A Swirling Vortex Is No Match for This Deep-Sea Sponge
A new paper finds a glass sponge has the power to eliminate destructive vortices that are created when fluid moves around a blunt object.,
At the bottom of the Pacific Ocean, cylindrical clusters of the glass sponge Euplectella aspergillum jut upward like skyscrapers in the deep sea. Some house tiny shrimp, to whom an 11-inch sponge is essentially a high-rise. And the sponge’s glass skeleton is certainly a feat of architecture, comprising a geometric latticework that gives the sponge the illusion of being wrapped in lace. Yet it is enduringly sturdy, able to stay rooted in the sea floor and weather currents without snapping or splintering.
Such structural superpowers leave many scientists eager to unravel whatever secrets this crystalline sponge contains. The answers could solve engineering problems, such as how to design a tall building that will not collapse in harsh winds. A study published Wednesday in the Journal of the Royal Society Interface reveals how the ridges in the sponge’s skeleton suppress a destructive phenomenon called vortex shedding, which can cause catastrophic damage to structures like chimneys and smokestacks.
“These works support the idea that the fluid dynamic properties of the glass sponges might be no less remarkable than their structural characteristics,” Giacomo Falcucci, a mechanical engineer at Tor Vergata University of Rome, who was not involved with the research, wrote in an email.
Under the glass sponge’s soft tissue, a tubular skeleton protects and supports the animal. The core skeleton comprises bundles of needly forms called spicules that are oriented vertically, horizontally and diagonally and fused together in a lattice structure that somewhat resembles a checkerboard. Surrounding this lattice are protruding clockwise and counterclockwise helical ridges that resemble a series of fire escapes winding around the tubular sponge and under its tissue. All together, the ridges look like a maze.
“It has this very dense, highly consolidated system,” said James Weaver, a senior scientist at Harvard University’s school of engineering and applied sciences and an author on the new paper. The study was also led by Katia Bertoldi and Matheus Fernandes, researchers at the same school.
Dr. Weaver started studying Euplectella aspergillum in the early 2000s. He first focused on sponge skeletons, investigating their diverse structures and mechanical properties.
For this paper, the researchers studied the sponge from a hydrodynamic perspective: how the fluids acted on and moved around its skeleton.
They pursued this question after noticing the sponge’s ridges bore an uncanny resemblance to helical strakes, ridge-like protrusions often used to protect the structural integrity of towers and other cylinders. When a fluid such as air moves around a smooth cylinder, vortices are shed alternately from one side to the other on the downwind side of the cylinder. These alternating vortices can cause the cylinder to vibrate, which leads to noise and safety concerns. In human architecture, helical strakes suppress the vortices by disrupting flow around the structure.
To understand if the glass sponge’s external ridges offered a similar hydrodynamic benefit, the researchers created a series of mechanical and computational models to visualize how the sponge’s anatomy affects the flow of surrounding fluids.
Their models showed the sponge’s maze of ridges completely eliminated vortex shedding. “What we find in the sponge structure is that it’s able to fully suppress it, rather than just delay or diminish it,” Mr. Fernandes said. One obvious application of the new research would be to design sponge-inspired helical strakes.
The authors hypothesize this highly complex skeleton helps keep the sponge anchored in the soft sediments of the seafloor, which could be excavated by the whirling vortices. “The sponge could be kickstanded,” Dr. Weaver said.
“This sponge skeleton fascinates material scientists,” Sally Leys, an invertebrate zoologist at the University of Alberta who was not involved with the research, wrote in an email. “However — a big however — they always neglect the animal’s tissues.”
Unlike past research that examined only the sponge’s skeleton, the new paper does include several models that attempt to reconstruct the soft, porous tissue of a living sponge.
In Dr. Leys’s eyes, some of the new paper’s models that show flow through a porous sponge are unrealistic. “Water does not move through a glass sponge passively,” Dr. Leys said. “They control the flow.”
Ocean sponges use an internal pump to channel water to nanometer-size openings where food and oxygen are exchanged and waste is excreted, and then the water exits through other pores and eventually leaves through the top of the sponge, Dr. Leys explained.
Dr. Leys also found the amount of flow the researchers chose to simulate around the sponge “wildly unrealistic,” because it was far greater than the highest flow a living Euplectella would ever experience, she said.
The researchers conceded that not all of their models were designed to reflect a living sponge in the wild. Rather, they simulated high levels of flow to demonstrate the potential utility of the sponge structure for engineering.
Dr. Leys worries the models could be misleading. “The real biology of these exotic animals needs to be given much higher consideration by materials scientists,” she said.
Though the precise vortex-suppressing qualities of living glass sponges may remain a mystery, the researchers’ results do illuminate the use of the internal skeleton as a proxy for human-made structures.
“It is important to realize the power of taking inspiration from nature,” Mr. Fernandes said.
In such a future, our terrestrial smokestacks might start looking a lot more like a bustling shrimp metropolis in the deep sea.