Understanding the power of mucus to reduce drag

Fish are covered by a thin layer of slimy mucus, which reduces drag while swimming. Sung et al. used the immersed boundary method to study the flow of water and the hydrodynamic effects of this slippery surface on a fish’s skin.

“Our investigation of the mechanisms behind the mucus layer and how it interacts with fish swimming in a basic configuration, provides significant insights into how fish benefit hydrodynamically by varying their local flexible surfaces,” said author Hyung Jin Sung.

Harnessing the drag reduction properties of mucus for underwater engineering applications has not been studied in detail. However, many simulations with the Navier slip surface have been done in recent years.

Self-propelled flexible plates are commonly used to simulate swimming fish. To mimic the mucus layer, the authors wrapped a self-propelled flexible plate in seaweed species, Saccharina japonica, to produce the Navier slip boundary condition.

The researchers found slip reduces input power by 8% and maximizes swimming speed and propulsion. The slip also increases propulsion efficiency by 6% and cruising speed by 5%. The authors compared the velocity profiles obtained from the experiment and immersed boundary method.

“The slip condition reduces the velocity gradient on the plate surface and saves more energy in the tangential direction,” said Sung.

The authors expect to continue their research on drag reduction caused by the fish mucus layers to help develop energy-saving technologies in aquatic environments.

Source: “A self-propelled flexible plate with a Navier slip surface,” Shuai Wang, Jaeha Ryu, Guo-qiang He, Fei Qin, and Hyung Jin Sung, Physics of Fluids (2020). The article can be accessed at https://doi.org/10.1063/1.5130698 .