Session ZC06: Biofluids: Sensing

Snake tongue flicking : A fluid mechanics approach

Why do snakes flick their tongues? Although well known as a chemosensory behaviour, the fluid dynamical underpinnings of this fast oscillatory movement remain obscure. Here, we present an experimental three-dimensional fluid mechanics investigation of tongue flicking in snakes. The snake tongue geometry was approximated as a simple y shape : two tapering cylinders emerged from a stalk, with varying angles between them. Each geometry was oscillated in a circular arc at two different peak velocities in a quiescient fluid (water). Two Reynolds numbers (Re 38 and 126) based on peak velocities were investigated. The lower Re corresponds to scaling calculations from previous data on tongue oscillations in air of banded water snake (Nerodia fasciata), and the larger is chosen arbitrarily to explore the change in physics with greater peak velocities. Using Lagrangian flow analysis, we observe complex three-dimensional vortical structures where longitudinal vortices trailing the tongue tines (branches of the Y shape) interact with those parallel to the direction of tongue movement. Vortex patterns indicate a flow field which might promote greater odorant sampling resulting from the y-shaped geometry of the snake’s tongue. To further understand the dynamics, we conducted a modal analysis. This involved decomposing the flow into spatial modes of varying length and time scales, capturing both small-scale and large-scale structures.   

Flow visualization around larval zebrafish

Larval zebrafish are a model organism in neuroscience for understanding action-perception cycles underlying behavior. Their small size, near optical transparency, and availability of fluorescently labeled transgenic lines facilitate whole brain calcium imaging. Here, we aim to study the flow sensing capabilities of larval zebrafish by combining behavioral assays with brain imaging and PIV analysis. Thus, we fix the fish’s head in agarose gel and subject the fish to controlled flow perturbations. Utilizing Fourier Light Field Microscopy (FLFM), we conduct PIV measurements around the fish and reconstruct the full 3D flow field and tail motion of the fish. Our results provide insights to the sensitivity and workings of the posterior lateral line system.   

Flow Structure Dynamics of Undulated Cylinder Array Configurations

An experimental study of seal whisker (phocid pinniped vibrissae) inspired undulated cylinders in various array configurations is conducted to compare the structure of the resultant wakes downstream of the arrays. The study is conducted in the Portland State University wind tunnel with a test section of 5 m in length, 1.2 m in width, and 0.8 m in height. The undulated cylinders are 3D-printed geometries scaled to a mean chord length, C, of 33.63 mm, a mean thickness of 17.53 mm, and a length of 720 mm. The arrays comprise nine (3x3) undulated cylinders oriented vertically and include an inline and staggered (streamwise) configuration with a 3C spanwise spacing and 3, 6, and 9C streamwise spacing. The wakes directly downstream of the arrays are characterized by a 6C square particle image velocimetry (PIV) window (streamwise-spanwise), an inflow of 3.7 m/s, and a Reynolds number of 8,600. Momentum and second-order moments are calculated to describe wake behavior and configuration effects. Results are compared to a single undulated cylinder and a comparable smooth cylinder. Streamwise spacing for the staggered array shows minimal impact on downstream wake behavior, while the inline configuration wake deficit significantly increases with closer streamwise spacing. The bio-inspired experimental study has wide-ranging application potential including reduced column vibration, fluid mixing applications, heat transfer control, and behavioral biological research.   

Understanding whisker array sensing using interpretable machine learning

Equipped with their whisker arrays, seals have exquisite sensitivity and intelligence for tactile and hydrodynamic sensing. While research efforts have improved the understanding of the hydrodynamically optimized whisker morphology, much less is known about how the signals from the whiskers are mapped to the environmental information being sensed. In this study, a one-way flow-structure interaction simulation setup is used to generate training data for a large-language-model based machine learning model. Whisker bending signals are collected using finite element simulations for whiskers in an array subjected to flow loading in the wake of various shapes. The flow features and their spatiotemporal correlation with whisker signals are analyzed. Arrays of whisker sensors are also being fabricated, calibrated, and assembled to generate training data to augment the training of the model. The model is trained to predict the shape and position of the upstream object based on whisker array signals. The trained model allows interpretability to gain insights into the sensing mechanism of the whisker array. It can be used in the future to power a locomotive agent for underwater applications such as trail tracking and environment mapping.   

Numerical Study on the Vibratory Response of Undulatory Seal Whiskers to Wakes

Harbor seals utilize their uniquely shaped whiskers, which have evolved as extraordinary hydrodynamic sensors, to track vortex wakes left by moving animals. Behavioral experiments have demonstrated seals' remarkable ability to differentiate and follow upstream wakes using these whiskers. Fundamental experiments revealed that the geometry of seal whiskers, when placed in the wake of a circular cylinder, generates stronger vibratory responses and better frequency synchronization compared to simple circular and elliptical cylinders. This study conducts a parametric analysis of the wake-induced vibration (WIV) of a whisker using direct numerical simulation (DNS). One segment of the whisker is modeled as an elastically mounted rigid body with periodic boundary conditions. The whisker is positioned downstream of a cylinder, with WIV analyzed in both tandem and staggered arrangements. Key parameters such as reduced velocity and cylinder relative size are also examined. Preliminary results indicate a slaloming motion, consistent with previous experimental findings. This study focuses on two mechanisms of WIV—wake stiffness-induced vibration and vortex force-induced vibration—and compares their effects on the whiskers' vibratory response. These comparisons elucidate the dominant factors contributing to WIV and enhance our understanding of how whisker geometry influences wake response.   

Flow past tandem vibrissae-inspired undulated cylinders in cross flow

The undulated vibrissae of harbor seals significantly impact flow physics, enhancing their prey-tracking abilities compared to the smooth vibrissae of California sea lions. These effects include modifications to the wake vortex sheet and reduced vortex-induced vibrations. While previous studies focused on single vibrissae, this research investigates tandem vibrissae using force measurements, particle image velocimetry, and smoke visualizations in a series of wind tunnel experiments. The normalized streamwise separation (L*) between the whiskers was varied from 2 to 5 for subcritical Reynolds numbers (Re = 1.2-2.2 x 10^4), examining two configurations: in-phase and out-of-phase undulations. Results indicate that wake profiles of undulated geometries become decorrelated at shorter streamwise separations (L*) compared to smooth cylinders. Our signal-to-noise analysis of the force spectra identified an optimal separation and phase configuration, providing insights for designing mechanical flow sensors utilizing an array of vibrissae.