Pipe flow regime classification – threshold based on Reynolds number The flow in a pipe is considered turbulent when the Reynolds number is:

Introduction:
Reynolds number (Re) classifies internal flows into laminar, transitional, and turbulent regimes. This question uses the conventional threshold values adopted in many practical handbooks for smooth pipes.

Given Data / Assumptions:

• Reynolds number is defined as Re = (rho * V * D) / mu for pipe flow.
• Geometry: circular pipe; fluid properties treated as constant.
• Typical thresholds: laminar at low Re, turbulent at higher Re.

Concept / Approach:
In many textbooks, laminar is Re < 2000, transitional is 2000–~2800 (sometimes 2000–4000), and turbulent is above the upper threshold. The options in this question take 2800 as the practical cutoff.

Step-by-Step Solution:
1) Identify regime ranges used in the options.2) Laminar: less than 2000 → not turbulent.3) Transitional: 2000 to 2800 → neither fully laminar nor fully turbulent.4) Above 2800 → turbulent by the convention implied in the options.

Verification / Alternative check:
Engineering references often cite laminar < 2000 and turbulent > 4000, with a gray zone in between. Using 2800 as a cutoff is a simplified convention consistent with many MCQs.

Why Other Options Are Wrong:

• Less than 2000: clearly laminar.
• Between 2000 and 2800: transitional, not definitively turbulent.
• None of these: incorrect because 'more than 2800' is acceptable for turbulent per the stated convention.

Common Pitfalls:
Forgetting that thresholds vary slightly by source; mixing criteria for pipes with external flows.

Final Answer:
more than 2800