Underwater or aerial vehicles with dimples like golf balls could be more efficient and maneuverable, researchers have demonstrated with a new prototype.
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.
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,” says Anchal Sareen, a University of Michigan 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.
Sareen and colleagues formed the prototype 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 researchers put the sphere to the test within a 3-meter-long wind tunnel, suspending it by a thin rod and subjecting it to different wind velocities.
For each flow condition, the researchers could finely adjust dimple depth by shifting the vacuum pump’s strength. They measured drag using a load cell, a sensor that detects force exerted by airflow on the object. At the same time, they sprayed an aerosol 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 adaptive skin setup is able to notice changes in the speed of the incoming air and adjust dimples accordingly to maintain drag reductions. Applying this concept to underwater vehicles would reduce both drag and fuel consumption,” says Rodrigo Vilumbrales-Garcia, a postdoctoral research fellow of naval architecture and marine engineering at UM and contributing author to the studies.
The smart 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.
“I was surprised that such a simple approach could produce results comparable to the Magnus effect, which requires continuous rotation,” says Putu Brahmanda Sudarsana, a UM graduate student in mechanical engineering and contributing author to the studies.
“In the long run, this could benefit, for example, compact spherical robotic submarines that prioritize maneuverability over speed for exploration and inspection. Typically, these submarines would require multiple propulsion systems, but this mechanism could help reduce that need.”
Looking ahead, Sareen anticipates collaborations that combine expertise in materials science and soft robotics, further advancing the capabilities of this dynamic skin technology.
“This smart dynamic skin technology could be a game-changer for unmanned aerial and underwater vehicles, offering a lightweight, energy-efficient and highly responsive alternative to traditional jointed control surfaces,” she says. “By enabling real-time adaptation to changing flow conditions, this innovation promises to enhance maneuverability, optimize performance and unlock new possibilities for vehicle design.”
Source: University of Michigan
