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: Identify the type of fluid motion.
Fluid motion can be categorized by how smoothly the fluid particles move.
In laminar flow, the fluid travels in well-defined, parallel layers, and there is no intermixing between adjacent layers.

Step 2: Introduce Reynolds number.
The nature of flow is commonly determined using the Reynolds number, a dimensionless parameter defined as:

\[ Re = \frac{\rho v D}{\mu} \] 

where:

• $\rho$ is the fluid density,
• $v$ is the average flow velocity,
• $D$ is the characteristic length (such as pipe diameter),
• $\mu$ is the dynamic viscosity of the fluid.

Step 3: Classify flow regimes in a pipe.
For flow through a circular pipe, experimental observations give the following ranges:

• $Re < 2000$: Flow remains smooth and orderly (laminar flow).
• $2000 < Re < 4000$: Flow enters a transitional state.
• $Re > 4000$: Flow becomes chaotic and irregular (turbulent flow).

Since the question asks specifically about laminar flow, we focus on the region where viscous forces dominate and the motion remains stable.

Step 4: Final conclusion.
Fluid flow is considered laminar when the Reynolds number is:

\[ \boxed{Re < 2000} \]