Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session H13: Biological Fluid Dynamics: Locomotion, High Reynolds, Number Swimming I |
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Chair: Keith Moored, Lehigh Room: North 127 ABC |
Monday, November 22, 2021 8:00AM - 8:13AM |
H13.00001: Harnessing Acoustic Black Hole Effect in Elastic Propulsors to Achieve Efficient Undulatory Locomotion Alexander Alexeev, Ersan Demirer, Oluwafikayo A Oshinowo, Alper Erturk Fish have evolved to various forms of swimming to better adapt to diverse environments and flow conditions. Anguilliform fish such as eel rely on travelling waves to achieve high efficiency in low to moderate Reynolds number flows. Tthunniform fish such as tuna almost exclusively use standing waves for fast locomotion at higher Reynolds numbers. Thus, traveling waves are of particular interest for developing efficient biomimetic robotic swimmers. However, generating traveling waves in finite-sized structures is far from being a trivial task. Here, we use the acoustic black hole (ABH) effect, a phenomenon arising in tapered structures that prevents wave reflection at the free end and therefore yields traveling flexural waves, to design biomimetic propulsors with a superior hydrodynamic efficiency. Using three-dimensional fully-coupled fluid structure interaction simulations, we demonstrate that ABH is an effective solution to generate travelling waves even in relatively short propulsors and can indeed drastically improve their hydrodynamic performance. We demonstrate this by modeling propulsors with different thickness profiles and characterizing their hydrodynamics as a function of their standing to traveling wave ratios. |
Monday, November 22, 2021 8:13AM - 8:26AM |
H13.00002: Dynamic Scaling of Metachronal Rowing David W Murphy, Kuvvat Garayev Metachronal rowing, in which adjacent appendages stroke sequentially, is a common locomotion technique among organisms with multiple swimming appendages. Organisms using metachronal rowing range in size from tiny copepod nauplii and paramecia to large mantis shrimp and ctenophores and swim at a wide range of Reynolds numbers. However, the fluid mechanics of how this locomotion technique is used across such a wide range of scales is not well understood. Drawing data from the literature for a wide variety of organisms, we present a scaling analysis of metachronal rowing. We examine body and appendage length, advance ratio, body and appendage Reynolds numbers, and Strouhal number. The ratio of body to leg length varies widely in the range of 2-60 and does not correlate with body size. We find a linear relationship between the body and appendage Reynolds numbers across several orders of magnitude. Further, the ratio of body to appendage Reynolds number, which may be recast as the product of the advance ratio and the ratio of the body length to appendage length, governs flow interactions among appendages. At relatively high Reynolds numbers, these interactions may include vortex interception, which may increase swimming efficiency. |
Monday, November 22, 2021 8:26AM - 8:39AM |
H13.00003: Bound states of asynchronously flapping tandem wings Manav Guzraty, Anand U Oza We present the results of a theoretical investigation into the dynamics of tandem flapping wings, a model system for studying schooling swimmers in relatively fast flows. We develop a discrete dynamical system in which the swimmers are modeled as wings that shed point vortices in an irrotational and inviscid flow. Our model may readily be generalized to account for configurations in which the wings' flapping amplitudes and frequencies are incommensurate. We find that the wings may either separate, collide, or form unsteady bound states, and we compare our theoretical predictions with recent experiments. Generally, our results indicate how hydrodynamics may influence the dynamics of schooling swimmers in biological contexts. |
Monday, November 22, 2021 8:39AM - 8:52AM |
H13.00004: Quasi-Steady and Wake-Induced Forces Balance to Generate Equilibrium Altitudes in Near-Ground Swimming Tianjun Han, Qiang Zhong, Daniel Quinn, Keith W Moored Neutrally-buoyant near ground swimmers experience alterations in their added mass, quasi-steady, and wake-induced forces compared to swimming far from a ground plane. In fact, using a simple freely-swimming pitching hydrofoil as a model near-ground swimmer it has been shown that a hydrofoil will be attracted to a stable equilibrium altitude due to competing hydrodynamic forces. Here, a potential flow decomposition method using the unsteady Bernoulli equation is presented and applied to understand the competing forces that give rise to equilibria. It is shown that many previous hypotheses do not hold and that equilibria are a balance between negative time-averaged quasi-steady lift and positive time-averaged wake-induced lift, while the time-averaged added mass lift is nearly zero across all ground proximities. Results that run counter to previous hypotheses are examined in detail. |
Monday, November 22, 2021 8:52AM - 9:05AM Not Participating |
H13.00005: Effect of Surface Air Entrapment in Small-Scale Flapping Fin Propulsion Abigail E Hays, Paul S Krueger Propulsive performance of flapping propulsion decays with decreasing Reynolds number (Re). Introducing apparent slip at the fin surface may serve to facilitate vortex formation at low Re and improve performance. This work investigates the effects of air entrapment on the fin to provide slip flow and its effects on vortex formation from a small-scale fin propulsion. The air entrapment fin uses a constant air supply to a perforated surface to keep air at the fin surface and relax the no-slip condition. Utilizing a water tunnel, Particle Image Velocimetry (PIV) measurements of the flow around a pitching and heaving 2D flat plate fin were made at Re=300. The circulation of the vortices and time-averaged thrust were determined from these measurements for a range of flapping parameters. Using this method, the results for a fin with air entrapment will be analyzed and compared with the results from a solid, control fin. |
Monday, November 22, 2021 9:05AM - 9:18AM |
H13.00006: Propulsive Performance of a Two Degree-of-Freedom Fish Platform Seth Brooks, Melissa A Green A two degree-of-freedom fish platform was used to investigate the relationship between simplified fish kinematics and propulsive performance. Its design, construction, and actuation provide control of maximum trailing-edge excursion; heave-to-pitch ratio; and phase offset between the tail and caudal fin while the oscillation frequency is fixed. Thrust and power input were measured to evaluate the performance of each parameter set while the flow field was measured using stereo particle image velocimetry. A parametric sweep of all parameters, excluding frequency, was used to evaluate the performance and flow field. This space included and extended beyond the known biological space. It was found that within the biological range of 0.20 < St < 0.40, the efficiency was maximized for a heave-to-pitch ratio of 0.75 and a phase offset of 115 degrees. For a given heave-to-pitch ratio, the thrust was maximized for a phase offset between 93 and 115 degrees and the power input was minimized for a phase offset between 115 and 135 degrees depending on the amplitude of the trailing-edge excursion. This work provides insight into the relationship between near-field vortex dynamics (finlet vortex, leading-edge vortex, and forming trailing-edge vortex) and propulsive performance. |
Monday, November 22, 2021 9:18AM - 9:31AM |
H13.00007: Role of appendage bending in metachronal paddling Mitchell P Ford, Nicholas A Battista, Arvind Santhanakrishnan Numerous species of aquatic invertebrates use metachronal paddling as their primary locomotion strategy. These invertebrates have widely varying body and appendage morphologies, ranging from soft-bodied organisms such as ctenophores that swim by beating many flexible cilia, to hard-bodied crustaceans that swim by oscillating pairs of jointed legs. We investigated how changing the bending angle and the time period needed to fully bend or straighten each paddle affect wake vorticity and momentum fluxes at realistic stroke frequencies and at paddle-based Reynolds numbers ranging from 0.1 to 1,000. A two-dimensional immersed boundary formulation (IB2D) was used to model the metachronal motion of flat plate paddles that each have a hinged joint approximately halfway along the length. We found that paddles that unbend quicker and have a smaller bending angle generate a more vertical wake, which can be useful for supporting the weight of negatively buoyant organisms. Comparisons between paddling and squirming strategies at different Reynolds numbers will also be presented. |
Monday, November 22, 2021 9:31AM - 9:44AM |
H13.00008: Burst and coast: on the intermittent tail-beat kinematics in steady-swimming fish Ramiro Godoy-Diana, Gen Li, intesaaf ashraf, Dmitry Kolomenskiy, Bill François, Frédéric Lechenault, Benjamin Thiria Body and caudal fin undulations are a widespread locomotion strategy in fish, and their swimming kinematics is usually described by a characteristic frequency and amplitude of the tail-beat oscillation. In some cases, fish use intermittent gaits, where a single frequency is not enough to fully describe their kinematics. Energy efficiency arguments have been invoked in the literature to explain this so-called burst-and-coast regime but well controlled experimental data are scarce. Here we report on an experiment with burst-and-coast swimmers and a numerical model based on the observations to show that the observed burst-and-coast regime can be understood as obeying a minimization of cost of transport. |
Monday, November 22, 2021 9:44AM - 9:57AM |
H13.00009: The physical mechanism behind the wake merging phenomena of pitching foils in side-by-side arrangement. Ahmet Gungor, Muhammad Saif Ullah Khalid, Arman Hemmati The unsteady hydrodynamics of two in-phase pitching foils in side-by-side arrangement is numerically investigated at Reynolds number of Re = 4000 and Strouhal numbers of St = 0.15 – 0.5. Separation distance between the foils varies between 0.5c (c being the chord length) and 2c. This study classifies the wake of the foils based on their deflected/horizontal and merged/separated characteristics in Strouhal number vs separation distance phase maps. The foils at higher Strouhal number initially produce two distinct vortex streets, which are deflected and parallel to each other. These streets merge with each other and form coherent vortical structures around the centerline after a particular number of pitching cycles that vary with Strouhal number and separation distance. The underlying physical mechanism for the merging phenomena is investigated, which reveals the presence of secondary structures shed from the main deflected vortex streets in the opposite direction. Interactions between secondary structures of the lower wake and the main vortex street of the upper wake triggers a rapprochement process. This eventually yields to the merging of two wakes. |
Monday, November 22, 2021 9:57AM - 10:10AM |
H13.00010: Schooling Hydrodynamics of 300 Fish Michail Chatzimanolakis, Pascal Weber, Michael Triantafyllou, Petros Koumoutsakos We present 2D and 3D Direct Numerical Simulations of the collective behaviour of 300 swimmers. |
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