Reynolds number and critical velocity The velocity corresponding to a Reynolds number of about 2000 for internal pipe flow is commonly termed what?

Introduction / Context:Reynolds number Re characterizes the ratio of inertial to viscous forces in a flow. In internal flows through pipes, specific Re thresholds indicate transitions between laminar, transitional, and turbulent regimes. The velocity associated with Re ≈ 2000 has a special name.

Given Data / Assumptions:

• Internal, fully developed, Newtonian flow in a circular pipe.
• Reynolds number Re = ρ V D / μ.
• Empirical transition: laminar for Re ≲ 2000, turbulent for Re ≳ 4000, transitional in between.

Concept / Approach:The velocity at which laminar flow tends to become unstable is called the lower critical velocity (corresponding to Re ≈ 2000). A higher threshold, sometimes termed higher critical Re (≈ 4000), marks where turbulence is fully sustained.

Step-by-Step Solution:

Verification / Alternative check:Experimental pipe-flow charts and Moody diagrams demarcate laminar, transitional, and turbulent zones near these Re values.

Why Other Options Are Wrong:

• Sub-sonic/super-sonic relate to Mach number, not Reynolds number.
• “Higher critical velocity” aligns with Re ≈ 4000, not 2000.

Common Pitfalls:Confusing Re thresholds with compressibility criteria; they are independent nondimensional groups.

Final Answer:Lower critical velocity