Critical velocities in pipe flow — meaning of Re ≈ 2800 The flow velocity that corresponds to a Reynolds number of about 2800 in a circular pipe is commonly termed the:

Introduction / Context:
In internal pipe flow, the Reynolds number indicates whether the flow is laminar, transitional, or turbulent. Two reference values are often used to bracket the transition region: a lower critical Reynolds number and a higher critical Reynolds number.

Given Data / Assumptions:

• Reynolds number Re = ρ V D / μ in a smooth circular pipe.
• Typical thresholds: laminar for Re ≲ 2000, transitional roughly between Re ≈ 2000 and Re ≈ 2800–4000, turbulent above the higher critical value.

Concept / Approach:
The lower critical Reynolds number marks the limit below which laminar flow is sustained (often cited near 2000). The higher critical Reynolds number marks the point above which fully developed turbulence is reliably established (often cited near 2800–4000 depending on sources and disturbances). Thus, associating Re ≈ 2800 with the “higher critical velocity” is a standard textbook convention in many question banks.

Step-by-Step Solution:
Recognize Re ≈ 2800 lies above the lower threshold (~2000).This value signifies the upper end of the transition window (higher critical).Therefore, choose “higher critical velocity.”

Verification / Alternative check:
Practical laboratory observations show that the exact transition range depends on inlet disturbances and pipe roughness. Nonetheless, exam conventions commonly use ~2000 (lower critical) and ~2800 (higher critical) as nominal markers.

Why Other Options Are Wrong:

• Sub-sonic and super-sonic relate to Mach number in compressible flow, not Reynolds number.
• Lower critical velocity corresponds to Re around 2000, not 2800.

Common Pitfalls:

• Assuming a single universal value for the higher critical Reynolds number; it varies with conditions, but the naming in this question follows common instructional values.

Final Answer:
higher critical velocity