Airfoil-like mechanics generate thrust on the anterior body of swimming fishes

Abstract

The anterior body of many fishes is shaped like an airfoil turned on its side. With an oscillating angle to the swimming direction, such an airfoil experiences negative pressure due to both its shape and pitching movements. This negative pressure acts as thrust forces on the anterior body. Here, we apply a high-resolution, pressure-based approach to describe how two fishes, bluegill sunfish (Lepomis macrochirus Rafinesque) and brook trout (Salvelinus fontinalis Mitchill), swimming in the carangiform mode, the most common fish swimming mode, generate thrust on their anterior bodies using leading-edge suction mechanics, much like an airfoil. These mechanics contrast with those previously reported in lampreys—anguilliform swimmers—which produce thrust with negative pressure but do so through undulatory mechanics. The thrust produced on the anterior bodies of these carangiform swimmers through negative pressure comprises 28% of the total thrust produced over the body and caudal fin, substantially decreasing the net drag on the anterior body. On the posterior region, subtle differences in body shape and kinematics allow trout to produce more thrust than bluegill, suggesting that they may swim more effectively. Despite the large phylogenetic distance between these species, and differences near the tail, the pressure profiles around the anterior body are similar. We suggest that such airfoil-like mechanics are highly efficient, because they require very little movement and therefore relatively little active muscular energy, and may be used by a wide range of fishes since many species have appropriately shaped bodies.

How zebrafish turn: analysis of pressure force dynamics and mechanical work

“…The proposed method is suitable for our problem, as it is able to cope with fluid-solid interfaces based on the median averaging that is used across different integration paths. Previous research has validated the pressure fields obtained using this approach for both static and dynamic examples in aquatic locomotion (Dabiri et al, 2014;Gemmell et al, 2015;Lucas et al, 2017Lucas et al, , 2020. The influence of the grid size on the spatial convergence of the underlying algorithm is discussed in appendix 1 of Dabiri et al (2014).…”

“…Furthermore, we made the assumption that pressure is equally distributed along the depth of the body, and thus that forces could be obtained by multiplying the computed pressure at the mid-section of the body with corresponding surface area slices perpendicular to the midline. This approach has been discussed further and validated by Lucas et al (2017) (see also Lucas et al, 2020).…”

“…However, the use of a laser comes with two key drawbacks: (1) non-transparent objects in the fluid cast shadows and occlude the movement of particles therein, which often requires the use of two aligned laser sheets (e.g. Lucas et al, 2020); (2) bright laser light sheets can alter fish behavior, especially in the case of smaller fish such as zebrafish, where laser light sheet thickness is of the order of 20-30% of body depth, and our goal was to study spontaneous turning behavior. Behavioral alteration of fish swimming behavior by laser light is insufficiently studied, and we observed zebrafish avoiding laser light sheets in preliminary experiments.…”

“…The force direction was determined by the normal direction to the body-fluid interface, because pressure-based forces act normal to the surface. The underlying assumption for this approach is that shear forces are small (Lucas et al, 2017(Lucas et al, , 2020, and that the hydrodynamic interaction forces largely result from pressure-based effects. This assumption is well justified for swimming at larger Re numbers (Re≈10,000).…”

“…However, to accomplish such pressure-based analyses it is necessary to obtain detailed measurements of both body motion (kinematics) and forces (dynamics) that result from body surface pressure changes, and surface pressures need to be estimated around the swimming fish throughout the maneuver. Fortunately, recent work has provided a method to successfully derive pressure fields from velocity fields in freely moving animals and has validated this approach against canonical cases (Dabiri et al, 2014;Gemmell et al, 2015;Lucas et al, 2017Lucas et al, , 2020. This paves the way for determining distributed forces along the body around swimming fish during maneuvers, estimating longitudinal patterns of positive and negative work, and quantifying overall dynamics and energetics during routine turning maneuvers in live animals.…”