What is critical velocity? Write its equation.

(N/A) Critical velocity is defined as the maximum velocity up to which the flow of a fluid in a pipe remains streamline (or laminar). Beyond this velocity,the flow becomes turbulent.
The equation for critical velocity $(v_{c})$ is given by:
$v_{c} = \frac{R_{e} \eta}{\rho d}$
Where:
$R_{e}$ is the Reynolds number,
$\eta$ is the coefficient of viscosity of the fluid,
$\rho$ is the density of the fluid,
$d$ is the diameter of the pipe.