In fluid mechanics, an orifice is defined as which of the following? Select the most accurate description of the opening and flow condition.
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AAn opening with a closed perimeter of regular shape through which water flows
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BAn opening where the upstream water level is below the top edge of the opening
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CAn opening that always carries partially full flow like an open channel
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DAn opening with sides prolonged 2–3 diameters (a mouthpiece in a thick wall)
Answer
Correct Answer: An opening with a closed perimeter of regular shape through which water flows
Explanation
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