Smart dimples slash drag, steer spherical vehicles with ease

Summary:

A new type of smart vehicle inspired by golf balls could transform how machines move through water and air, thanks to a sphere that morphs its surface in real time to cut drag and steer. Developed at the University of Michigan, the prototype features a thin latex skin stretched over a perforated sphere. When a vacuum pump is activated, the latex pulls inward, forming precise dimples; when the pump is turned off, the surface returns to smooth.

The research, published in Flow ¹ and Physics of Fluids ², demonstrates how changing the dimple depth can reduce drag by up to 50% compared to a smooth surface across a range of flow speeds. At high speeds, shallower dimples worked best; deeper ones were more effective at lower speeds. The adaptive system also allows for real-time control, adjusting to changes in surrounding flow conditions.

By selectively dimpling just one side of the sphere, researchers generated lift forces up to 80% of the drag force, enabling precise steering without moving parts. This mechanism, inspired by flow separation dynamics, could pave the way for highly maneuverable, compact vehicles for ocean exploration, inspection, or surveillance, with fewer external components and lower energy demands.

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Nimble dimples: Agile underwater vehicles inspired by golf balls

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.

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 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 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,” said Rodrigo Vilumbrales-Garcia, a postdoctoral research fellow of naval architecture and marine engineering at U-M 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,” said Putu Brahmanda Sudarsana, U-M 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 said. “By enabling real-time adaptation to changing flow conditions, this innovation promises to enhance maneuverability, optimize performance and unlock new possibilities for vehicle design.”

Journal References:

Article Source:
Press Release/Material by University of Michigan
Featured image: A spherical prototype with adjustable surface dimples is suspended in a wind tunnel, ready to test drag. Credit: Jeremy Little | Michigan Engineering