Vorticity

Author: Joseph Myers Hill

Many real-world fluid flows exhibit a rotating or ‘spinning’ type motion. From the small whirlpool you see when draining a bath to the largest tornados and tropical hurricanes, each flow has a region where fluid spins round and round. We call this region a vortex and say that the flow has vorticity, a measure of how fast the fluid is spinning. Have a go at creating your own vortices using this interactive tool from George Corney.

Fig 1: Vorticity in a flow through a sink drain and a tornado. Despite the size difference, these are the same phenomena!

We use our knowledge of the drivers and effects of vorticity is countless fluid applications. When we stir milk into tea, the circular motion of the teaspoon creates a small vortex which rapidly mixes the two fluids. Knowledge of how vorticity enhances mixing is leveraged in many kitchen devices, from hand mixers to blenders. Can you think of any more examples?

Vortices aren’t always a desirable fluid flow phenomenon, however. So called wingtip vortices form around the wings of fast-moving aircraft and can contribute to drag. This slows down the aircraft, meaning more fuel must be used to reach the destination. On humid days, these vortices can even be visible to the naked eye!

Fig 2: Left - wingtip vortices on the wings of a fighter jet. Right - red winglets on the end of a plane wing.

Here, knowledge of vorticity aids in the design of winglets, which manipulate the size and location of the vortices to reduce drag. This in turn reduces fuel consumption, vital for reducing the carbon footprint of modern-day air travel.

So, look out for vorticity in your bath tonight, next time you take a flight, or fix yourself a quick bite!