The influence of wall effects on self-propelled performance of brown trout swimming

Harbor seals can exquisitely perceive and distinguish different hydrodynamic signals through their distinctive undulatory whiskers. In this study, an elastically supported Harbor seal whisker model undergoing one degree-of-freedom vibration in the cross-flow direction was positioned right downstream of a fixed cylinder and a flapping caudal fin model, respectively, as wake generators. The vibration responses, spectral frequencies, and fluid forces of the whisker model at different angles of attack (0° ≤ α ≤ 90°) are experimentally investigated. The reduced velocity, Ur, ranges from 4.0 to 26.0, and the Reynolds number, Re, varies within the range of 730–4770. The whisker model exhibits a combined vortex-induced vibration (VIV)—wake-induced vibration (WIV) response in the cylinder wake flow,whereas in the caudal fin wake flow, it displays a combination of VIV—wake-induced galloping response at α ≤ 45° and VIV–WIV response at α ≥ 60°. The frequency features of the wake generated by the fixed cylinder and the flapping caudal fin model, along with the increased VIV influence due to the angle of attack, cause the whisker model to exhibit different vibration characteristics. Understanding the vibration responses and fluid forces of the whisker model at various angles of attack in different wake flows is pivotal for future studies on developing whisker-inspired sensors.

Dynamics of harbor seal whiskers at different angles of attack in wake flow

Zhao,

Zhang,

et al. 2024

Numerical simulation of manta ray swimming using a smoothed-particle hydrodynamics method and investigation of the vortical structures in the wake

Gao,

Sun,

Huang

et al. 2024

A three-dimensional smoothed-particle hydrodynamics (SPH) method is used to study the moving boundary problem of a swimming manta ray, focusing on Eulerian and Lagrangian coherent structures. The manta ray's boundary motion is predefined by a specific equation. The calculated hydrodynamic results and Eulerian coherent structures are compared with data from the literature. To improve computational stability and efficiency, the δ+-SPH model used in this study incorporates tensile instability control and an improved adaptive particle-refinement technique. By comparing and analyzing the Eulerian and Lagrangian coherent structures, the relationship between these vortex structures and hydrodynamic force generation is examined, revealing the jet mechanism in the manta ray's wake. The SPH method presented herein is robust and efficient for calculating biomimetic propulsion problems involving moving boundaries with large deformations, and it can accurately identify vortex structures. The approach of this study provides an effective simulation tool for investigating biomimetic propulsion problems such as bird flight and fish swimming.

Numerical simulation on the interaction of median fins for enhancing vortex dynamics and propulsion performance in fish self-propelled swimming

Yang,

Li,

et al. 2024

Studies have shown that fish can enhance propulsion performance by utilizing the interaction between median fins (dorsal, anal, and caudal fins), compared to fish with only caudal fin. However, most of the current studies are based on the fish oscillating in-place, and the analysis of median fins interaction to improve swimming propulsion performance is still insufficient, and the mechanism needs further study. This study applied three-dimensional numerical simulation methods to solve the process of grass carp accelerating from a stationary state to cruising state under different body and median fins combination, as well as different motion parameter models. A comparative quantitative analysis of different models was conducted to assess the impact of median fins interactions on enhancing swimming performance, with a detailed analysis of the hydrodynamic mechanisms and their relationship with vortex dynamics. The results indicated that interactions between median fins could generate significant hydrodynamic benefits, with the fish's average swimming speed increasing up to 4.6 times, thrust up to 33.47%, and swimming efficiency up to 25.48%. This study found that the enhancement of propulsion performance was due to the formation of high-intensity and persistent posterior body vortices by the movements of the dorsal and anal fins, which were captured by the leading-edge of the caudal fin, greatly enhancing the strength of the leading-edge vortex. This study elucidates the hydrodynamic mechanisms of the interaction between median fins and could provide new insights into the efficient swimming mechanism of fish in nature.