Hydrodynamics of tandem flapping pectoral fins with varying stroke phase offsets

TL;DR

This study investigates how the phase difference between tandem flapping fins affects thrust and wake dynamics, revealing optimal phasing for enhanced propulsion in bioinspired underwater vehicles.

Contribution

It provides detailed experimental and computational analysis of fin phase effects on thrust and wake interactions, informing design strategies for underwater propulsion systems.

Findings

Rear fin thrust varies with phase offset, peaking at 25% more thrust when lagging by a quarter cycle.

Wake coherence and vortex structures are significantly influenced by fin phase differences.

Flow interactions and vortex shedding patterns are characterized, impacting propulsion efficiency.

Abstract

We show how phasing between tandem bioinspired fins flapping at high-stroke amplitudes modulates rear fin thrust production and wake characteristics. Load cell thrust measurements show that the rear fin generates 25% more thrust than the front fin when it lags the latter by a quarter cycle, and performs 8% worse when it leads the front fin by the same amount. The flow interactions between the fins responsible for these observations are analyzed using two-dimensional particle image velocimetry measurements and three-dimensional computational fluid dynamics simulations. Distributions of velocity elucidate variations in the effective flow induced on the rear fin for different phase offsets. Vortex structure interactions and particle rakes reveal the contributions of the leading- and trailing-edge vortices shed by the front fin in modulating the suction at the rear fin leading edge. Furthermore, the wake structure far downstream of the fins changes in its coherence, axial and radial extents for the different phase offsets. These findings are relevant for the design and performance optimization of various unmanned underwater vehicles that utilize such tandem systems.