The water swirls as it flows down a sink due to the Coriolis force. This force is caused by the rotation of the Earth and creates whirlpools clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.
When water flows down a sink, it often takes a swirling trajectory instead of falling straight down. This whirlpool is mainly due to the conservation of angular momentum, a kind of rotational energy that moving fluids possess. As the water converges toward the drain, its speed increases as the available space decreases, somewhat like a skater speeding up by bringing their arms in while spinning. This movement causes a visible rotation of the water, creating the typical whirlpool. In practice, even if the water seems calm at first, slight initial disturbances are enough to start this spiral circulation.
Contrary to a widely held belief, the Coriolis force has very little influence on the flow of water in a household sink. This force, due to the rotation of the Earth, comes into play for large-scale phenomena such as hurricanes or ocean currents, but it is so weak on a small scale, like your sink, that it becomes negligible compared to other more direct factors. To clearly observe its effect, very controlled conditions, almost laboratory-like, are necessary, so be cautious of hasty everyday conclusions! In reality, it is more the shape of the sink, the initial direction of the flow, or the imperfections of the surface that truly determine the direction of the whirlpool.
The shape of the sink plays a significant role. A perfectly symmetrical and smooth sink causes few disturbances, creating a light and random whirlpool. But if the sink has an off-center, sloped, or asymmetrical shape, the water naturally follows the slope, accelerating the appearance of a beautiful whirlpool. The same goes for tilt: even a slight incline encourages the water to swirl in a specific direction, dictated by the quickest descent toward the drain. A flat sink like a pancake will limit the rotation, with the water draining more slowly, whereas an inclined basin significantly enhances the swirling effect.
The way water behaves at the start significantly changes the final result of the whirlpool. If at first you pour the water very quickly or somewhat abruptly, it will create an initial movement that will influence the entire flow afterward. Even a slight impulse or an object in the water can also modify the rotation. The temperature of the water also plays a small role, as it affects the viscosity (basically, the resistance of the water to flow). Another often overlooked element: the irregularities of the sink or the presence of dirt can disrupt the flow and alter the final appearance of the whirlpool. These side effects, even minimal, make each flow unique, even in the same sink.
The vortices observed in nature, known as von Kármán vortices, are responsible for amazing phenomena such as the regular formation of clouds and the vibrations of cables and beams subjected to wind.
The largest known oceanic whirlpool on Earth measures over 150 kilometers in diameter! These oceanic vortices influence the weather and facilitate heat exchange between the oceans and the atmosphere.
In physics, the official term for describing vortex motion is "vortex flow." Pilots constantly seek to understand and avoid these dangerous vortices left behind by aircraft in flight.
The Coriolis force, responsible for many large atmospheric movements on Earth, is named after the French scientist Gaspard-Gustave Coriolis, who described it mathematically for the first time in 1835.
Although the Coriolis force does indeed influence large-scale movements such as those of oceans or weather systems, it is very weak and often negligible at the scale of a household sink. What you are observing is more related to local specifics such as the shape of the sink and the initial water currents.
The formation of a vortex naturally optimizes the flow of water by creating a central zone of low pressure, thereby facilitating rapid drainage towards the outlet. It is this dynamic spiral structure that enables faster drainage.
Even with a perfectly symmetrical sink in theory, a slight initial disturbance or minor instabilities in the movement of the water are enough to trigger the vortex phenomenon. In practice, it would be nearly impossible to completely eliminate any rotational movement.
Yes, vortices are very common in nature. They can be observed especially in atmospheric tornadoes, tropical cyclones, as well as in ocean currents and even in astrophysical phenomena such as spiral galaxies or black holes surrounded by an accretion disk.
Contrary to popular belief, the whirlpool in a sink can rotate in both directions. In most everyday cases, the Coriolis force is too weak to impose a short-term direction; instead, factors such as the shape of the sink, the incline, and the initial movement of the water influence the direction of the whirlpool.
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