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 R38: Biofluids: Flexible Swimmers III |
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Chair: George Lauder, Harvard University Room: Sheraton Back Bay B |
Tuesday, November 24, 2015 12:50PM - 1:03PM |
R38.00001: Application of PIV-based pressure measurements to the study of aquatic propulsion Kelsey Lucas, John Dabiri, George Lauder Although it is relatively straightforward to image how fluid moves around a swimmer, translation of these motions to mechanisms that generate forces for propulsion is more difficult. This process is greatly facilitated by a recently developed technique for non-invasive pressure measurements that generate 2D pressure fields. Here, we explore how accurate a purely pressure-based calculation of propulsive forces can be. By comparing these calculations to forces and torques measured directly using a sensor on a robotic flapping foil system, we characterize the effects of motion frequency and out-of-plane flows on the calculation's accuracy. We then apply this calculation to study the dynamics of fish-like swimming of a foil model with non-uniform flexural stiffness, and to those of a freely swimming fish. [Preview Abstract] |
Tuesday, November 24, 2015 1:03PM - 1:16PM |
R38.00002: Non-invasive 3D geometry extraction of a Sea lion foreflipper Chen Friedman, Martha Watson, Pamela Zhang, Megan Leftwich We are interested in underwater propulsion that leaves little traceable wake structure while producing high levels of thrust. A potential biological model is the California sea lion, a highly maneuverable aquatic mammal that produces thrust primarily with its foreflippers without a characteristic flapping frequency. The foreflippers are used for thrust, stability, and control during swimming motions. Recently, the flipper's kinematics during the thrust phase was extracted using 2D video tracking. This work extends the tracking ability to 3D using a non-invasive Direct Linear Transformation technique employed on non-research sea lions. marker-less flipper tracking is carried out manually for complete dorsal-ventral flipper motions. Two cameras are used (3840 $\times$ 2160 pixels resolution), calibrated in space using a calibration target inserted into the sea lion habitat, and synchronized in time using a simple light flash. The repeatability and objectivity of the tracked data is assessed by having two people tracking the same clap and comparing the results. The number of points required to track a flipper with sufficient detail is also discussed. Changes in the flipper pitch angle during the clap, an important feature for fluid dynamics modeling, will also be presented. [Preview Abstract] |
Tuesday, November 24, 2015 1:16PM - 1:29PM |
R38.00003: Resistive and reactive force production in actuated elastic swimmers Ramiro Godoy-Diana, Miguel Pineirua, Benjamin Thiria We study the force production dynamics of undulating elastic plates as a model for fish-like inertial swimmers. Using a beam model coupled with Lighthill's large-amplitude elongated-body theory, we explore different localized actuations at one extremity of the plate (heaving, pitching, and a combination of both) in order to quantify the reactive and resistive contributions to the thrust. The latter has only recently been pointed out as a crucial element in the force balance of large Reynolds number swimmers [Pi\~neirua et al. Phys. Rev. E (2015)]. We show that this balance is modified as the frequency of excitation changes and the response of the elastic plate shifts between different resonant modes. In the heaving case for instance, higher frequencies and thus higher modes are associated to a stronger resistive contribution to the thrust, while in pitching case, at all frequencies, thrust production comes mostly from the reactive term. [Preview Abstract] |
Tuesday, November 24, 2015 1:29PM - 1:42PM |
R38.00004: Optimality Principles of Undulatory Swimming Nishant Nangia, Rahul Bale, Neelesh Patankar A number of dimensionless quantities derived from a fish's kinematic and morphological parameters have been used to describe the hydrodynamics of swimming. In particular, body/caudal fin swimmers have been found to swim within a relatively narrow range of these quantities in nature, e.g., Strouhal number or the optimal specific wavelength. It has been hypothesized or shown that these constraints arise due to maximization of swimming speed, efficiency, or cost of transport in certain domains of this large dimensionless parameter space. Using fully resolved simulations of undulatory patterns, we investigate the existence of various optimality principles in fish swimming. Using scaling arguments, we relate various dimensionless parameters to each other. Based on these findings, we make design recommendations on how kinematic parameters for a swimming robot or vehicle should be chosen. [Preview Abstract] |
Tuesday, November 24, 2015 1:42PM - 1:55PM |
R38.00005: The Hydrodynamics of Plesiosaurs Luke Muscutt, Bharathram Ganapathisubramani, Gareth Dyke, Gabriel Weymouth Plesiosaurs are extinct marine reptiles that existed at the same time as the dinosaurs, and are the only known animals to swim by actively flapping their four wing-like flippers. This can be viewed as a tandem flapping wing problem, where the hind wing is operating in the wake of the fore wing. Experiments using full-scale robotic plesiosaur flippers in a large flume tank have been used to investigate the kinematics and interaction of the flippers. The flippers are actuated in heave and pitch, and a combination of force measurements and flow visualization are used to analyze the characteristics of the vortex interaction between the flippers. Previous two-dimensional numerical simulations have shown that certain kinematics give an increase in thrust of the hind flipper of up to 50{\%}. The current experiments determine if such thrust augmentation is present for a three-dimensional flowfield and determine the kinematics that give the highest possible thrust. This will help to answer paleo-biological questions about the function and evolution of the plesiosaur flippers, along with helping to determine if tandem flapping wings could be a viable propulsion system for autonomous underwater vehicles. [Preview Abstract] |
Tuesday, November 24, 2015 1:55PM - 2:08PM |
R38.00006: Fluid Dynamics of Clap-and-Fling with Highly Flexible Wings inspired by the Locomotion of Sea Butterflies Zhuoyu Zhou, Kourosh Shoele, Deepak Adhikari, Jeannette Yen, Donald Webster, Rajat Mittal This study is motivated by the locomotion of sea butterflies (L. Helicina) which propel themselves in the water column using highly flexible wing-like parapodia. These animals execute a complex clap-and-fling with their highly flexible wings that is different from that of insects, and the fluid dynamics of which is not well understood. We use two models to study the fluid dyamics of these wings. In the first, we use prescribed wing kinematics that serve as a model of those observed for these animals. The second model is a fluid-structure interaction model where wing-like parapodia are modeled as flexible but inextensible membranes. The membrane properties, such as bending and stretching stiffness are modified such that the corresponding motion qualitatively matches the kinematics of L. helicina. Both models are used to examine the fluid dynamics of the clap-and-fling and its effectiveness in generating lift for these animals. Acknowledgement -- research is supported by a grant from NSF. [Preview Abstract] |
Tuesday, November 24, 2015 2:08PM - 2:21PM |
R38.00007: The role of spanwise-flexible propulsors in swimming and flying Jaime Wong, David Rival Natural swimmers and flyers span several orders of magnitude in both Reynolds number and fluid-propulsor mass ratio. Intuitively, one would expect different aeroelastic strategies to be employed across these regimes. However, similar magnitudes of spanwise bending, as measured by flexion angle, have been observed across this entire range for cruise conditions$^1$. In this study, it is hypothesized that propulsor flexion has converged to generate similar spanwise vorticity transport in order to control the dynamic-stall vortex ubiquitous in natural swimming and flight. In particular, it is believed that vorticity convection and vortex stretching delay vortex detachment by balancing vorticity generated at the leading-edge and by reducing the overall vortex size, respectively, as recently shown for flapping profiles$^2$. Moving forward, passive spanwise flexibility of propulsors is now abstracted as a spanwise-variation in effective incidence. This abstraction is realized as a pitching-flapping motion. By comparing passively flexible cases to rigid cases, the role of flexibility on controlling vorticity transport, and thus delaying vortex detachment is elucidated.\\ 1 Lucas, K. N. et al., Nat. Commun. 5, 3293 (2014)\\ 2 Wong, J. G., Rival, D. E., J. Fluid Mech. 766, 611-625 (2015) [Preview Abstract] |
Tuesday, November 24, 2015 2:21PM - 2:34PM |
R38.00008: Strouhal number for free swimming Mehdi Saadat, Tyler Van Buren, Daniel Floryan, Alexander Smits, Hossein Haj-Hariri In this work, we present experimental results to explore the implications of free swimming for Strouhal number (as an outcome) in the context of a simple model for a fish that consists of a 2D virtual body (source of drag) and a 2D pitching foil (source of thrust) representing cruising with thunniform locomotion. The results validate the findings of Saadat and Haj-Hariri (2012): for pitching foils thrust coefficient is a function of Strouhal number for all gaits having amplitude less than a certain critical value. Equivalently, given the balance of thrust and drag forces at cruise, Strouhal number is only a function of the shape, i.e. drag coefficient and area, and essentially a constant for high enough swimming speeds for which the mild dependence of drag coefficient on the speed vanishes. Furthermore, a dimensional analysis generalizes the findings. A scaling analysis shows that the variation of Strouhal number with cruising speed is functionally related to the variation of body drag coefficient with speed. {\em Ref:} Saadat, M. and Haj-Hariri, H. Role of Strouhal number (St) in free swimming. American Physical Society, 65th Annual DFD Meeting, San Diego, CA, Nov 18–20, 2012. [Preview Abstract] |
Tuesday, November 24, 2015 2:34PM - 2:47PM |
R38.00009: A mechanism for efficient swimming Hossein Haj-Hariri, Mehdi Saadat, Aaron Brandes, Vishaal Saraiya, Hilary Bart-Smith We present experimental measurements of hydrodynamic performance as well as wake visualization for a freely swimming 3D foil with pure pitching motion. The foil is constrained to move in its axial direction. It is shown that the iso-lines for speed and input power (or economy) coincide in the dimensional frequency versus amplitude plane, up to a critical amplitude. The critical amplitude is independent from swimming speed. It is shown that all swimming gaits (combination of frequency and amplitude) share a single value for Strouhal number (for amplitudes below the critical amplitude), when plotted in non-dimensional frequency vs. amplitude plane. Additionally, it is shown that the swimming gaits with amplitudes equal to the critical amplitude are energetically superior to others. This finding provides a fundamental mechanism for an important observation made by Bainbridge (1958) namely, most fish (such as trout, dace, goldfish, cod and dolphins) maintain constant tail-beat amplitude during cruise, and their speed is correlated linearly with their tail-beat frequency. The results also support prior findings of Saadat and Haj-Hariri (2013). {\em Ref1:} Bainbridge, R. Journal of Experimental Biology 35, 109 (1958). {\em Ref2:} Saadat, M., Haj-Hariri, H., APS DFD66, 2013. [Preview Abstract] |
Tuesday, November 24, 2015 2:47PM - 3:00PM |
R38.00010: Thickness-varying flexible plunging fins swim more efficiently Yuanda Li, Peter Yeh, Alexander Alexeev We use three dimensional computer simulations to probe the hydrodynamics of oscillating flexible fins with varying thickness. The fin is modeled as an elastic rectangular plate with the thickest section at the leading edge, decreasing linearly until the trailing edge. The plate is modeled as infinitely thin, and we assume that the thickest part of the fin is much smaller compared to its other length scales. Therefore, we simulate the swimmer as two dimensional plate and introduce the effect of the thickness gradient by including an appropriate mass gradient and stiffness gradient along the length of the plate. The flexible fin is actuated by a plunging motion at its leading edge. We evaluate the performance of the swimmer by measuring the steady state thrust, free swimming velocity, input power, and swimming economy as a function of driving frequency and the magnitude of the thickness gradient. We find a wideband frequency range in which the swimming economy is increased as compared to a uniformly thick swimmer. These findings may shed insight into some of the physical mechanisms that allow fish to have high swimming efficiency. [Preview Abstract] |
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