What is a drain vortex?
When a basin empties through a single hole, the water almost never flows straight in. Instead the streamlines spiral, the surface dips, and a small vortex forms over the drain — the funnel you see every time you pull a plug. A vortex is simply flow that circles an axis while also moving along it, so the water both spins and sinks at the same time.
Reading the diagram
The basin holds a body of water whose free surface is flat at the walls but pulled down at the centre. The curved blue lines are spiral streamlines: they show water travelling inward and downward towards the drain outlet. Along the axis a narrow air core reaches down to the hole, and the dashed ring marks the overall rotation — the direction in which the whole body of water is swirling.
Why it matters
The same mechanics appear far beyond the kitchen sink: in bath and toilet drains, in reservoir and tank intakes, in the swirling air of a tornado or dust devil, and in the vortices a wing leaves behind. Recognising that the spin is concentrated, not created, explains why vortices are strongest right at their centre.
1. Water flows towards the drain
Once the plug is out, water from every side of the basin heads for the single outlet. It arrives with a tiny amount of sideways motion — a faint swirl left by how the basin was filled, by its shape, or by a passing current.
2. The radius shrinks
As the water moves inward it has to squeeze into a smaller and smaller circle. The same parcel of water is now travelling around a tighter loop than it was before, even though nothing has given it an extra push.
3. Angular momentum makes it spin faster
Angular momentum depends on radius multiplied by speed. Because it has to stay roughly constant while the radius falls, the speed has to rise. Halve the radius and the water turns roughly twice as fast — the gentle drift becomes a deliberate whirl.
4. Fast spin pulls the surface down
The quicker the water circles, the stronger the outward centrifugal effect. It raises pressure at the rim and lowers it near the axis, and the free surface answers by dropping towards the centre. The flat surface becomes a funnel.
5. An air core opens to the outlet
With the pressure at the axis pulled very low, the air above can push down into the middle of the funnel. A continuous air column — the air core — now runs from the surface straight to the drain, and the spiralling water empties out around it.
What causes a whirlpool over a drain?
Water draining from all directions carries a small amount of rotation. As it converges towards the outlet, conservation of angular momentum makes each parcel spin faster, and that turning flow sweeps into a spiralling vortex above the drain.
Why does the water speed up near the drain?
Angular momentum stays roughly constant as water moves inward. Because it depends on radius times speed, halving the radius roughly doubles the speed, so water that drifted in slowly ends up whirling quickly right at the outlet.
Why does the surface dip into a funnel?
Faster rotation means a stronger outward centrifugal effect, which pushes the pressure up at the rim and lowers it near the axis. The free surface responds by sinking towards the centre, forming the funnel you see.
Does the drain direction depend on the hemisphere?
No. The tiny Coriolis force is far too weak to set the direction of a basin draining in seconds; the spiral is set by whatever small swirl, shape or current the water already had.
What is the air core in a vortex?
It is the column of air that reaches down the centre of the funnel to the outlet. Once the spin is strong enough, pressure at the axis drops so far that a continuous air passage opens from the surface straight to the drain.