What is Reynolds number and what does it indicate?

Reynolds number (Re) is a dimensionless ratio of inertial forces to viscous forces: Re = rho*v*D/mu, where rho is density, v is velocity, D is characteristic length, and mu is dynamic viscosity. It indicates flow regime: Re < 2100 for laminar flow (smooth, parallel streamlines), Re > 4000 for turbulent flow (chaotic, mixing), and 2100-4000 for transitional flow. Reynolds number is fundamental to predicting friction factors, heat transfer coefficients, and mass transfer rates.