&Bullet; physics 14, 42
A simple network of pipes based on the structures of a bird’s lungs converts the reciprocating flow into a one-way flow.
Inspired by structures in a bird’s airway, researchers have shown that an oscillating flow can be converted into a one-way flow by passing it through a properly connected network of pipes  . Elements of the concept have already been suggested by others studying the lungs of birds. However, the new work directly reviews the idea in the laboratory and includes simulations that explain the underlying principles. The results could lead to new approaches for the manipulation of high velocity flows in fluid circuits, for example in chemical processing and machine cooling systems.
A century ago physiologists made a remarkable observation. While the direction of air flow in the lungs of most mammals varies when they inhale and exhale, the lungs of birds differ – the air flow through the oxygen-absorbing tissue is one-way. Systems that achieve such rectification of flow – converting an oscillating flow to unidirectional – usually rely on components such as diodes or valves to prevent backflow. Such components are absent in avian lungs, but researchers have suggested that the inertia of air moving through a connection between two pipes can create a valve-like effect.
Leif Ristroph of New York University and his colleagues realized that the valve effect can only lead to a correction if the airway network of the lungs has a suitable structure. The connectivity of this network in birds causes air to flow in complete cycles, which is different from mammalian lungs. However, it was not clear what combination of closed loops and transitions would be required to create rectification and how the basic mechanism could be fully explained.
To find out, the team built simple pipe networks, starting with a closed loop of rubber tubing. They filled it with water and connected it to a pump that forcibly made the water flow clockwise and counterclockwise. Next, the team used T-joints to join a smaller segment of pipe in two places, but the two T’s were in different orientations at a 90-degree angle to each other. This asymmetry was critical to the rectification. With the pump running, rectification occurred in the loop containing the segment and part of the original loop. Particles placed in the water vibrated but gradually moved around this loop, moving further forward than backward in each cycle. Meanwhile, the particles in the remainder of the original loop did not evolve. “This is essentially what happens in the lungs. Now we can actually see and measure what’s going on, ”says Ristroph.
Using rigid pipes 1.6 cm in diameter, the team built a larger network with a different structure that worked on the same principle. By monitoring the microparticles in the water, the researchers were able to measure the flow for a wide range of frequencies and amplitudes of the pump vibration.
Computer simulations of the simpler pipe structure resulted in an effect that explained the rectification: at every T-junction, vortices occur that block the “side street” during one flow direction, but not in the opposite direction. This valve-like effect occurs at the two T-junctions at different times during each oscillation cycle.
The rectification effect is based on eddies that occur more frequently at higher flow velocities – what liquid researchers describe as a higher Reynolds number. In their simulations and experiments, Ristroph and his colleagues found that the effect was most pronounced with Reynolds numbers above 2000 (liquid velocity of about 10 cm / s for a water-filled pipe with a diameter of 1.6 cm). However, it was still observed for flows with Reynolds numbers as low as 100 (liquid velocity of around 1 cm / s in the same experiments).
“It turns out that with high Reynolds numbers, new physics occurs that opens up exciting possibilities for flow control,” says Ristroph. The effect could be used, for example, to create a fluid circuit that pumps large amounts of coolant or lubricant into a machine. Vibrations from the machine can create an oscillating drive in a grinding network that is tailored to pump the fluid through the system.
“This paper is a fascinating demonstration of how counterintuitive high Reynolds numbers can be,” says Eleni Katifori of the University of Pennsylvania, who studies structures inspired by biological transport networks. “The work is simple, elegant and quite original.”
Correction (March 23, 2021): In an earlier version of this story, the fluid velocities were accidentally switched for the low and high Reynolds number cases.
Rachel Berkowitz is Corresponding Editor for physics based in Vancouver, Canada.
- QM Nguyen et al., “Flow correction in loop-shaped network models of the bird lungs”, Phys. Rev. Lett.126114501 (2021).