What is a venturi tube?

• A. A long pipe with a constricted inner surface
• B. A pipe whose cross-section narrows then widens
• C. A pipe with a uniform diameter
• D. A short pipe with a constricted inner surface

Answer: (D)

This question tests how a Venturi tube uses a constricted region to change flow speed and pressure. As fluid enters the tube and its cross-section narrows to a throat, the velocity increases to conserve mass, and the static pressure drops according to Bernoulli’s principle. After passing the throat, the tube widens again, which helps recover some of the pressure as the flow decelerates slightly.

The description that best matches this behavior is a pipe whose cross-section narrows then widens. That shape—converging to a throat and then diverging—captures the essential Venturi geometry and the resulting pressure-velocity changes.

If a pipe is described as simply long with a constricted surface, it omits the widening portion needed to recover pressure. A uniform-diameter pipe has no constriction to accelerate the flow, and a short pipe with a constriction describes only the narrowing, not the full Venturi form.