Name: flapping soft fin deformation modeling using planar laser-induced fluorescence imaging
Uploaded: 2022-04-28T13:30:00
Description: Watch this Scientific Journal Video about Flapping Soft Fin Deformation Modeling using Planar Laser-Induced Fluorescence Imaging at JoVE.com

This research was supported by the Office of Naval Research through a US Naval Research Laboratory (NRL) 6.2 base program and performed while Kaushik Sampath was an employee of the Acoustics Division at NRL and Nicole Xu held an NRC Research Associateship award in the Laboratories for Computational Physics and Fluid Dynamics at NRL. The authors would like to acknowledge Dr. Ruben Hortensius (TSI Inc.) for technical support and guidance.

1. Fin fabrication
<ol>
	<li>Build a fin mold based on the desired shape design.
	<ol>
		<li>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.</li>
		<li>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.</li>
	</ol>
	</li>
	<li>Mi...

<ol>
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Planar laser-induced fluorescence is typically used to visualize aqueous flows by seeding the fluid with dye, which fluoresces when exposed to a laser sheet25,26. 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...

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 techniques1,2,3,4,5,6,7,8,9,10,11,12. The importance of characterizing tip vortex generation and wake inclination to understand thrust generation in single- and multi-fin systems has been documented in numerous studies, both computational and experimental13,14,15,16,17,18. For fin mechanisms made of compliant materials, shown in various studies to reduce wake inclination and increase thrust17, it is also essential to capture and accurately model their deformation time-history to pair with the flow structure analysis. These results can then be used to validate computational models, inform fin design and control, and facilitate active research areas in unsteady hydrodynamic loading on flexible materials, which need validation19. Studies have used direct high-speed image-based shape tracking in shark fins and other complex objects20,21,22, but the complex 3D fin shape often blocks optical access, making it difficult to measure. Thus, there is a pressing need for a simple and effective method to visualize flexible fin motion.
A material widely used in compliant fin mechanisms is polydimethylsiloxane (PDMS) due to its low cost, ease of use, ability to vary stiffness, and compatibility with underwater applications23, as described extensively in a review by Majidi et al.24. In addition to these benefits, PDMS is also optically transparent, which is conducive to measurements using an optical diagnostic technique such as planar laser-induced fluorescence (PLIF). Traditionally within experimental fluid mechanics25, PLIF has been used to visualize fluid flows by seeding the fluid with dye or suspended particles or taking advantage of quantum transitions from species already in the flow that fluoresce when exposed to a laser sheet26,27,28,29. This well-established technique has been used to study fundamental fluid dynamics, combustion, and ocean dynamics26,30,31,32,33.
In the present study, PLIF is used to obtain spatiotemporally resolved measurements of shape deformation in flexible fish-inspired robotic fins. Instead of seeding the fluid with dye, the underwater kinematics of a PDMS fin are visualized at various chordwise cross-sections. Although planar laser imaging can be performed on regular cast PDMS without additional fluorescence, modifying PDMS to enhance fluorescence can improve the signal-to-noise ratio (SNR) of the images by reducing the effects of background elements, such as the fin mounting hardware. PDMS can be made fluorescent by employing two methods, either by fluorescent particle seeding or pigmentation. It has been reported that, for a given part ratio, the former alters the stiffness of the resultant cast PDMS34. Therefore, a nontoxic, commercially available pigment was mixed with transparent PDMS to cast fluorescent fins for the PLIF experiments.
To provide an example of using these fin kinematics measurements for computational model validation, the experimental kinematics are then compared with values from the coupled fluid-structure interaction (FSI) models of the fin. The FSI models used in the computations are based on the first seven eigenmodes computed using the measured material properties for the fins. Successful comparisons validate fin models and provide confidence in using the computational results for fin design and control. Further, the PLIF results demonstrate that this method can be used to validate other numerical models in future studies. Additional information about these FSI models can be found in prior work35,36 and in fundamental texts of computational fluid dynamics methods37,38. Future studies can also allow for simultaneous measurements of solid deformations and fluid flows for improved experimental studies of FSI in robotic fins, bioinspired soft robots, and other applications. Furthermore, because PDMS and other compatible elastomers are widely used in various fields, including sensors and medical devices, visualizing deformations in flexible solids using this technique can benefit a larger community of researchers in engineering, physics, biology, and medicine.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>ADMET controller</td>
			<td>ADMET</td>
			<td>MTESTQuattro</td>
			<td></td>
		</tr>
		<tr>
			<td>Axon II</td>
			<td>Society of Robots</td>
			<td></td>
			<td>Microcontroller for the fin hardware</td>
		</tr>
		<tr>
			<td>Berkeley Nucleonics Delay Generator</td>
			<td>Berkeley Nucleonics Corp</td>
			<td>Model 525</td>
			<td>BNC delay generator and software</td>
		</tr>
		<tr>
			<td>BobCat Cam Config</td>
			<td>Imperx</td>
			<td></td>
			<td>Camera settings software</td>
		</tr>
		<tr>
			<td>CCD camera</td>
			<td>Imperx</td>
			<td>B2340</td>
			<td>4 MegaPixel</td>
		</tr>
		<tr>
			<td>COMSOL</td>
			<td>COMSOL Inc</td>
			<td></td>
			<td>Commercial structural dynamics software for fluid-structure interaction modeling</td>
		</tr>
		<tr>
			<td>D646WP Servo</td>
			<td>Hitec</td>
			<td>36646S</td>
			<td>32-Bit, Digital, High Torque, Waterproof Servo for the fin pitch rotation</td>
		</tr>
		<tr>
			<td>D840WP Servo</td>
			<td>Hitec</td>
			<td>36840S</td>
			<td>32-Bit, Multi Purpose, Waterproof, Steel Gear Servo for the fin stroke rotation</td>
		</tr>
		<tr>
			<td>Electric Pink fluorescent pigment</td>
			<td>Silc Pig</td>
			<td>PMS812C</td>
			<td></td>
		</tr>
		<tr>
			<td>EverGreen (532 nm dual pulsed Nd:YAG laser system)</td>
			<td>Quantel</td>
			<td>EVG00070</td>
			<td>Laser head and power supply, 70 mJ</td>
		</tr>
		<tr>
			<td>Force transducer</td>
			<td>ADMET</td>
			<td>SM-10-961</td>
			<td>10 lbf load cell</td>
		</tr>
		<tr>
			<td>FrameLink Express</td>
			<td>Imperx</td>
			<td></td>
			<td>Camera capture software</td>
		</tr>
		<tr>
			<td>Longpass fluorescence filter</td>
			<td>Edmund Optics</td>
			<td></td>
			<td>560 nm</td>
		</tr>
		<tr>
			<td>MATLAB</td>
			<td>MathWorks</td>
			<td></td>
			<td>Software for image analysis</td>
		</tr>
		<tr>
			<td>Planetary centrifugal mixer</td>
			<td>THINKY MIXER</td>
			<td>AR-100</td>
			<td></td>
		</tr>
		<tr>
			<td>Silicone rubber compounds</td>
			<td>Momentive</td>
			<td>RTV615</td>
			<td>Clear PDMS</td>
		</tr>
		<tr>
			<td>Stratasys J750</td>
			<td>Stratasys</td>
			<td></td>
			<td>3D printer, polyjet</td>
		</tr>
		<tr>
			<td>Universal testing machine</td>
			<td>ADMET</td>
			<td>eXpert 2611</td>
			<td>Table top model</td>
		</tr>
		<tr>
			<td>VeroBlack</td>
			<td>Stratasys</td>
			<td></td>
			<td>3D printer material to build the molds</td>
		</tr>
		<tr>
			<td>VeroGray</td>
			<td>Stratasys</td>
			<td></td>
			<td>3D printer material to build the molds</td>
		</tr>
	</tbody>
</table>

flapping soft fin deformation modeling using planar laser-induced fluorescence imaging

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.

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...

Watch this Scientific Journal Video about Flapping Soft Fin Deformation Modeling using Planar Laser-Induced Fluorescence Imaging at JoVE.com

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

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.

Propulsive mechanisms inspired by the fins of various fish species have been increasingly researched, given their potential for improved maneuvering and ...

The ability to measure deformations of soft robotic fins non-intrusively is important because we can better inform fin design and control for underwater vehicles and validate computational models. We can repurpose an existing tool within fluid dynamics, called planer laser induced fluorescence and expand it for solids, which could then be used for simultaneous measurements of solids and fluids. This method is generalizable to soft robotic systems and materials.We can use this technique, not only to validate fluid structure interactions, but also to study flexible materials for sensors and medical applications. Begin by designing and building a custom 3D printed gloss finished mold of fin shape. Prepare an experimental setup by mounting a pulsed laser system on a rectangular glass water tank to generate a planer light sheet intersecting the tank at its midplane at 30 Hertz.Install a four megapixel charge coupled device, or CCD camera, with a 35 millimeter lens and a long pass fluorescence filter of 516 nanometers. After installation, perform a calibration of the micrometer to pixel conversion by taking a single image from the CCD camera with a ruler placed in the laser sheet plane. Then select two positions on the camera and divide the distance in micrometers by separating pixels.Ensure the micrometer to pixel ratio is small enough or in the sub millimeter range for the application. Synchronize the laser pulses and camera images with the flapping fin using trigger outputs from the fin software and signals from a delay generator and associated software. Ensure that all laser safety is per the institutional guidelines.To set the laser system, turn the laser system on with the power key that runs the chiller to cool the laser heads. The fault lights blink until the system is ready to power the lasers. Set the trigger source to external lamp, external Q switch.For both laser heads, set the laser energy to 60 to 80%of the full power and press each Q switch button. Then turn the lasers on by pressing the power button. Next, plug in the power cables to the camera and ensure proper connections to the computer before opening the camera setting software and selecting the proper port.After turning the delay generator on, connect the external gate channel to the fin trigger, channel E to the camera, and channels A to D to the laser. Then, open the delay generator software to select the pulse mode to burst and system resolution to four nanoseconds. Set the period in seconds.Adjust the external trigger/gate mode to triggered, threshold to 0.20 volts and trigger edge as rising. Also, set channels as described in the script. Align the fin, so the laser sheet passes through one cord wise section of the fin at a span wise position, and then secure the fin platform with the mounting hardware.Connect the power to the fin control hardware and fin motors to begin fin flapping with the selected kinematics, and turn off all the ambient lights. In the delay generator software, press run to begin the synchronized experiments and acquire images of the intersection of the laser sheet with the fin throughout the stroke cycle. Observe the fin flapping in the tank with laser sheet on and ambient lights off.When done, press stop before disconnecting the fin from the power source. Move the fin platform, so the laser sheet crosses at a new span wise position. Replace the fin with additional desired fin membranes and perform experiments as demonstrated before to acquire the images for the desired number of measurements.Analyze the image by extracting all the white objects representing fin cross sections from the BW area, filt dot M binary image and displaying the image with the M show dot M.Then create a trace of the binary image boundary for each image to obtain a 2D shape, by selecting all the fin pixels that touch the black background pixels. Compare the resulting fin shapes with the 3D fluid structure interaction, or FSI models, generated from the center lines, to showcase how the process can be used as high fidelity validation. The programmed fin kinematics yielded a stroke amplitude of 43 degrees and a pitch amplitude of 17 degrees.The image illustrates the comparisons at two positions in the stroke. One in the middle of the upstroke and one in the middle of the downstroke. Furthermore, the comparisons were made between the shape deformations of the PDMS 10 to one, and PDMS 20 to one fin.The 3D fin shapes were reconstructed from the planer laser induced fluorescence, FSI, and rigid cases in the mid upstroke, demonstrating the capability of the present technique for providing high fidelity validation for the FSI simulations. In the experiment, the fin cross section was not visible at every step due to the opaque rigid spar. The image shows the result where the fin was not visible.The most important thing to remember is to test the synchronization of the components before running full experiments. Once the timing is set, perform a test run first.

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