with Osborne Reynolds, whose 1883 experiments demonstrated the transition between laminar and turbulent pipe flow, leading to the naming of the quantity.

Limitations include that Re is a simplifying, geometry-dependent measure. For non-Newtonian or compressible flows, heat transfer effects, multiphase flows, or flows with significant chemical reactions, the simple Re expression may be insufficient and alternative formulations or additional dimensionless groups are used.