What is critical velocity? Write a formula for critical velocity.

(N/A) Critical velocity is the maximum velocity up to which the flow of a fluid remains streamlined or laminar. When the velocity of the fluid exceeds this value,the flow becomes turbulent.
The formula for critical velocity $(v_c)$ is given by:
$v_c = \frac{R_e \cdot \eta}{\rho \cdot D}$
Where:
$R_e$ is the Reynolds number (a dimensionless quantity),
$\eta$ is the coefficient of viscosity of the fluid,
$\rho$ is the density of the fluid,
$D$ is the diameter of the pipe.