Tethering and depth of submergence affect the swimming performance of undulatory robots

TL;DR

This study investigates how tethering and submergence depth influence the swimming performance of undulatory robots, revealing that submerged and free swimming improve efficiency and speed, with implications for biomimetic design.

Contribution

It provides experimental insights into the effects of tethering and depth on robotic undulatory swimming performance, explaining discrepancies in previous studies.

Findings

Tethered surface swimming has similar speed but lower energy cost than free swimming.

Deeper submergence improves speed and energy efficiency by over 10%.

Increased wave drag near the surface reduces performance.

Abstract

Over the past few decades, biomimetic robotic experiments have significantly advanced our understanding of undulatory swimming. Compared to animal experiments, robotic experiments offer repeatability and controlled parameter variations, but the robots operate under constraints that differ from those experienced by their natural counterparts. Freely swimming robots often remain on the surface, whereas most undulatory fish, including eels, are typically fully submerged during locomotion. Studies focusing on submerged swimming commonly rely on tethered robots to maintain depth control. This study examines the performance implications of surface versus submerged swimming, and tethered versus free swimming, using the robotic undulatory swimmer 1-guilla. The robot was tested in two configurations: free swimming in a pool and tethered swimming in a water channel at the surface and at varying depths down to three body heights. We varied kinematic input parameters and quantified performance in terms of swimming speed, cost of transport, and body kinematics. Our results reveal that at the surface, tethered swimming achieves speeds comparable to free swimming but at a lower energetic cost. This reduction in cost of transport is attributed to the suppression of body roll during tethered operation. Increasing submergence depth improved both the maximum speed and energy efficiency by more than 10% relative to the surface swimming performance. As the body kinematics remained unchanged when submerged, the performance deficit near the surface is attributed to increased wave drag. Overall, our findings provide explanations and insights into discrepancies in results obtained for tethered and free-swimming robotic studies, they highlight the hydrodynamic challenges of surface locomotion, and can help explain why natural undulatory swimmers predominantly favor submerged propulsion.