Have you ever sat waiting for someone in a coffee shop, slightly bored? Resisting the urge to check your email or Twitter on the phone (perhaps the battery is dead), you have been stirring your coffee and playing with the vortices that form behind the spoon. Have you wondered why they form? Or played with detaching a vortex from the spoon and getting it to ‘bounce’ off of the side of the cup?
Such vortices form behind objects in a flow of liquid when either the speed of the liquid, or the size of the object, reaches a critical value. The research about how and why these vortices form is a huge field. From improvements to plane design, through understanding insect flight and even into how wind instruments such as flutes work, understanding these vortices is a challenging topic. It is also useful to know about the behaviour of these vortices when designing chimneys in order to prevent their collapse.
Chimneys are of course stationary, but when they are in high winds, vortices form around the chimney just like the vortices behind the spoon (rather than the spoon moving through the coffee, the wind moves past the chimney). At relatively low speeds, the wind forms small whirlwinds as we see behind the spoon in the coffee. At higher wind speeds, the vortices forming behind the chimney can start to detach and form a pattern known as a Karman vortex sheet. As each vortex detaches from the chimney it subjects the chimney to a small force. Under some conditions and around some objects, this can result in the rather beautiful sounds of the Aeolian harp. Under more extreme conditions, it can result in the collapse of chimneys. The Ferrybridge C cooling towers collapsed in 1965 in high winds as a result of the turbulence around the cooling towers. To minimise the chances of such vortex sheets forming, chimneys are now designed with a spiral pattern (pictured) around them. Far from being an aesthetic feature, this spiral channels the wind so that vortex sheets cannot form behind the chimney.
Something to think about next time you’re waiting for someone in a cafe.