Dimensionless number describing the effect of viscosity in different flow scenarios. Ratio between the largest and smallest eddies. $ Re = \frac{\rho uL}{\mu} $ If viscosity is large, $Re$ is small, and vice versa For $Re >> 1$, can rewrite the [[Navier-Stokes equations]] as: $ \frac{D\vec{u}}{Dt}=-\nabla p+\frac{1}{Re}\mu\nabla^2\vec{u} $ For $Re << 1$: $ Re\frac{D\vec{u}}{Dt}=-\nabla p+\mu\nabla^2\vec{u} $ Flow with $Re<2000$ considered ***laminar***, while flow with $Re >2000$ considered ***turbulent***