Computational analysis of vortex dynamics and performance enhancement due to body–fin and fin–fin interactions in fish-like locomotion

Abstract

Numerical simulations are used to investigate the hydrodynamic benefits of body–fin and fin–fin interactions in a fish model in carangiform swimming. The geometry and kinematics of the model are reconstructed in three-dimensions from high-speed videos of a live fish, Crevalle Jack (Caranx hippos), during steady swimming. The simulations employ an immersed-boundary-method-based incompressible Navier–Stokes flow solver that allows us to quantitatively characterize the propulsive performance of the fish median fins (the dorsal and the anal fins) and the caudal fin using three-dimensional full body simulations. This includes a detailed analysis of associated performance enhancement mechanisms and their connection to the vortex dynamics. Comparisons are made using three different models containing different combinations of the fish body and fins to provide insights into the force production. The results indicate that the fish produces high performance propulsion by utilizing complex interactions among the fins and the body. By connecting the vortex dynamics and surface force distribution, it is found that the leading-edge vortices produced by the caudal fin are associated with most of the thrust production in this fish model. These vortices could be strengthened by the vorticity capture from the vortices generated by the posterior body during undulatory motion. Meanwhile, the pressure difference between the two sides of posterior body resulting from the posterior body vortices (PBVs) helps with the alleviation of the body drag. The appearance of the median fins in the posterior region further strengthens the PBVs and caudal-fin wake capture mechanism. This work provides new physical insights into how body–fin and fin–fin interactions enhance thrust production in swimming fishes, and emphasizes that movements of both the body and fins contribute to overall swimming performance in fish locomotion.

Tuna locomotion: a computational hydrodynamic analysis of finlet function

“…A few previous numerical studies [13][14][15] on the hydrodynamics of simplified finlets have been conducted in fish-like propulsion, in which finlets were modelled as rigid strip-like elongated fins that were not independently mobile. Similar elongated dorsal/anal fins were studied in crevalle jack (Caranx hippos) swimming [16]. Among these results, finlets/fins were found to operate in the local flow that is converging to the posterior body, mainly induced by the posteriorly narrowed body of the fishes [14,16].…”

“…Similar elongated dorsal/anal fins were studied in crevalle jack (Caranx hippos) swimming [16]. Among these results, finlets/fins were found to operate in the local flow that is converging to the posterior body, mainly induced by the posteriorly narrowed body of the fishes [14,16]. Enhanced mean thrust and propulsive efficiency attributed to the simplified finlets were found in a tuna-like model [13,15].…”

The fish body functions as an airfoil: surface pressures generate thrust during carangiform locomotion

“…where thrust was concentrated on the caudal region (21,22). But, the timing of the peaks in positive pressure thrust on the leading side of the caudal fin and in negative pressure thrust on the trailing side differs (Fig.…”

“…This thrust production mechanism means that the anterior body produces less drag than it might otherwise, but it is still net drag producing. Dubois et al (42)(43)(44), Anderson et al (19), Borazjani and Sotiropoulos (21), and Liu et al (22) all find that the anterior body produces net drag forces. Our work does not contradict these findings; indeed, we find that, on the anterior body, the magnitude of the negative pressure thrust forces is smaller than the sum of drag forces (positive pressure drag on the tip of the snout, segment 1, 0-10% L, and negative pressure drag on the midbody, segment 3, 20-40% L) ( Figs 5, S2).…”

“…Some previous researchers hypothesized that fish could benefit from this effect, with local thrust greatly reducing the impact of the net drag expected on a carangiform swimmer's anterior body (20). Indeed, in computational models, one can see areas of negative pressure on the anterior body (21,22), but this effect has never been studied systematically or in living fishes. We therefore used a recent set of tools (23,24) to quantify the pressure and forces produced during swimming for two fish species that both have airfoil-shaped anterior bodies, bluegill sunfish (Lepomis macrochirus Rafinesque) and brook trout (Salvelinus fontinalis Mitchill), at high temporal and spatial resolution, the first such experimental test for negative pressure thrust production in living carangiform swimmers.…”