Jumping Archer Fish Hydrodynamics in 3D
Archer fish (genus Toxotes) exhibit multi-modal prey capture strategies combining spitting, rapid in-water pursuit, and jumping to feed in competitive environments. Archer fish can successfully jump to capture prey located several body lengths out of the water from directly below the surface with zero initial velocity. This rapid maneuver leaves limited space (one body length) to accelerate before fully exiting the water. The biomechanics and hydrodynamics of the fin and body motions for propelling, steering, and stabilizing the fish is highly three-dimensional and of interest to engineers aiming to replicate these aquatic launches. I will present results from 3D PIV studies on live jumping archer fish over a range of jump heights. Experiments focus on the interactions between three posterior fins (anal, dorsal, and caudal) and the role of these fins in stability and propulsion. The timing, interactions, and relative contributions to thrust and lateral forces from each fin show direct interactions between the upstream momentum generated by the anal and dorsal fins and the caudal fin in subsequent tail strokes. Strong wake features from the anal fin are additionally observed independently of the caudal fin wake. We find that fin interactions vary with the height of the jump as a result timing and body posture changes. When the initial posture is closer to horizontal (e.g., lower jump heights) the entire span of the anal fin is oriented such that its undulation produces a downward jet. As the body rotates toward vertical throughout the jump, the tail passes through a majority of the anal fin’s wake. At higher jump heights the initial body posture is more vertical and the tail passes through the wake of the posterior lobe of the downward facing anal fin and a similar region behind the dorsal fin.
Time-resolved, volumetric measurements of the unsteady hydrodynamic wake structures are acquired using a light-field imaging and synthetic aperture refocusing methodology. The novel three-dimensional, three-component (3D-3C) light field particle imaging velocimetry (PIV) method uses a multi-camera array to resolve volumetric flow fields, from vortex rings to multiphase flows. Light Field PIV system represents the next generation of 3D PIV techniques and can have significant impact on the community due to the relatively low cost of image reconstruction and the ability to resolve densely seeded flow fields, to image near deformed interfaces, such as the archer fish fins. Results from 3D PIV studies on live jumping archer fish over a range of jump heights will be discussed in the context of propulsive performance and jumping statistics.