Recently, Professor Ding Yang and his team from Beijing University of Posts and Telecommunications published a significant achievement in the Proceedings of the National Academy of Sciences (PNAS), for the first time discovering the "propulsion reversal" phenomenon in propeller systems — under specific scenarios, objects will retreat regardless of whether the propeller rotates forward or backward. This discovery breaks through traditional fluid dynamics understanding of propulsion, issuing a warning and possible solutions for the challenges that propeller propulsion may face in practical applications of micro-robots in medical and environmental fields. The related paper was officially published on October 3rd, Beijing Time.

Screen shot of the relevant paper

In daily life, propellers are the "main force" of propulsion systems, with ocean-going vessels and helicopters relying on them to provide forward power. However, the experiment conducted by Ding Yang's team presents an "abnormal scenario." They placed a miniature submarine model, 7.5 centimeters long, into both clean water and silicone oil. In clean water, the submarine moved forward when the propeller rotated forward and backward when it rotated backward, as expected. But when placed in silicone oil, which is about 100 times more viscous than water, the submarine only retreated no matter how the propeller rotated, as if being dragged by an invisible force.

"This phenomenon, where the submarine moves backward even though the propeller rotates forward, we call it 'propulsion reversal,'" explained a team member. This seemingly "malfunctioning" phenomenon actually reveals a scientific field that has not been previously focused on — the unique rules of propeller propulsion in medium Reynolds number (Re) fluid environments.

The Reynolds number is a "ruler" for measuring the movement of objects in fluids. For large objects such as ships and airplanes moving in water or air, inertial forces dominate, placing them in the high Reynolds number category. Microorganisms like E. coli and Helicobacter pylori, moving in blood or water, have dominant viscous forces, placing them in the low Reynolds number category. However, future micro-robots working in human blood vessels or micro-vehicles for industrial pipeline inspections have dimensions between these two, experiencing both inertial and viscous forces during motion, making them fall into the medium Reynolds number environment. Previously, there were mature theories for high and low Reynolds number propeller propulsion, but the medium Reynolds number area remained a "blind spot."

Why does the propeller produce "reverse thrust" in a medium Reynolds number environment? The team found the "invisible hand" behind this through three-dimensional computer simulations, examining both force fields and flow fields.

According to the introduction, when the propeller rotates in silicone oil, it simultaneously generates two opposing "forces." One is the "centrifugal suction effect," where the rotating propeller causes the silicone oil to form a vortex similar to a tornado, creating a negative pressure area near the propeller. This negative pressure area acts like a vacuum cleaner, continuously pulling the silicone oil behind the propeller forward, and the pulled silicone oil produces a reactive force that pulls the submarine backward — this is the key force causing the submarine to retreat. The other is the "backward fluid acceleration effect," where the inclined angle of the propeller blades causes them to collide with the silicone oil when rotating, creating a jet of water flowing backward, producing a forward thrust along the propeller axis, which is the same principle as how ordinary ships move forward.

Experimental equipment

In a medium Reynolds number environment, the backward pulling force caused by the "centrifugal suction effect" far exceeds the forward thrust produced by the "backward fluid acceleration effect," like a "tug-of-war," where the backward force wins completely, ultimately leading to the submarine's "reverse movement." In high Reynolds number or low Reynolds number environments, the forward thrust always dominates, so this "abnormal" situation does not occur.

"Now that we have understood the mechanism of 'propulsion reversal,' we can target the problem," said Ding Yang. For example, when designing medical robots for use in blood vessels in the future, we can avoid 'reverse thrust' by increasing the distance between the propeller and the body, optimizing the shape of the propeller, and allowing the robot to move in the intended direction accurately.

Next, the team plans to improve the fluid dynamics theory of propeller propulsion under medium Reynolds number conditions, aiming to bridge the theoretical system of propeller propulsion in high and low Reynolds numbers. "We hope to conduct joint research with related disciplines such as micro-robots, providing support for the practical application of micro-vehicles and other systems," said Ding Yang.

Original link:

https://www.pnas.org/doi/10.1073/pnas.2504153122

Source: Guangming Network

Original: https://www.toutiao.com/article/7558411148496880147/

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