Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session A06: High Reynolds Number Swimming I |
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Chair: Alexander Alexeev, Georgia Institute of Technology Room: 134 |
Sunday, November 20, 2022 8:00AM - 8:13AM |
A06.00001: Collective locomotion of two-dimensional lattices of flapping plates Silas D Alben We study the propulsive properties of rectangular and rhombic lattices of flapping plates at O(10--100) Reynolds numbers in incompressible flow. We vary five parameters: flapping amplitude, frequency (or Reynolds number), horizontal and vertical spacings between plates, and oncoming fluid stream velocity. Lattices that are closely spaced in the streamwise direction produce intense vortex dipoles between adjacent plates. The lattices transition sharply from drag- to thrust-producing as these dipoles switch from upstream to downstream orientations at critical flow speeds. The flows assume a variety of periodic and nonperiodic states, with and without up-down symmetry, and multiple stable self-propelled speeds can occur. With small lateral spacing, rectangular lattices yield net drag, while rhombic lattices may generate net thrust efficiently. As lateral spacing increases, rectangular lattices eventually achieve higher efficiencies than rhombic lattices, and the two types of lattice flows converge. At Re = 70, the maximum Froude efficiencies of time-periodic lattice flows are about twice those of an isolated plate. At lower Re, the lattices' efficiency advantage increases until the isolated flapping plate no longer generates thrust. |
Sunday, November 20, 2022 8:13AM - 8:26AM |
A06.00002: Using thickness tapering to enhance hydrodynamic performance of elastic propulsors Ersan Demirer, Alexander Alexeev Using computational modelling, we probe the hydrodynamic performance of bio-inspired elastic propulsors with tapered thickness. The simulations reveal that thickness tapering drastically increases the hydrodynamic efficiency and thrust generated by oscillating elastic propulsors. Furthermore, tapered propulsors can maintain high performance for a wide range of actuation frequencies. We relate the hydrodynamic benefit of thickness tapering to the emerging acoustic black hole effect at the propulsor trailing edge that minimizes the wave reflection and promotes the development of the traveling waves propagating along the propulsor length. We compare propulsors with different thickness profiles and demonstrate that propulsors that exhibit traveling waves yield the best hydrodynamic performance. The results have implications for the development of highly efficient bio-mimetic robotic swimmers and, more generally, enhance our understanding of undulatory locomotion. |
Sunday, November 20, 2022 8:26AM - 8:39AM |
A06.00003: Albatrosses of rivers: Inertial mechanism behind dynamic station holding in fish Petr Denissenko, Valentine Muhawenimana, Stephanie Muller, Sam Tucker Harvey, Catherine Willson Many aquatic and aerial animal species are known to utilise their surrounding flow field and/or the induced flow field of a neighbour to reduce their physical exertion, however, the mechanism by which such benefits are obtained has remained elusive. In this work, we investigate the swimming dynamics of rainbow trout in the wake of a thrust-producing oscillating hydrofoil. Despite the higher flow velocities in the inner region of the vortex street, some fish maintain position in this region, while exhibiting an altered swimming gait. Estimates of energy expenditure indicate a reduction in the propulsive cost when compared to regular swimming. By examining the accelerations of the fish, an explanation of the mechanism by which energy is harvested from the vortices is proposed. Similar to dynamic soaring employed by albatross, the mechanism can be linked to the non-equilibrium hydrodynamic forces produced when fish encounter the cross-flow velocity generated by the vortex street. |
Sunday, November 20, 2022 8:39AM - 8:52AM |
A06.00004: Induced leading-edge flow separation is critical to the formation of one-dimensionally stable positions in in-line schooling Tianjun Han, Amin Mivehchi, Keith W Moored Stable positions, speed and propulsive efficiency benefits have all been found in the schooling of in-line swimmers. However, the mechanisms behind school cohesion and how the key variables of Lighthill number and dimensionless amplitude can modulate the cohesion and associated hydrodynamic benefits are not fully understood. Here new simulations of two purely pitching teardrop hydrofoils in an in-line arrangement are examined with a potential flow advanced boundary element method (ABEM) solver augmented by a leading-edge suction parameter based separation model. The foils freely swim in the streamwise direction and have a fencing scheme to prevent penetration of the impinging wake vortex elements into the downstream hydrofoil. The ABEM solver is validated against a wide array of previous simulations and experiments. When leading-edge separation is suppressed it’s discovered that the follower always collides into the leader showing no school cohesion across a wide range of initial conditions, Lighthill numbers, and dimensionless amplitudes. When leading-edge separation is allowed, one-dimensionally stable arrangements form and agree with previous experiments. For swimmers in stable equilibrium arrangements increasing the Lighthill number not only makes them be spaced closer together, but also distorts the linear relationship between stable positions and the phase lag. |
Sunday, November 20, 2022 8:52AM - 9:05AM |
A06.00005: Self-propelled swimming and propulsive performance of bio-inspired pitching panels Justin T King, Melissa A Green Swimming animals using oscillatory propulsive mechanisms propel themselves through the water using either a caudal fin or a fluke located at the rear of their body. The propulsive appendages of swimming animals display a wide diversity in planform, including those with different trailing edge shapes. The current work expands upon prior experimental work on the time-averaged performance of bio-inspired pitching propulsors as measured in fixed velocity water tunnel experiments. Rather than using a fixed velocity free stream flow and pure pitching kinematics, the current work investigates mean propulsive performance using experiments conducted at self-propelled swimming speeds for combined pitching and heaving motions. Trailing edge shape and kinematic parameters are varied for a series of bio-inspired panels with a nominally trapezoidal planform. In total, five unique panel geometries, each with a different trailing edge shape, were actuated with multiple motion profiles until the resultant self-propelled swimming speed was determined. The measured self-propelled swimming results are discussed in the context of changes to planform shape and kinematics. Prior work on the same panel geometries at fixed swimming velocity are compared and contrasted to the findings measured at self-propelled swimming speeds. The current work also focuses on the implications of these findings on the design of bio-inspired vehicles relying on novel propulsive mechanisms and the effects that propulsor geometry and kinematics may have on animals swimming at constant velocity. |
Sunday, November 20, 2022 9:05AM - 9:18AM |
A06.00006: Fine-tuning near-boundary swimming equilibria using asymmetric kinematics Leo Liu, Qiang Zhong, Tianjun Han, Keith W Moored, Daniel Quinn When swimming near a solid planar boundary, bio-inspired propulsors can naturally equilibrate to certain distances from that boundary. We discovered that asymmetric pitch kinematics can fine-tune those equilibrium distances. We present a study of near-boundary pitching hydrofoils based on water channel experiments and potential flow simulations. By varying the bias angle (spatial asymmetry), stroke-speed ratio (temporal asymmetry), and normalized ground distance, we examined how near-ground thrust, lift, and efficiency were affected by asymmetric kinematics. We found that asymmetric pitch kinematics do affect near-boundary equilibria, resulting in the equilibria shifting either closer to or away from the planar boundary. The magnitude of the shift depends on whether the pitch kinematics have spatial asymmetry or temporal asymmetry. Swimming at stable equilibrium requires less active control, while shifting the equilibrium closer to the boundary can result in higher thrust with no measurable change in propulsive efficiency. Our work reveals how asymmetric kinematics could be used to fine-tune a hydrofoil's interaction with a nearby boundary, and it offers a starting point for understanding how fish and birds use asymmetries to swim near substrates, water surfaces, and sidewalls. |
Sunday, November 20, 2022 9:18AM - 9:31AM |
A06.00007: Efficient self-propelled locomotion by an elastically supported rigid foil actuated by a torque Ramon Fernandez-Feria, Pablo E Lopez-Tello, Enrique Sanmiguel-Rojas A new theoretical model is presented for an aquatic vehicle self-propelled by a rigid foil undergoing pitching oscillations generated by a torque of small amplitude applied at an arbitrary pivot axis at which the foil is elastically supported to allow for passive heaving motion. The model is based on 2D linear potential-flow theory coupled with the self-propelled dynamics of the semi-passive flapping foil elastically mounted on the vehicle hull through translational and torsional springs and dampers. It is governed by just three ordinary differential equations, whose numerical solutions are assessed with full viscous numerical simulations of the self-propelled foil. Analytical approximate solutions for the combined effect of all the relevant non-dimensional parameters on the swimming velocity and efficiency are also obtained by taking advantage of the small-amplitude of the applied torque. Thus, simple power laws for the velocity and efficiency dependencies on Lighthill number and torque intensity are obtained. It is found that the swimming velocity and transport efficiency can be greatly enhanced by selecting appropriately the non-dimensional constants of the translational and torsional springs, which are mapped for typical values of the remaining parameters n aquatic locomotion. These resonant values serve to select optimal frequencies of the forcing torque for given structural and geometric parameters. Thus, the present model and analysis provides a useful guide for the design of an efficient flapping-foil underwater vehicle. |
Sunday, November 20, 2022 9:31AM - 9:44AM |
A06.00008: Demonstrating Targeted Navigation and Emergent Behaviour in Turbulent Flows via Deep Reinforcement Learning. Advay Mansingka, Roberto Zenit Understanding navigation in the presence of a turbulent flow has many applications including designing autonomous drones, creating targeted drug delivery technologies, and modelling weather patterns. Given the unpredictable nature of turbulence, it is challenging to build control schemes for efficient navigation. This study considers autonomous navigation in turbulent flows by training deep reinforcement learning algorithms in computer simulations of turbulent flows. Our swimmers learn to navigate a two-dimensional flow, and to exploit vortices in the fluid field to traverse their environments. We demonstrate that the RL based swimmers outperform simpler control theory-based approaches. We build on prior work by demonstrating emergent multi-agent behavior in independently trained swimmers, and showcase their ability to 'slipstream' each other in the flows to further optimize their movements. |
Sunday, November 20, 2022 9:44AM - 9:57AM |
A06.00009: How phase synchrony affects the stable equilibrium of two free swimming pitching foils Amin Mivehchi, Tianjun Han, Pedro C Ormonde, Keith W Moored Aquatic animals swim efficiently in different schooling formations, with different spacing and phase synchrony. For a simplified two-dimensional pitching foil with out-of-phase synchrony, Kurt et al.(2019 JFM, 2022 Arxiv) show the existence of an equilibrium position, which can be tuned with change in reduced frequency(k) and Strouhal number(St). Following these findings and by using two free-swimming pure pitching foils, we present new simulations to examine the effect of phase synchrony on the equilibrium position and performance. We further examine how variations in Lighthill number, reduced frequency, and Strouhal number also play a role. These results are relevant in biology and the development of bio-inspired underwater vehicles. |
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