Question

In case of fluid flowing on horizontal surface the flow of fluid becomes unsteady when Reynolds number (Rn) is

• A. less than 499
• B. between 500 - 999
• C. between 1000 - 1999
• D. greater than 2000

Hint:

In the Std XI classification, flow is streamline below 1000, unsteady between 1000 and 2000, and turbulent above 2000.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The Reynolds number \(R_n = \dfrac{\rho vD}{\eta}\) compares inertial forces with viscous forces. Its value tells us the type of flow in a pipe.

Step 2: Key Formula or Approach:
In the Maharashtra Board Std XI classification:
1. \(R_n < 1000\): the flow is streamline (steady).
2. \(1000 < R_n < 2000\): the flow is unsteady.
3. \(R_n > 2000\): the flow is turbulent.

Step 3: Detailed Explanation:
The question asks when the flow becomes unsteady, which is the intermediate band between streamline and turbulent flow.
Option (A) less than 499 and option (B) 500 to 999 lie below 1000, so the flow there is streamline.
Option (D) greater than 2000 is the turbulent range.
Option (C), between 1000 and 1999, is the unsteady range.

Final Answer:
The flow is unsteady for \(R_n\) between 1000 and 1999, option (C). \[ \boxed{1000 < R_n < 2000 \text{ (C)}} \]