Difficulty: Easy
Correct Answer: Circulation strengthens with speed; eventually a central vortex forms at the free surface
Explanation:
Introduction / Context:Baffles suppress swirling and vortex formation. In unbaffled vessels, however, the absence of baffles allows bulk rotation and a vortex at the free surface as speed increases. Understanding this progression helps avoid gas entrainment, poor mixing, and erratic level control.
Given Data / Assumptions:
Concept / Approach:At low speeds, flow is laminar or transitional with gentle circulation. As N increases, tangential velocity rises and the free surface depresses at the center due to radial pressure gradients, eventually forming a stable vortex. The Froude number Fr = N^2 * Di / g indicates the propensity for vortexing; larger Fr favors deeper vortices and potential air entrainment.
Step-by-Step Solution:Increase N → stronger circulation and swirling motion.Surface depression grows with centrifugal effects.At sufficient Fr, a well-defined vortex forms, possibly entraining air.Conclusion: circulation precedes vortex; vortex deepens as speed rises.
Verification / Alternative check:Visual observations and surface profile measurements show vortex depth scaling with Fr; adding baffles (reducing swirl) markedly reduces vortexing at the same N.
Why Other Options Are Wrong:Immediate turbulence after small speed increase is unrealistic; regime depends on Rei and geometry.Ignoring surface effects misses the defining unbaffled behavior.Vortex at low speed with no prior circulation is not typical; circulation occurs at all speeds.
Common Pitfalls:Confusing turbulence onset with vortex formation; assuming vortex depth is solely a function of speed rather than Fr and liquid properties.
Final Answer:Circulation strengthens with speed; eventually a central vortex forms at the free surface
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