Internal (Borda) Mouthpiece – Meaning of “Running Free” For an internal mouthpiece, if the jet contracts inside and does not touch the mouthpiece wall (vena contracta persists), the mouthpiece is said to be running free; if the jet expands to fill the mouthpiece, it is running full. Hence the given statement is incorrect.

Introduction:
Mouthpieces are short tubes fitted to orifices. Their discharge behavior depends on whether the jet forms a vena contracta that remains detached (running free) or expands and attaches to the tube walls (running full). The distinction affects the coefficient of discharge and head–flow relation.

Given Data / Assumptions:

• Internal (Borda) mouthpiece mounted flush with tank wall.
• Sharp entrance, short length compared with diameter.
• Steady head over the mouthpiece.

Concept / Approach:

With a sharp-edged entrance, the jet contracts after entry (vena contracta). If the jet stays contracted and does not touch the tube, the mouthpiece is said to run free and typically exhibits a lower discharge coefficient. At higher heads or with certain geometries, the jet may expand and fill the tube, i.e., run full, yielding a larger discharge coefficient.

Step-by-Step Solution:

Verification / Alternative check:

Textbook data show Cd_free roughly half that of the theoretical jet, while Cd_full is substantially larger due to pressure recovery and reduced contraction losses.

Why Other Options Are Wrong:

Truth qualifiers (only at low head, with air, etc.) do not change the fundamental definitions of “free” and “full”.

Common Pitfalls:

Confusing internal with external mouthpieces; assuming any filling implies cavitation rather than simply running full.

Final Answer:

False