Hydrodynamic benefits of fish swimming near riverbeds

Many freshwater fish swim near the bottom of riverbeds or hard rocks. While this behavior can be partially explained by increased food in those locations, the solid boundaries also provide hydrodynamic benefits from the wall effect.

Yang et al. simulated brown trout swimming near river sidewalls and floor to determine the wall effects impact.

“We aimed to maintain similarity with real conditions by setting the model’s boundary conditions, initial water flow velocities, and various parameters based on actual flow conditions,” said author Guang Yang. “We selected the brief motion state of fish accelerating their swimming near the side and bottom walls to a straight-line cruising state as the benchmark for our research.”

The team found that the wall effect becomes noticeable when the distance from the wall to the fish is half the length of the fish’s body. At one half fish-lengths from the side wall, the fish obtains more thrust and achieves a higher swimming speed. As a tradeoff, the wall effect also increase the lateral and vertical forces of the fish body, reducing efficiency. Swimming near the floor requires more energy.

This work can help inform efforts to recreate fish habitats that have been destroyed by human development and pollution.

“We can apply our results to create a more preferred living environment for fish and provide significant hydrodynamic benefits for their swimming,” said Yang. “In fact, we have carried out a similar engineering practice in the upper reaches of the Yangtze River in China. We placed cement reefs in the habitat of rare fish, such as Dabry’s sturgeon and suckerfish, and used the wall effect to create a habitat for these fish.”

Source: “The influence of wall effects on self-propelled performance of brown trout swimming,” by Guang Yang, Wen-jie Li, Hong-bo Du, Yu Wan, Rui Jia, Sheng-fa Yang, and Peng Zhang, Physics of Fluids (2024). The article can be accessed at https://doi.org/10.1063/5.0202100 .