Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session A26: Biofluids: Flexible Swimmers I |
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Chair: Eric Tytell, Tufts University Room: 306 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A26.00001: The hydrodynamics of linear accelerations in bluegill sunfish, \textit{Lepomis macrochirus} Tyler Wise, Alex Boden, Margot Schwalbe, Eric Tytell As fish swim, their body interacts with the fluid around them in order to generate thrust. In this study, we examined the hydrodynamics of linear acceleration by bluegill sunfish, \textit{Lepomis macrochirus}, which swims using a carangiform mode. Carangiform swimmers primarily use their caudal fin and posterior body for propulsion, which is different from anguilliform swimmers, like eels, that undulate almost their whole body to swim. Most previous studies have examined steady swimming, but few have looked at linear accelerations, even though most fish do not often swim steadily. During steady swimming, thrust and drag forces are balanced, which makes it difficult to separate the two, but during acceleration, thrust exceeds drag, making it easier to measure; this may reveal insights into how thrust is produced. This study used particle image velocimetry (PIV) to compare the structure of the wake during steady swimming and acceleration and to estimate the axial force. Axial force increased during acceleration, but the orientation of the vortices did not differ between steady swimming and acceleration, which is different than anguilliform swimmers, whose wakes change structure during acceleration. This difference may point to fundamental differences between the two swimming modes. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A26.00002: Effect of aspect ratio in free-swimming plunging flexible plates Peter Yeh, Alexander Alexeev Using three dimensional fully-coupled fluid-structure interaction simulations, we investigate the free swimming of plunging elastic rectangular plates with aspect ratios ranging from 0.5 to 5 in a viscous fluid with Reynolds number 250. We find that maximum velocity occurs near the first natural frequency regardless of aspect ratio, while the maximum swimming economy occurs away from the first natural frequency and corresponds to a specific swimmer bending pattern characterized by reduced displacement of the swimmer's center of mass. Furthermore, we find that swimmers with wider span are both faster and more economical than narrow swimmers. These faster speeds are due to decreased drag for low aspect ratio plunging swimmers, which is in agreement with a recently proposed vortex-induced drag model that suggests that the smaller relative size of side vortices in low aspect ratio swimmers creates less drag per unit width. Our results are useful for the design of small autonomous micro-swimming devices and also provide insights on the physics of aquatic locomotion using oscillating fins. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A26.00003: Numerical simulations of chordwise flexible pitching foils: are expanding or contracting forms more efficient? Kai Schneider, Thomas Engels, Dmitry Kolomenskiy, Joern Sesterhenn We present three-dimensional direct numerical simulations of chord-wise flexible plates of different shape with driven pitching motion. We focus on the tip vortices originating from three-dimensional effects due to the finite span. These vortices are important when predicting the swimmers cruising velocity, since they contribute significantly to the drag force. First we consider rectangular swimmers with different aspect ratios and compare with an experimental study (Raspa et al. , Phys. Fluids 26, 2014). Then we study expanding and a contracting shapes. We find the cruising velocity of the contracting swimmer to be higher than the rectangular one, which in turn is higher than the expanding one, while the power requirements are the lowest for the contracting shape. We provide evidence that this finding is due to the tip vortices interacting differently with the swimmer. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A26.00004: Numerical Investigation of the C-start in an Elastic Plate Daniel Canuto, Jeff Eldredge, Roberto Zenit The C-start is a swimming mechanism employed by certain fish to achieve rapid acceleration from rest. In addition to its relatively low energy cost, the agility that this mechanism permits makes its understanding important for the design of biomimetic swimmers. To investigate the dynamics of C-starts, we conduct two-dimensional numerical simulations of an elastic plate rotated about its edge through a specified angle. The plate is free to translate in one direction, normal to its rotational axis and parallel to the plate's final orientation. The results obtained are compared with experimental data. Based on the swimming velocity, it is found that the C-start can be divided into three distinct stages: motion begins with a period of nearly constant acceleration, continues with a transient period as the tail's rotation ends, and concludes with a very gradual deceleration, or ``coasting". These stages are analyzed, as are the effects of important design parameters (e.g., body density, bending stiffness) on the dynamics observed in each stage. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A26.00005: 3D Kinematics and Hydrodynamic Analysis of Freely Swimming Cetacean Yan Ren, Dustin Sheinberg, Geng Liu, Haibo Dong, Frank Fish, Joveria Javed It's widely thought that flexibility and the ability to control flexibility are crucial elements in determining the performance of animal swimming. However, there is a lack of quantification of both span-wise and chord-wise deformation of Cetacean's flukes and associated hydrodynamic performance during actively swimming. To fill this gap, we examined the motion and flexure of both dolphin fluke and orca fluke in steady swimming using a combined experimental and computational approach. It is found that the fluke surface morphing can effectively modulate the flow structures and influence the propulsive performance. Findings from this work are fundamental for understanding key kinematic features of effective Cetacean propulsors, and for quantifying the hydrodynamic force production that naturally occurs during different types of swimming. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A26.00006: Investigation of the Role of Planform Shape and Swimming Gait in Cetacean Propulsion Fatma Ayancik, Frank E. Fish, Keith W. Moored Dolphins and whales, known as cetaceans, have morphological characteristics associated with enhanced thrust production, high propulsive efficiency and reduced drag. These animals oscillate their moderate aspect ratio flukes in a heaving and pitching motion to propel themselves through the water. Surprisingly, these animals display a large variation in their fluke shape and swimming gait. The present study aims to probe the connection between the fluke shape and swimming gait in high performance swimming. The planform shape of cetacean flukes is parameterized with a NACA-inspired function where the coefficients are fit to several species. An unsteady three-dimensional boundary element method is used to identify the thrust production, energetics and wake structure of free-swimming flukes with an added virtual body drag. The shape and gait parameters of the different species are exchanged to gain a broader understanding of the connection between shape and gait. The numerical results are compared with lunate tail theory to assess the limitations of the theory and its predictions of force and energetic scalings. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A26.00007: Performance of an unsteady plate with a two-dimensional body attached upstream Rodrigo Lisazo, Tyler Van Buren, Daniel Floryan, Devon Hartsough, Emile Oshima, Clarence W. Rowley, Alexander Smits We present results from experiments and simulations on a biologically-inspired two-dimensional heaving and pitching rigid plate. Two cases are considered: an isolated pitching plate actuated sinusoidally at the leading edge, and a pitching plate attached at the trailing edge of a stationary, streamlined two-dimensional panel. Experiments were conducted in a water channel facility equipped with a six component load cell and particle image velocimetry (PIV). Simulations were performed using an immersed boundary method. Of particular interest are (1) the impact of leading edge separation on propulsive efficiency, (2) the influence of the incoming boundary layer on the trailing vortex structure and (3) the effects of the unsteady motion on the behavior of the boundary layer. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A26.00008: A robotic device with a passive undulating ribbon fin: kinematics and propulsive performance Hanlin Liu, Oscar Curet Many aquatic animals swim with high maneuverability using undulating ribbon fins. In this type of swimming, the organism propels by sending one or multiple traveling waves along an elongated fin. In previous work, robotic models with fully actuated fins where the parameters of the traveling waves are fully prescribed have been used to study the propulsive performance and fluid dynamics of this type of propulsion. However, less work has been done in ribbon fins with passively undulating waves. In this work, we use a robotic device to study the kinematics and propulsive performance of a passively undulating ribbon fin. The physical model is composed of fifteen rays interconnected with a membrane. Only two rays are actuated while the other rays are free to rotate through a common axis. The robotic fin was tested in a flume at different flow conditions. In a series of experiments we measured fin kinematics, propulsive forces and power consumption. As the leading two rays are actuated, a traveling wave with decaying amplitude passes through the passive rays. As the frequency of the actuated rays increases, the enclosed area of the undulating wave and the traveling wave frequency increase while the wavelength decreases. Our data also show that the propulsive force generated by the fin scaled with the enclosed area and the square of the relative velocity between incoming flow and traveling wave. These results suggest that both natural swimmers and underwater vehicles using ribbon-fin-based propulsion can potentially take advantage of passive undulating waves. [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A26.00009: Dynamic Surface Morphing of Sunfish Caudal Fin Enhances Its Propulsive Efficiency in Steady Swimming Geng Liu, Chengyu Li, Haibo Dong, George Lauder In this work, an integrated experimental and computational approach has been used to investigate the correlation between the propulsive performance and surface morphology of bluegill sunfish's caudal fin in steady swimming. 3D sunfish caudal fin kinematics and surface morphing were reconstructed based on the output of a high-speed photogrammetry system. Hydrodynamic performance and wake structures were numerically studied by an in-house immersed-boundary-method flow solver. It is found that the spanwise surface morphing enhances both the thrust and the propulsive efficiency by more than 30{\%}. Further investigation of the near-field and far-field wakes has shown that the enhanced span edge vortices were responsible for the performance improvement. Vortex dynamics analyses of such unsteady flow are expected to provide physical insight into the understanding of a potential performance enhancement mechanism in bluegill sunfish caudal fin propulsion. [Preview Abstract] |
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