Question

Flow in a pipe is laminar if Reynolds number is ____

• A. Less than \(2000\)
• B. Between \(4000\) and \(6000\)
• C. Between \(2000\) and \(4000\)
• D. Above \(6000\)

Hint:

Pipe flow classification: \[ \boxed{ \begin{aligned} Re\lt 2000 &\rightarrow \text{Laminar}\\ 2000\lt Re\lt 4000 &\rightarrow \text{Transition}\\ Re\gt 4000 &\rightarrow \text{Turbulent} \end{aligned} } \]

Answer 1

The Correct Option is A

Step 1: Recall what Reynolds number actually compares. \[ Re = \frac{\rho V D}{\mu} \] which is really just inertial forces divided by viscous forces in the flow.
Step 2: Think about what happens at low Re.
When viscous forces dominate over inertial forces, they damp out any tiny disturbance in the flow before it can grow, keeping fluid layers moving smoothly parallel to each other.
Step 3: Recall the experimentally fixed boundary.
Extensive pipe flow experiments place this smooth, orderly regime below a Reynolds number of 2000, beyond which disturbances start to grow into turbulence.
\[ \boxed{Re < 2000} \]