Internal Mouthpiece — Running Full Criterion An internal (re-entrant) mouthpiece is said to be running full if the length of the mouthpiece exceeds which condition relative to the orifice diameter?

Introduction:
A mouthpiece is a short tube attached to an orifice. For internal or re-entrant mouthpieces, the flow pattern depends on the length-to-diameter ratio. When the jet fills the passage and issues without vena contracta inside the tube, the mouthpiece is said to run full, which affects the applicable coefficients used in discharge formulas.

Given Data / Assumptions:

• Internal mouthpiece attached to a thin-plate orifice.
• Steady, incompressible liquid; gravitational discharge.
• Criterion stated in terms of length vs. orifice diameter.

Concept / Approach:
Empirically, an internal mouthpiece runs full when its length L is sufficiently large compared to the orifice diameter d. The classical rule used in textbooks is: if L > 3d, the mouthpiece runs full. In that state, the flow behaves more like through a short tube than through an orifice with vena contracta, changing Cd and head-loss characteristics.

Step-by-Step Solution:
Identify the type: internal/re-entrant mouthpiece.Apply the criterion: L > 3d implies running full.Therefore the descriptive term to select is ‘‘full’’.

Verification / Alternative check:
Hydraulics lab observations show that as L increases beyond ~3d, the contraction diminishes and the jet adheres to the walls, filling the mouthpiece.

Why Other Options Are Wrong:
‘‘Free’’ refers to a jet discharging freely with a vena contracta; ‘‘partially’’ is non-standard terminology for the classification.

Common Pitfalls:
Confusing external (Borda) mouthpieces with internal; mixing the ‘‘running free’’ vs ‘‘running full’’ regimes.

Final Answer:
full