Re-entrant (Borda's) mouthpiece is classified as which type of mouthpiece?

Introduction:
Mouthpieces are short tubes attached to orifices to control and measure discharge. Classification into internal and external types affects jet formation and discharge coefficients. The re-entrant or Borda's mouthpiece has a distinct geometry and flow pattern.

Given Data / Assumptions:

• Name: Re-entrant (Borda's) mouthpiece.
• Short-tube element projecting into the reservoir.

Concept / Approach:
A re-entrant mouthpiece projects inward from the tank wall, creating a vena contracta inside the mouthpiece. Because it protrudes into the reservoir, it is called an internal mouthpiece. This geometry yields characteristic coefficients (coefficient of discharge often near 0.5 to 0.62 under running full orifice conditions, depending on regime).

Step-by-Step Solution:
1) Identify geometry: tube projects inward (re-entrant).2) Classify based on projection direction: inward implies internal.3) Therefore, select 'internal' as the correct classification.

Verification / Alternative check:
Standard hydraulics texts depict Borda's mouthpiece as an internal projection creating a sharper contraction than external mouthpieces, confirming the classification.

Why Other Options Are Wrong:

• External: Would project outward from the tank wall into the discharge region.
• Converging-diverging (Venturi): A long, smoothly varying nozzle/venturi, not a short mouthpiece.
• Bell-mouthed: Flared at entry, opposite of re-entrant behavior.

Common Pitfalls:

• Confusing the flow regime 'running free' vs 'running full' with the geometric classification.
• Assuming all mouthpieces are external because the jet exits to atmosphere.

Final Answer:
internal