Active tail flexion in concert with passive hydrodynamic forces improves swimming speed and efficiency

Abstract

Fish typically swim by periodic bending of their bodies. Bending seems to follow a universal rule; it occurs at about one-third from the posterior end of the fish body with a maximum bending angle of about $30^{\circ }$ . However, the hydrodynamic mechanisms that shaped this convergent design and its potential benefit to fish in terms of swimming speed and efficiency are not well understood. It is also unclear to what extent this bending is active or follows passively from the interaction of a flexible posterior with the fluid environment. Here, we use a self-propelled two-link model, with fluid–structure interactions described in the context of the vortex sheet method, to analyse the effects of both active and passive body bending on the swimming performance. We find that passive bending is more efficient but could reduce swimming speed compared with rigid flapping, but the addition of active bending could enhance both speed and efficiency. Importantly, we find that the phase difference between the posterior and anterior sections of the body is an important kinematic factor that influences performance, and that active antiphase flexion, consistent with the passive flexion phase, can simultaneously enhance speed and efficiency in a region of the design space that overlaps with biological observations. Our results are consistent with the hypothesis that fish that actively bend their bodies in a fashion that exploits passive hydrodynamics can at once improve speed and efficiency.

Locomotion performance for oscillatory swimming in free mode

“…This behaviour is perfectly consistent with the physical meaning of the two contributions related to the added mass and to the vortex shedding, respectively. The sample flow fields reported in figure 9, inspired by previous analyses [41,42], may give a simple idea to understand their counteracting role. As illustrated in figure 9(a), the potential acyclic field u ϕ generated by the caudal fin downstroke leads to a counterclockwise angular velocity Ω ϕ in the opposite direction with respect to tail motion.…”

Inviscid modeling of unsteady morphing airfoils using a discrete-vortex method

“…A 30% total chord flap length was chosen following values from the literature on the topic, 19 and observed in flapping-like means of locomotion in nature. 17 Results for both configurations are included in Sect. 4.…”

Research on the effects of complex terrain on the hydrodynamic performance of a deep-sea fishlike exploring and sampling robot moving near the sea bottom

“…Ogata et al (2017) have simulated the swimming processes of fishlike swimmers at various Reynolds numbers (Re) and analyzed several data sets of flow field using Q-criterion isosurfaces to build a prediction model for the terminal swimming speed at different Re. Hang et al (2022) have designed a self-propelled two-link model to analyze the effects of both active and passive body bending on the swimming performance of robotic fish, and the results showed that speed and efficiency could be improved simultaneously when fish actively bend their bodies in a fashion that exploited passive hydrodynamics. Xing et al (2022) have proposed a novel bionic pectoral fin and experimentally studied its hydrodynamic performance, and the results indicated that the hydrodynamic performance was closely related to the motion equation parameters including the amplitude, frequency, and phase difference.…”