In fluid mechanics, an orifice is defined as which of the following? Select the most accurate description of the opening and flow condition.

Introduction / Context:
Understanding what constitutes an orifice versus a mouthpiece or open-channel opening is fundamental in hydraulics. Exam questions often test whether learners can distinguish these devices by geometry and flow conditions, which directly affect discharge equations and coefficients.

Given Data / Assumptions:

• The opening is in a tank or vessel wall or base.
• Flow issues due to a head difference across a thin boundary (thin plate idealization).
• No requirement for the jet to run full in a long tube (that would be a mouthpiece).

Concept / Approach:
An orifice is a sharp-edged opening of closed perimeter in a relatively thin wall. The jet forms immediately and contracts to a vena-contracta. The upstream water level for standard orifice flow is above the top edge of the opening, creating a positive head. Mouthpieces, by contrast, have appreciable length (2–3 diameters or more) that alters jet development and coefficients.

Step-by-Step Solution:
Identify the defining geometry: a sharp-edged opening with closed perimeter → orifice.Check upstream level condition: it must be above the top for pressurized orifice flow.Recognize prolonged sides as a mouthpiece, not an orifice.

Verification / Alternative check:
Standard texts define an orifice by the thin-plate opening with a sharp edge; introducing a length (tube) changes the device to an internal or external mouthpiece and changes coefficients Cd, Cc, Cv.

Why Other Options Are Wrong:
Upstream level below top edge: that indicates weir/open-channel behavior, not classic orifice head.Partially full flow: describes open-channel conditions, not a pressurized jet through a plate.Prolonged sides 2–3D: that is a mouthpiece, not a simple orifice.

Common Pitfalls:

• Confusing weirs (free surface control) with orifices (pressurized jet).
• Assuming any opening is an orifice even when there is a tube length.

Final Answer:
An opening with a closed perimeter of regular shape through which water flows