Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session H28: Hydrodynamic Locomotion |
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Chair: Megan Leftwich, Los Alamos National Laboratory Room: Ballroom II |
Monday, November 21, 2011 10:30AM - 10:43AM |
H28.00001: Hydrodynamics of self-propulsion near boundaries Saverio Spagnolie, Eric Lauga The swimming kinematics and trajectories of many microorganisms are altered by the presence of nearby boundaries, be they solid or deformable, and often in perplexing fashion. When an organism's swimming dynamics vary near such boundaries a question arises naturally: is the change in behavior biological, fluid mechanical, or perhaps mediated by other physical laws? Of interest then is a general framework for determining the extent to which fluid mechanics passively alter swimming behaviors in the presence of a wall. To this end, we explore a far-field description of swimming organisms and provide a general framework for studying the fluid-mediated modifications to swimming trajectories, and consider trapped/escape trajectories and equilibria for model organisms of varying shape and propulsive activity. This framework may help to explain surprising behaviors observed in the swimming of many microorganisms and synthetic swimming devices. [Preview Abstract] |
Monday, November 21, 2011 10:43AM - 10:56AM |
H28.00002: Benefits of unsteady swimming near a wall Daniel Quinn, Peter Dewey, Keith Moored, Alexander Smits The benefits of flying and swimming near the ground have been well-documented for fixed-wing vehicles, and have led to 'wing-in-ground' craft (WIG) with higher efficiencies than their conventional counterparts. Here it is examined whether unsteady propulsion techniques experience these same enhancements. Experimental particle image velocimetry was conducted in the wake of a rigid pitching panel and a flexible triangular fin, both actuated at several distances from a fixed wall. In both cases, an increase in momentum flux behind the trailing edge was observed, suggesting thrust amplification is present. A finite core vortex array model was developed to model the wake behind these propulsors.~Mirror image vortex cores were placed across the wall to satisfy the zero flux boundary condition, and the effects of viscosity were estimated by assuming a slowly expanding Gaussian distribution of vorticity around each core. The model offers insight into the origins of the momentum amplification due to the presence of the wall. Supported by ONR MURI Grant N00014-08-1-0642. [Preview Abstract] |
Monday, November 21, 2011 10:56AM - 11:09AM |
H28.00003: Wake interactions of panels swimming in a side-by-side configuration Birgitt Boschitsch, Peter A. Dewey, Keith W. Moored, Alexander J. Smits A pair of pitching panels arranged in a side-by-side configuration are experimentally examined under free swimming conditions. The panels were pitched about their leading edges by shafts located just behind the trailing edge of a NACA 0012-64 airfoil to suppress the formation of leading edge vortices. A recirculating water channel is set with a flow speed that matches the free swimming speed of the panel system. Power measurements are used to determine the energy consumption of the panel system per distance traveled. Finite and infinite aspect ratio panels are examined for a range of Strouhal numbers, transverse panel spacings, and oscillation phase shifts between the two panels. It is found that under certain operating conditions, a pair of panels maintain a higher free swimming velocity in comparison to a single isolated panel. To assess the wake generated by the panels, digital particle image velocimetry (DPIV) and hydrogen bubble visualization are used. [Preview Abstract] |
Monday, November 21, 2011 11:09AM - 11:22AM |
H28.00004: Interaction of in-phase and out-of-phase flexible filament in fish schooling Emad Ud din, Hyung Jin Sung Fish schooling is not merely a social behavior; schooling improves the efficiency of movement within the fluid environment. Inspired by the schooling from a hydrodynamic perspective, a group of aquatic animals is modeled as a collection of individuals arranged in a combination of tandem and side-by-side (diamond) formation. The downstream bodies are strongly influenced by the vortices shed by the upstream body shown by vortex-vortex and vortex-body interactions. Trailing fish takes advantage of this flow pattern for energy economy. To investigate the interactions between flexible bodies and vortices, in the present study three flexible flags in viscous flow are solved by numerical simulation using an improved version of the immersed boundary method for in-phase and out-of-phase filaments. The drag coefficient of the downstream filaments drops even below the value of a single flag. Such drag variations are influenced by the interactions between vortices shed by the upstream flexible body and vortices surrounding the downstream filaments. Interaction of the flexible flags is investigated as a function of the gap distance between flags and different bending coefficients, for in-phase and out-of-phase cases at intermediate Reynolds numbers. [Preview Abstract] |
Monday, November 21, 2011 11:22AM - 11:35AM |
H28.00005: Turns and maneuvers during swimming Amneet Bhalla, Noah Mosberg, Rahul Bale, Neelesh Patankar In this work we use fully resolved fluid dynamics computations based on an immersed body approach to study fish turns and maneuvers. We present a numerical method to control the trajectory of fish during turns and maneuvers. Fish tracking a prey is presented as an example case. Numerical simulations are carried out on spatially adaptive grid for speed and accuracy. The effect of deformation kinematics and Reynolds number (Re), on the turn radius of an undulatory swimmer, is studied. Power spent during turning at different turn radii and Re is also reported. These results can be used to quantify the cost of various maneuvers and to identify efficient maneuvers to attain the same objective, e.g., reaching a target location during prey tracking. [Preview Abstract] |
Monday, November 21, 2011 11:35AM - 11:48AM |
H28.00006: Analyzing the fast-start performance of northern pike using a mechanical fish Yahya Modarres-Sadeghi, Chengcheng Feng, Brian Bonafilia, Andrew Costain The northern pike is able to achieve an instantaneous acceleration of 245 m/s$^{2}$ through a two-stage motion. In the first stage the fish curls its body into either a C-shaped or an S-shaped curve (preparatory stage), and in the second stage uncurls it very quickly (propulsive stage) generating high accelerations due to the vortices shed from its tail. We have built a mechanical fish, based on the body profile of a pike, which is capable of performing this two-stage fast-start motion. Movement is governed by servo motors, which pull on cables attached to certain ribs, bending the fish into a C- or an S-shape. The degree of bending and timing of strokes can be controlled, and the fish can perform either a propulsive stroke only or a full stroke consisting of both the preparatory stage and the propulsive stage. The mechanical fish is capable of achieving peak accelerations of around 4 m/s$^{2}$. We use this fish in order to study the influence of various variables on the observed acceleration. Although the maximum accelerations observed in our mechanical fish are smaller than those of a live fish, the form of the measured acceleration signal as function of time is quite similar to that of a live fish. The hydrodynamic efficiencies are observed to be around 12{\%}, and it is shown that the majority of the thrust is produced at the rear part of the mechanical fish -- similarly to the live fish. [Preview Abstract] |
Monday, November 21, 2011 11:48AM - 12:01PM |
H28.00007: Hydrodynamic optimality of ribbon fin shapes Rahul Bale, Malcolm MacIver, Neelesh Patankar The primary mode of propulsion in gymnotiform and balistiform swimmers is via the undulation of anal and/or dorsal fins, commonly referred to as ribbon fins, attached to a more or less rigid body. Ribbon fins usually have a convex shape as opposed to a rectangular or concave profile. In this work we investigate if there is a hydrodynamic basis underlying this observation. Fully resolved fluid dynamics computations are performed to calculate the mechanical cost of transport (COT) as a measure of swimming efficiency of the fin. We find that the ribbon fin of a black ghost knifefish has lower COT compared to a hypothetical rectangular ribbon fin. In order to quantify this difference in COT between the two fin shapes, we obtain scaling for COT in terms of various parameters which affect the swimming performance of the fin. Using scaling arguments we address the question of how a convex profile, commonly observed in gymnotiform and balistiform swimmers, is optimal compared to rectangular or concave shapes. [Preview Abstract] |
Monday, November 21, 2011 12:01PM - 12:14PM |
H28.00008: Wake topology of under-actuated rajiform batoid robots Pablo Valdivia y Alvarado, Gabriel Weymouth, Dilip Thekoodan, Nicholas Patrikalakis Under-actuated continuous soft robots are designed to have modes of vibration that match desired body motions using minimal actuation. The desired modes of vibration are enabled by flexible continuous bodies with heterogenous material distributions. Errors or intentional approximations in the manufactured material distributions alter the achieved body motions and influence the resulting locomotion performance. An under-actuated continuous soft robot designed to mimic rajiform batoids such as stingrays is used to investigate the influence that fin kinematics variations have on wake topology, and the trade-offs that simplifying the body material structure has on achievable swimming performance. Pectoral fin kinematics in rajiform batoids are defined by traveling waves along the fin cord with particular amplitude envelopes along both the fin cord and span. Digital particle image velocimetry (DPIV) analysis of a prototype's wake structure and immersed-boundary numerical simulations are used to clarify the role of traveling wave wavelength, fin flapping frequency, and amplitude envelope characteristics on the resulting wake topology and swimming performance. [Preview Abstract] |
Monday, November 21, 2011 12:14PM - 12:27PM |
H28.00009: Kinematics and dynamics of sphenisciform wings Flavio Noca, Fabien Crisinel, Pierre Munier Three-dimensional scans of three different species of taxidermied penguins ({\sl Aptenodytes patagonicus}, {\sl Pygoscelis papua}, and {\sl Spheniscus magellanicus}) have been performed. A three-dimensional reproduction of an African penguin ({\sl Sphenicus demersus}) wing was manufactured and tested in a hydrodynamic channel. A six-degree-of-freedom robot was programmed to perform the three dimensional kinematics, obtained from actual footage. A six-component force balance was used to retrieve the dynamics of the wing motion. Results will be presented and discussed. [Preview Abstract] |
Monday, November 21, 2011 12:27PM - 12:40PM |
H28.00010: Flow Field Characteristics of Finite-span Hydrofoils with Leading Edge Protuberances Derrick Custodio, Charles Henoch, Hamid Johari Past work has shown that humpback whale-like leading edge protuberances can significantly alter the load characteristics of both 2D and finite-span hydrofoils. To understand the mechanisms responsible for observed performance changes, the flow field characteristics of a baseline hydrofoil and models with leading edge protuberances were examined using the Stereo Particle Image Velocimetry (SPIV) technique. The near surface flow field on the hydrofoils was measured along with the tip vortex flow field on finite-span hydrofoils. Angles of attack ranging from 6 to 24 degrees were examined at freestream velocities of 1.8 m/s and 4.5 m/s, corresponding to Reynolds numbers of 180 and 450 thousand, respectively. While Reynolds number does not play a major role in establishing the flow field trends, both the protuberance geometry and spatial proximity to protuberances affect the velocity and vorticity characteristics near the foil surface, and in the wake and tip vortex. Near surface measurements reveal counter-rotating vortices on protuberance shoulders, while tip vortex measurements show that streamwise vorticity can be strongly affected by the presence of protuberances. The observed flow field characteristics will be presented. [Preview Abstract] |
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