[Paper Review] Bio-inspired underwater propulsors

Tyler Van Buren, Daniel Floryan+1 more|arXiv (Cornell University)|Jan 29, 2018

Micro and Nano Robotics4 citations

TL;DR

This paper proposes bio-inspired underwater propulsion systems by analyzing biological swimming mechanisms—oscillatory, undulatory, pulsatile jet, and drag-based—demonstrating that drag-based propulsion, when optimized with impulsive, high-drag shapes like half-cylinders, can achieve high speed and efficiency. The key contribution is a novel amphibious vehicle design using spring-loaded, impulsively exposed half-cylinders on tracks, achieving high thrust-to-weight ratios and enabling dual-mode land and water mobility.

ABSTRACT

Here we present a general overview of bio-inspired propulsion. We identify the major types of aquatic swimmers (oscillatory, undulatory, pulsatile, and drag-based) and break down their mechanisms for thrust production (drag-based, lift-based, added mass, and momentum injection). For each swimmer, we (1) discuss wake characteristics; (2) derive expressions for thrust and efficiency; and (3) design a specific bio-inspired aquatic vehicle concept. This document will be a book chapter, which at the moment is unpublished.

Motivation & Objective

• To identify and analyze the physical mechanisms behind high-performance biological swimming techniques for engineering inspiration.
• To challenge the assumption that drag-based propulsion is inherently inefficient, demonstrating its potential for high-speed, high-efficiency underwater vehicles.
• To design a novel amphibious underwater vehicle that leverages drag-based propulsion with impulsive, high-drag propulsors for dual land and water operation.
• To optimize propulsion efficiency by analyzing thrust generation mechanisms, including added mass, momentum injection, and phase-locked pitching/heaving motions.
• To provide a design framework for next-generation underwater vehicles that integrate biological principles without strict biomimicry constraints.

Proposed method

• Classifies underwater swimmers into four types: oscillatory (e.g., tuna), undulatory (e.g., eel), pulsatile jet (e.g., jellyfish), and drag-based (e.g., turtle), based on propulsion mechanisms.
• Analyzes thrust generation via four mechanisms: drag-based, lift-based, added mass forces, and momentum injection, with equations derived for steady-state drag and thrust balance.
• Applies the drag-based thrust equation $ F_d = \frac{1}{2} \rho U_\infty^2 A C_D $ to model force from bluff bodies, emphasizing the role of frontal area and drag coefficient.
• Derives the velocity ratio efficiency $ \eta_s = \frac{1}{1 + \sqrt{\xi}} $, where $ \xi = \frac{C_{D_b} A_b}{C_{D_p} A_p} $, to quantify system efficiency in paddling systems.
• Designs a concept amphibious vehicle with tank-like tracks housing spring-loaded half-cylinders that are impulsively exposed to fluid, maximizing drag during short thrust phases.
• Uses phase-locked pitching and heaving motions at ~270° offset to enhance lift-based thrust, and incorporates flexibility to improve efficiency in oscillatory systems.

Experimental results

Research questions

• RQ1 How can drag-based propulsion be optimized to achieve high speed and efficiency in underwater vehicles, contrary to the common belief of inherent inefficiency?
• RQ2 What is the optimal geometric and dynamic configuration for impulsive drag-based propulsors to maximize average thrust and system efficiency?
• RQ3 How do phase-locked pitching and heaving motions influence thrust and efficiency in oscillatory swimmers, and what is the ideal phase offset?
• RQ4 What role does flexibility play in enhancing thrust and efficiency in bio-inspired propulsors, particularly in oscillatory and undulatory systems?
• RQ5 Can a single vehicle design effectively transition between land and water propulsion using a unified drag-based mechanism with variable exposure of propulsors?

Key findings

• Drag-based propulsion, when implemented with impulsively exposed high-drag shapes like half-cylinders, can achieve high thrust and efficiency, challenging the perception of drag-based systems as inefficient.
• The theoretical system efficiency $ \eta_s = \frac{1}{1 + \sqrt{\xi}} $ shows that high efficiency is achieved when the propulsor has high drag and low frontal area, and the vehicle has low drag.
• Quasi-two-dimensional shapes produce nearly twice the drag of low aspect ratio 3D shapes, indicating that planar geometry maximizes drag for a given size.
• Limiting the flow development time from an impulsive start increases average thrust, demonstrating that short, high-acceleration thrust phases outperform steady motion.
• A phase offset of approximately 270° between pitching and heaving motions maximizes thrust and efficiency in oscillatory systems by minimizing peak angle of attack.
• The proposed amphibious vehicle design enables dual-mode operation: tracks with exposed half-cylinders allow high-drag propulsion in water, while flipping the vehicle enables land mobility, with potential for shrouding to improve hydrodynamics when submerged.