Performance and optimization of multi-fin oscillatory propulsion

Multi-fin bioinspired propulsion systems have seen a rise in interest recently due to natural occurrence in animal systems and emerging technology. These systems make use of fin-fin interaction to improve swimming performance where the downstream fins use the wake of the upstream fin to their benefit. While 2-fin systems are relatively well understood, it is unclear how performance is impacted as the number of fins grow. Using an immersed boundary method solver, we model the multi-fin systems to analyze performance across several key parameters: spacing between fins, phase offset, freestream velocity, and number of fins per system. We characterize performance via system thrust and propulsive efficiency. We show that a dimensionless phase parameter incorporating inter-fin spacing, oscillation frequency, and freestream velocity approximately collapses the thrust response across different operating conditions, providing a useful predictor of favorable wake-interaction timing. As more fins are added ($>2$), they can further enhance the group thrust and efficiency without modifying the performance of the fins upstream. Through optimization, we found this enhancement to peak with a 3-4 fin system, after which performance diminishes with additional fins due to the growing wake complexity. Optimal conditions led to boosts in thrust or efficiency of 30% and 20%, respectively. From this study, we show that there are performance gains realized in larger fin systems (though with diminishing returns) and optimizing system performance of multi-fin systems can be done sequentially to save computational resources.