# Effect of Caudal Keel Structure on the Head Stability of a Bionic Dolphin Robot

## TL;DR

This study explores how the structure of a robotic dolphin's caudal keel affects its head stability during movement, offering design insights for biomimetic underwater robots.

## Contribution

The study introduces a dual stabilization mechanism through caudal keel geometry, combining experimental and CFD methods to quantify design trade-offs.

## Key findings

- The optimal keel configuration reduced head pitch angle deviation by 20.9% at 2 Hz.
- Non-dimensional height (h/c) was identified as the dominant stabilizing parameter with diminishing returns beyond an optimal range.
- The most stable keel design increased the Cost of Transport by approximately 9.1%.

## Abstract

To address the challenge of head stability in a biomimetic robotic dolphin during self-propulsion, this study systematically investigates the passive stabilization mechanism of a bio-inspired caudal keel. A combined experimental and computational fluid dynamics (CFD) approach was employed to evaluate four keel geometries across a tail oscillation frequency range of 0.5–2 Hz. The experimental results demonstrate that the optimal keel configuration reduced the standard deviation of the head pitch angle by 20.9% at 2 Hz. CFD analysis revealed a dual stabilization mechanism: an effective keel not only attenuates the intensity of the primary disturbance moment at the driving frequency but, more critically, also enhances the spectral purity of the signal by suppressing high-frequency harmonics and broadband stochastic noise through the systematic reorganization of caudal vortices. A systematic investigation of keel geometry identified non-dimensional height (h/c) as the dominant parameter, with its stabilizing effect exhibiting diminishing returns beyond an optimal range. Furthermore, a quantifiable design trade-off was established, showing an approximate 9.1% increase in the Cost of Transport (CoT) for the most stable configuration. These findings provide quantitative design principles and a deeper physical insight into the passive stabilization of biomimetic underwater vehicles, highlighting the importance of both disturbance intensity and spectral quality.