4. Critical Reynolds Number

Critical Reynolds number approximates the location where the flow transitions from laminar to turbulent flow. \[\begin{aligned} \text{Re}_{x,c} \approx 10^5 \qquad & \text{external (flat plate) flow} \\ \text{Re}_{D,c} \approx 10^3 \qquad & \text{internal (duct) flow} \end{aligned}\]