Question

Flow is said to be laminar when the Reynolds number is:

• A. Less than 1000
• B. Less than 2000
• C. Between 2000 and 4000
• D. Greater than 4000

Hint:

Reynolds number represents the ratio of inertial forces to viscous forces.
High Viscosity \( \implies \) Low \( Re \) \( \implies \) Laminar Flow.
High Velocity \( \implies \) High \( Re \) \( \implies \) Turbulent Flow.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Fluid flow can be classified based on the nature of the motion of the fluid particles.
Laminar flow (or streamline flow) occurs when fluid particles move in smooth layers or paths, with no mixing between layers.
Step 2: Key Formula or Approach:
The Reynolds number (\( Re \)) is a dimensionless quantity used to predict flow patterns.
\[ Re = \frac{\rho v D}{\mu} \]
Where:
\( \rho \) = density of the fluid.
\( v \) = flow velocity.
\( D \) = characteristic linear dimension (e.g., pipe diameter).
\( \mu \) = dynamic viscosity.
Step 3: Detailed Explanation:
For flow through a circular pipe:
1. If \( Re<2000 \), the flow is Laminar.
2. If \( 2000<Re<4000 \), the flow is in a Transition region.
3. If \( Re>4000 \), the flow is Turbulent.
Since the question asks when the flow is laminar, we look for the criteria representing the stable laminar regime. Although the transition starts around 2100-2300, in standard competitive exam contexts, "Less than 2000" is the accepted threshold for purely laminar flow.
Step 4: Final Answer:
Flow is laminar when the Reynolds number is less than 2000.