Underwater or aerial vehicles with dimples like golf balls could be more efficient and maneuverable, a new prototype developed at the University of Michigan has demonstrated.
Golf ball dimples cut through pressure drag, the resistance force an object meets when moving through a fluid, propelling the ball 30% further than a smooth ball on average. Taking this as inspiration, a research team developed a spherical prototype with adjustable surface dimples and tested its aerodynamics in a controlled wind tunnel.
“A dynamically programmable outer skin on an underwater vehicle could drastically reduce drag while eliminating the need for protruding appendages like fins or rudders for maneuvering. By actively adjusting its surface texture, the vehicle could achieve precise maneuverability with enhanced efficiency and control,” said Anchal Sareen, U-M assistant professor of naval architecture and marine engineering and mechanical engineering and corresponding author of two studies published in Flow and The Physics of Fluids.
These nimble vehicles could access typically hard-to-reach areas in the ocean while conducting surveillance, mapping new areas or collecting data on water conditions.
The prototype was founded by stretching a thin layer of latex over a hollow sphere dotted with holes, resembling a pickleball. A vacuum pump depressurizes the core, pulling the latex inwards to create precise dimples when switched on. Turning off the pump makes the sphere smooth again.
To find out how the dimples affected drag, the sphere was put to the test within a 3-meter-long wind tunnel, suspended by a thin rod and subjected to different wind velocities.
For each flow condition, the dimple depth could be finely adjusted by shifting the vacuum pump’s strength. Drag was measured using a load cell, a sensor that detects force exerted by airflow on the object. At the same time, an aerosol was sprayed into the wind tunnel while a high-speed laser and camera captured the motion of the tiny particles as they flowed around the sphere.
For high wind speeds, shallower dimples cut the drag more effectively while deeper dimples were more efficient at lower wind speeds. By adjusting dimple depth, the sphere reduced drag by 50% compared to a smooth counterpart for all conditions.
The morphable sphere can also generate lift, allowing for controlled movement. Often thought of as the upwards force responsible for keeping planes in the air, lift can work in any direction as long as it is perpendicular to the direction of the flow.
To achieve this, researchers designed the inner skeleton with holes on only one side, causing the sphere to develop one smooth and one dimpled side when activated.
This created asymmetric flow separation on the two sides of the sphere, deflecting the wake toward the smooth side. By Newton’s third law, the fluid applies an equal and opposite force toward the rough side, effectively pushing the sphere in the direction of the dimples. Dimples on the right generate force to the right while those on the left push left. This enables precise steering by selectively activating dimples on the desired side.
The team tested the new sphere in the same wind tunnel setup with varying wind velocity and dimple depth. With the optimal dimple depth, the half rough/half smooth sphere generated lift forces up to 80% of the drag force. The lift generation was as strong as the Magnus effect, but instead of using rotation, it was created entirely by modifying the surface texture.
Looking ahead, the team anticipates collaborations that combine expertise in materials science and soft robotics, further advancing the capabilities of this dynamic skin technology.