Holey Moley! Programmable Dimples Slice Drag

Dimples are not just an attractive facial feature, they also play a big role in the field of fluid dynamics. Well, maybe not the dimples on people’s faces, but consider the dimples on golf balls, for instance. You didn’t think they were all about looks, did you? A dimpled golf ball travels a whopping 30% further through the air than an equivalent ball with a smooth surface. The key to this performance is in the way that dimples reduce drag when moving through a fluid.

Engineers at the University of Michigan realized that these properties might be useful beyond the golf course. They thought that if drag can be reduced so significantly on a ball, perhaps similar effects could be seen with underwater or aerial vehicles. But rather than putting the skin of a golf ball on such a vehicle straight away, they instead developed a spherical prototype with adjustable surface dimples.

Making the depth of the dimples adjustable is an important innovation, because as the speed of an object moving through a fluid changes, so does the way the dimples interact with the fluid. For optimal drag reduction, the dimples may need to be shallower or deeper depending on how fast the vehicle moves through it. Moreover, by selectively adjusting a subset of the dimples, navigation is also possible. A mechanism like this could potentially replace the fins and wings in use today.

To build the prototype, the researchers stretched a thin sheet of latex over a hollow, pickleball-like sphere riddled with small holes. A vacuum pump was then used to depressurize the inside of the sphere. When turned on, the pump pulled the latex inward through the holes, forming dimples. When the vacuum was turned off, the latex returned to its smooth state. This clever design allowed for real-time control of dimple formation and depth.

The sphere was tested inside a 3-meter-long wind tunnel, where it was suspended by a thin rod and exposed to varying wind speeds. Using a load cell to measure drag and high-speed lasers and cameras to track airflow, the team observed how different dimple depths influenced the behavior of air around the sphere. They found that at high speeds, shallower dimples were more effective at cutting drag, while deeper dimples performed better at lower speeds. In all cases, dynamically adjusting the dimples led to drag reductions of up to 50% compared to a smooth sphere.

Beyond just reducing drag, the dimpled surface also enabled the generation of lift, without any moving fins or propellers. By activating dimples on only one side of the sphere, the team was able to create an asymmetric flow pattern. This caused the airflow to separate unevenly, producing a sideways force similar in magnitude to the lift generated by a spinning ball via the Magnus effect. But unlike the Magnus effect, this method required no rotation, only surface texture changes.

Using data from the tests, the team also created a predictive model that relates optimal dimple depth to flow speed (measured by Reynolds number), allowing for closed-loop control. This means the surface can automatically adjust itself during operation for maximum efficiency.

This deceptively simple innovation might one day play a big role in a new generation of smart, agile, and fuel-efficient vehicles exploring our skies and oceans.