Convergent–divergent mouthpiece characteristics Which statements correctly describe a convergent–divergent mouthpiece running full under suitable submergence?

Introduction / Context:
Mouthpieces are short tubes fitted to orifices to modify jet formation and recover pressure. A well-designed convergent–divergent mouthpiece can run full and deliver discharge close to the ideal by minimizing losses and recovering kinetic energy in the diffuser section.

Given Data / Assumptions:

• Short convergent followed by a diffuser section.
• Steady incompressible flow; sufficient submergence to avoid air entrainment.
• Proper diffuser angle to limit separation and recover pressure.

Concept / Approach:

The convergent accelerates the flow to the throat (near vena contracta), while the diffuser converts velocity head back to pressure head. With good design, the discharge coefficient C_d approaches 1.0 and losses due to sudden enlargement are avoided because expansion is gradual, not abrupt.

Step-by-Step Solution:

Verification / Alternative check:

Comparative tests show higher C_d for convergent–divergent mouthpieces than for sharp orifices or re-entrant mouthpieces under similar heads.

Why Other Options Are Wrong:

Each of (a), (b), and (c) is correct; hence (d) is the most complete answer, while (e) omits the key performance metric C_d ≈ 1.0.

Common Pitfalls:

Using too steep a diffuser angle causing separation; insufficient submergence that leads to cavitation or air ingestion.

Final Answer:

All the above