A golf ball has dimples on its surface to increase the efficiency of airflow over it and let it fly further. Why isn鈥檛 the skin of aircraft and cars similarly dimpled? Would it compromise their structural integrity?
鈥� The answer relies on knowing there are two elements to 鈥渄rag鈥�, the resistance to motion when an object moves through a fluid.
The first is called friction (or skin) drag. This is the force generated due to viscous shear as a fluid moves tangentially across the surface of the object. Skin drag is minimised by making the object鈥檚 surface as smooth as possible. So with streamlined things such as aircraft wings and dolphins, where skin drag is a significant component of overall drag, the lowest drag force comes by having a smooth surface.
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鈥淚n streamlined shapes like dolphins, the lowest drag force comes from having a smooth surface鈥�
The second type of drag is called pressure drag. This is the force due to the difference in the fluid pressure between the front and back of the object. Under ideal flow conditions, the loss in pressure as a fluid accelerates around to an object鈥檚 maximum cross-sectional area is matched by a recovery of this pressure as the fluid decelerates again towards the rear of the shape. In most situations, however, the boundary layer separates from the object before the pressure fully recovers. This creates a region of low-pressure eddies behind the object, and a large pressure drag.
Generally, a streamlined shape is the best way to reduce pressure drag because it delays the detachment of the boundary layer. But in the case of a golf ball, the shape is fixed and pressure drag is dominant. Although the dimples do increase skin drag, they also cause the boundary layer to become turbulent. Turbulent boundary layers have more momentum and are better able to stick to the ball鈥檚 surface. So there is less pressure drag and the ball travels further.
Most cars are constrained by the need to have space for luggage and passengers, and so are fairly angular in shape. This means pressure drag is their major drag component. However, unlike the golf ball, the abrupt corners of their shape control where the boundary layer detaches. Dimpling the surface to make this layer turbulent would have little effect on where detachment takes place. Hence dimples on cars would not significantly reduce the pressure drag, but would increase skin drag.
Simon Iveson, Chemical engineering discipline, The University of NewcastleNew South Wales, Australia
鈥� Textured surfaces have been tried on aircraft, and they do reduce drag at high speed. In the 1990s, Airbus experimented with a coating that had a texture similar to sharkskin. In 2013, the German airline Lufthansa tried a paint embossed with grooves parallel to the direction of airflow. But there are problems of durability with such surfaces, and it is hard to keep dirt from building up in the grooves.
Ralph Hancock, London, UK
鈥� The MythBusters TV show did this to a car. They measured its coasting distance from a set speed on a straight, deserted road. They then coated the car in large dimples and reran the test. There was a measurable and repeatable difference. Unfortunately, the car was not particularly attractive.
If yachts had dimpled hulls, particularly made of fibreglass, this could lead to extra weight cancelling any speed gain. And the cost of altering the moulds for small runs might not make financial sense. Given that most club racing involves handicapping, there is no incentive to gain speed at extra cost. Hull cleaning would be another challenge.
鈥淎 coasting car covered in large dimples showed a measurable difference in distance travelled鈥�
Andrew Doherty, Tongala, Victoria, Australia
鈥� Golf balls are not the only objects that use dimples to improve airflow. I have a pair of cycling shoes and several sets of wheels with dimples. The carbon fibre wheels are lightweight and have a reputation for strength, so it is unlikely the dimples compromise structural integrity.
David Cox, Adelaide, South Australia
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