Internal mouthpieces (Borda type) – identifying flow regime In an internal mouthpiece fitted flush with a tank wall, if the jet after the vena contracta does not touch the sides of the mouthpiece along its length, the mouthpiece is said to be:

Introduction / Context:
Short-tube or internal (Borda) mouthpieces can operate in two distinct regimes: running free and running full. Correctly identifying the regime is essential because coefficients of discharge and velocity differ between them.

Given Data / Assumptions:

• Sharp-edged entrance causing a vena contracta immediately downstream of the orifice plane.
• Steady incompressible flow with modest heads.
• Tube length small (a few diameters).

Concept / Approach:
In the running free regime, the jet contracts and remains detached from the tube wall, exiting the mouthpiece without wetting the internal surface. If the tube is long enough (or pressure recovery causes expansion), the jet attaches to the wall and the mouthpiece runs full, behaving like a short pipe with additional friction and different coefficients.

Step-by-Step Solution:
1) Observe the jet behaviour after contraction: does it contact the tube wall?2) If no contact occurs, the flow is detached; classify as running free.3) If contact occurs along the length, classify as running full.4) Recognize that length-to-diameter ratio strongly influences the regime (running free commonly when L/D < about 3).

Verification / Alternative check:
Laboratory visualizations show a clear annular space between the contracted jet and the tube wall when running free; discharge coefficients then approximate orifice flow values rather than short-pipe values.

Why Other Options Are Wrong:

• Running full / partially running full: require wall contact and wetting.
• Partially running free: nonstandard term; regime is typically categorized as either free or full.
• Choked: not a standard term for incompressible mouthpiece flow.

Common Pitfalls:
Assuming any internal tube implies pipe-flow behaviour; in running free, friction inside the tube is negligible because the jet does not contact the wall.

Final Answer:
running free