Transition in Pipe Flow — Name the Velocity What is the term for the velocity at which laminar flow ceases (i.e., the upper limit of laminar regime before fully turbulent flow establishes)?

Introduction / Context:
Flow in pipes is characterized by the Reynolds number Re = ρ V D / μ. As velocity increases, flow transitions from laminar to transitional to turbulent. Two commonly referenced thresholds bound the laminar and turbulent regimes for internal flows.

Given Data / Assumptions:

• Constant properties and a circular pipe of diameter D.
• Laminar regime exists at sufficiently low Reynolds number.
• Definitions: lower and higher critical velocities bound transitional behavior in many texts.

Concept / Approach:
The lower critical velocity marks the onset of deviation from ideal laminar behavior (Re near about 2000 in smooth pipes). The higher critical velocity marks the end of the transitional regime (Re near about 4000), beyond which flow is fully turbulent. Thus, the velocity where laminar flow stops is the higher critical velocity in the sense that above this value, turbulence dominates.

Step-by-Step Reasoning:

Verification / Alternative check:
Experimental pipe-flow maps show friction factor correlations deviating from laminar law f = 64/Re once Re exceeds the lower critical value and becoming fully turbulent past the higher threshold.

Why Other Options Are Wrong:
“Velocity of approach” is unrelated; “Lower critical velocity” denotes the start of transition, not the end; “None” contradicts standard terminology.

Common Pitfalls:
Treating the transition as a single sharp value; actual thresholds depend on disturbances, roughness, and entry effects.

Final Answer:
Higher critical velocity.