Reynolds number is defined as $\mathrm{Re} =\frac{u L}{\nu} = \frac{\rho u L}{\mu}=\frac{\dot{m}L}{A\mu}$ ## 1. Internal pipe flows Laminar flow for $Re_D < 2300$ Turbulent flow for $Re_D > 2900$ $L = D$ diameter of the pipe. $L = Area/Perimater$ for non circular pipes ## 2. External Flow ### 2.1. Flat Plate Turbulent flow for $Re_L > 10^5$ ### 2.2. Cylinder For external flow over a cylinder, different regions are observed at various Reynolds number affecting how the boundary layer attaches to the body. From Wikipedia <img src = "https://upload.wikimedia.org/wikipedia/commons/thumb/3/3f/Reynolds_behaviors.png/330px-Reynolds_behaviors.png">