The flow of water in wash hand basin when it is being emptied through a central opening, is an example of
- AFree vortexCorrect
- BForced vortex
- CRotational vortex
- DRankine vortex
Solution
Concept
Free vortex flow
Formula
Free vortex: v · r = constant (irrotational)
Step-by-step solution
1. Water leaving a basin through a central hole rotates without any external torque. 2. The velocity increases towards the centre (v ∝ 1/r). 3. This is a free (irrotational) vortex.
Explanation
In a free vortex the fluid rotates by itself, e.g. water draining from a basin or a cyclone. In a forced vortex an external torque rotates the fluid as a solid body (water in a rotating cylinder).
Shortcut
Drain / tornado = free vortex. Rotating tank = forced vortex.
Common mistake
Choosing Rankine vortex because the centre of a drain looks like a core.
Correct answer: A) Free vortex