Critical velocity is the speed and direction at which the flow of a liquid through a tube changes from smooth to turbulent. Determining the critical velocity depends on multiple variables, but it is the Reynolds number that characterises the flow of the liquid through a tube as either turbulent or laminar. The Reynolds number is a dimensionless variable, which means that it has no units attached to it. In this article, we shall be discussing the critical velocity formula.

## How to Calculate Critical Velocity?

The formula to calculate the critical velocity of a liquid flowing through a tube is given by

Critical velocity (Vc) = K η / ρ r

where

V_{c }is the critical velocity

K is the Reynold’s number

r is the radius of the tube through which the liquid flows

ρ is the density of the liquid

Depending on the value of Reynold’s number, the flow type can be decided as follows:

- If K is between 0 to 2000, the flow is laminar or streamlined.
- If K is between 2000 to 3000, the flow is turbulent or unstable
- If K is above 3000, the flow is highly unstable

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