Geometric Characteristics of Flapping Foils for Enhanced Propulsive Efficiency

Mechanisms for hydrodynamic benefit via fluid interactions in large planar fish schools ( n ≥ 10) are investigated by two-dimensional numerical simulations of carangiform fish swimming. It is observed that the average swimming efficiency of the 10-fish school is increased by 30% over a single swimmer, along with a thrust production improvement of 114%. The performance and flow analyses characterize the associated hydrodynamic interaction mechanisms in large dense schools leading to enhanced performance. First, anterior body suction arises from the proximity of the suction side of the flapping tail to the head of the following fish. Next, the block effect is observed as another fish body blocks the flow behind a fish. Finally, the wall effect enhances the flow of momentum downstream where the body of a neighboring fish acts as a wall for the flapping of a fish tail moving toward it. Because these primary body–body interactions are based on the arrangement of surrounding fish, a classification of the individual fish within the school is presented based on the intra-fish interactions and is reflected in the performance of the individuals. It is shown that the school can be separated as front fish, middle fish, edge fish, and back fish based on the geometric position, performance, and wake characteristics. Finally, groupings and mechanisms observed are proven to be consistent over a range of Reynolds numbers and school arrangements.

Hydrodynamics of body–body interactions in dense synchronous elongated fish schools

Kelly

Menzer

2023

Wave-assisted propulsion: An experimental study on traveling ships

Chan,

Wang,

Yang

et al. 2024

A submerged hydrofoil interacting with incoming waves produces combined heaving and pitching motion, facilitating the conversion of wave energy into thrust. When the foil is attached to the ship hull, the generated “green” power from wave energy could assist the ship's propulsion system and significantly reduce fuel costs. This study experimentally assesses thrust generation from a fixed mid-hull foil by comparing towing force at different wave and traveling speeds. The optimal mid-hull foil demonstrates a fuel cost reduction ranging from 10.3% to 20.4% at diverse traveling speeds and wave parameters. Thrust generation increases at higher traveling speeds. Additionally, this study mathematically describes the hydrofoil motion with an outer pivot, which better suits the ship–foil model. This study then introduces a Strouhal number (StA,S) specifically for the ship–foil model, considering ship travel, ship response, and the hydrofoil's rotation around its outer pivot.

Effects of body shape on hydrodynamic interactions in a dense diamond fish school

Kelly,

Dong

2024

To examine the effect that body shape plays in the interactions between fish in a dense fish school, fish-like undulating foils are numerically studied in a high-density diamond school. Shape parameters of leading edge radius, boattail angle, and maximum thickness location along the body are independently varied to control the body shape. A traveling wave is prescribed to the body, and the flow around the school is solved using an immersed boundary method-based incompressible Navier–Stokes flow solver. Our findings indicate that body shape does play a significant role in the performance of the school and varies school efficiency and thrust productions by as much as 7% and 40%, respectively, and changes the efficiency in individual swimmers within the school by up to 25%. The leading edge radius drives the scale of the anterior body suction experienced throughout the school. A rounder leading edge generates more suction but consequently consumes more power. Changes to the location of the maximum thickness along the foil significantly impact the wall effect in the school by changing the shape of the “wall.” A maximum thickness that occurs at or in front of the closest point of interaction between the body and tail is favored. Finally, changes to the boattail angle manipulate the shape of the active channels in the school altering vortex–body interactions and the wall effect. A large boattail angle leads to a pinching that prohibits significant flow in the channels, reducing performance.