Flapping Soft Fin Deformation Modeling using Planar Laser-Induced Fluorescence Imaging

Summary

The present protocol involves the measurement and characterization of 3D shape deformation in underwater flapping fins built with polydimethylsiloxane (PDMS) materials. Accurate reconstruction of these deformations is essential for understanding the propulsive performance of compliant flapping fins.

Abstract

Propulsive mechanisms inspired by the fins of various fish species have been increasingly researched, given their potential for improved maneuvering and stealth capabilities in unmanned vehicle systems. Soft materials used in the membranes of these fin mechanisms have proven effective at increasing thrust and efficiency compared with more rigid structures, but it is essential to measure and model the deformations in these soft membranes accurately. This study presents a workflow for characterizing the time-dependent shape deformation of flexible underwater flapping fins using planar laser-induced fluorescence (PLIF). Pigmented polydimethylsiloxane fin membranes with varying stiffnesses (0.38 MPa and 0.82 MPa) are fabricated and mounted to an assembly for actuation in two degrees of freedom: pitch and roll. PLIF images are acquired across a range of spanwise planes, processed to obtain fin deformation profiles, and combined to reconstruct time-varying 3D deformed fin shapes. The data are then used to provide high-fidelity validation for fluid-structure interaction simulations and improve the understanding of the performance of these complex propulsion systems.

Introduction

In nature, many fish species have evolved to use a variety of body and fin motions to achieve locomotion. Research to identify the principles of fish locomotion has helped drive the design of bioinspired propulsion systems, as biologists and engineers have worked together to develop capable next-generation propulsion and control mechanisms for underwater vehicles. Various research groups have studied fin configurations, shapes, materials, stroke parameters, and surface curvature control techniques^1,2,3,4,5<....

Protocol

1. Fin fabrication

1. Build a fin mold based on the desired shape design.
   1. Design and build a custom 3D-printed gloss-finished mold of fin shape (Figure 1). See STL files for fabricating the mold in Supplementary Coding Files 1-4.
   2. Insert structural elements into the mold, such as a 3D-printed rigid plastic leading-edge spar. See the STL file of the spar in Supplementary Coding File 2.
2. Mix PDMS (see Table of Materials) in the desired part ratio.
   1. Select the part ratio of base elastomer to curing agent (i.e.,....

Results

A trapezoidal fish-inspired artificial pectoral fin was cast in two different materials (PDMS 10:1 and 20:1, both mixed with fluorescent dye) out of a mold, each with a rigid leading-edge spar inserted into the leading quarter chord (Figure 2 and Figure 3). Tensile testing of the two fin materials (Figure 3) yielded elastic moduli of 0.38 MPa and 0.82 MPa for the PDMS 20:1 and PDMS 10:1 fins, respectively, with an R² of 0.......

Discussion

Planar laser-induced fluorescence is typically used to visualize aqueous flows by seeding the fluid with dye, which fluoresces when exposed to a laser sheet^25,26. However, using PLIF to visualize deformations in compliant materials has not been previously reported, and this study describes an approach for obtaining time history measurements of high-resolution shape deformation in flexible solid fins using PLIF. Comparing these fin measurements with FSI simulation.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
ADMET controller	ADMET	MTESTQuattro
Axon II	Society of Robots		Microcontroller for the fin hardware
Berkeley Nucleonics Delay Generator	Berkeley Nucleonics Corp	Model 525	BNC delay generator and software
BobCat Cam Config	Imperx		Camera settings software
CCD camera	Imperx	B2340	4 MegaPixel
COMSOL	COMSOL Inc		Commercial structural dynamics software for fluid-structure interaction modeling
D646WP Servo	Hitec	36646S	32-Bit, Digital, High Torque, Waterproof Servo for the fin pitch rotation
D840WP Servo	Hitec	36840S	32-Bit, Multi Purpose, Waterproof, Steel Gear Servo for the fin stroke rotation
Electric Pink fluorescent pigment	Silc Pig	PMS812C
EverGreen (532 nm dual pulsed Nd:YAG laser system)	Quantel	EVG00070	Laser head and power supply, 70 mJ
Force transducer	ADMET	SM-10-961	10 lbf load cell
FrameLink Express	Imperx		Camera capture software
Longpass fluorescence filter	Edmund Optics		560 nm
MATLAB	MathWorks		Software for image analysis
Planetary centrifugal mixer	THINKY MIXER	AR-100
Silicone rubber compounds	Momentive	RTV615	Clear PDMS
Stratasys J750	Stratasys		3D printer, polyjet
Universal testing machine	ADMET	eXpert 2611	Table top model
VeroBlack	Stratasys		3D printer material to build the molds
VeroGray	Stratasys		3D printer material to build the molds