Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session A5: Biofluids: From Idealized Swimming to Boundary Layer Flow |
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Chair: Alexandra Techet, MIT Room: 3008 |
Sunday, November 23, 2014 8:00AM - 8:13AM |
A5.00001: Effect of longitudinal ridges on the hydrodynamic performance of a leatherback turtle model Kyeongtae Bang, Jooha Kim, Sang-im Lee, Haecheon Choi Leatherback sea turtles (\textit{Dermochelys coriacea}) known as the fastest swimmer and the deepest diver among marine turtles have five longitudinal ridges on their carapace, and these ridges are the most remarkable morphological features distinguished from other marine turtles. To investigate the effect of these ridges on the hydrodynamic performance of the leatherback turtle, we model a carapace with and without ridges using a stuffed leatherback turtle in the National Science Museum, Korea. We measure the drag and lift forces on the ridged model in the ranges of real leatherback turtles' Reynolds number (\textit{Re}) and angle of attack ($\alpha$), and compare them with those of non-ridged model. At $\alpha$ $<$ 6$^{\circ}$, longitudinal ridges decrease drag on the ridged model by up to 32\% compared to non-ridged model. On the other hand, at $\alpha$ $>$ 6$^{\circ}$, the drag and lift coefficients of the ridged model are higher than those of the non-ridged model, and the lift-to-drag ratio of the ridged model is higher by about 7\% than that of the non-ridged model. We also measure the velocity field around both models using a particle image velocimetry and explain the hydrodynamic role of ridges in relation to diving behaviors of leatherback sea turtles. [Preview Abstract] |
Sunday, November 23, 2014 8:13AM - 8:26AM |
A5.00002: Characterization of the Boundary Layer on Full-Scale Bluefin Tuna Brian Amaral, Kimberly Cipolla, Charles Henoch The physics that enable tuna to cross large expanses of ocean while feeding and avoiding predators is not presently understood, and could involve complex control of turbulent boundary layer transition and drag reduction. Typical swimming speeds of Bluefin tuna are 1-2 m/s, but can be higher during strong accelerations. The goal of this work is to experimentally determine the approximate lateral location at which transition to turbulence occurs on the tuna for various speeds. The question is whether laminar flow or an advanced propulsion mechanism (or both) allows them to swim at high speeds. Uncertainties include the surface roughness of the skin, local favorable and adverse pressure gradients, and discontinuities such as the open mouth or juncture at the fins. Historically, much of the fluid mechanics work in the area of fish locomotion has focused on vortex shedding issues rather than the boundary layer. Here, the focus is obtaining information on the boundary layer characteristics of a rigid tuna model. A full scale model of a Pacific Bluefin tuna was fabricated using a mold made from an actual deceased tuna, preserving the surface features and details of the appendages. The model was instrumented with 32 wall pressure sensors and experiments performed in a tow tank. Results from flow visualization, drag and wall pressure measurements over a range of speeds and varying angles of attack will be presented. [Preview Abstract] |
Sunday, November 23, 2014 8:26AM - 8:39AM |
A5.00003: Passive appendages aid locomotion through symmetry breaking Shervin Bagheri, Ugis Lacis, Andrea Mazzino, Hamid Kellay, Nicolas Brosse, Fredrik Lundell, Francois Ingremeau Plants and animals use plumes, barbs, tails, feathers, hairs, fins, and other types of appendages to aid locomotion. Despite their enormous variation, passive appendages may contribute to locomotion by exploiting the same physical mechanism. We present a new mechanism that applies to body appendages surrounded by a separated flow, which often develops behind moving bodies larger than a few millimeters. We use theory, experiments, and numerical simulations to show that bodies with protrusions turn and drift by exploiting a symmetry-breaking instability similar to the instability of an inverted pendulum. Our model explains why the straight position of an appendage in flowing fluid is unstable and how it stabilizes either to the left or right of the incoming fluid flow direction. The discovery suggests a new mechanism of locomotion that may be relevant for certain organisms; for example, how plumed seeds may drift without wind and how motile animals may passively reorient themselves. [Preview Abstract] |
Sunday, November 23, 2014 8:39AM - 8:52AM |
A5.00004: On the Hydrodynamic Function of Sharkskin: A Computational Investigation Aaron Boomsma, Fotis Sotiropoulos Denticles (placoid scales) are small structures that cover the epidermis of some sharks. The hydrodynamic function of denticles is unclear. Because they resemble riblets, they have been thought to passively reduce skin-friction--for which there is some experimental evidence. Others have experimentally shown that denticles increase skin-friction and have hypothesized that denticles act as vortex generators to delay separation. To help clarify their function, we use high-resolution large eddy and direct numerical simulations, with an immersed boundary method, to simulate flow patterns past and calculate the drag force on Mako Short Fin denticles. Simulations are carried out for the denticles placed in a canonical turbulent boundary layer as well as in the vicinity of a separation bubble. The computed results elucidate the three-dimensional structure of the flow around denticles and provide insights into the hydrodynamic function of sharkskin. [Preview Abstract] |
Sunday, November 23, 2014 8:52AM - 9:05AM |
A5.00005: Proprioceptive gait and speed selection in a slender inertial swimmer Mederic Argentina, Mattia Gazzola, L. Mahadevan We study the dynamics of a slender inertial swimmer accounting for hydrodynamics, mechanics, muscle activity and sensory feedbacks. Our theory elucidates how elastic properties and proprioception contribute to selecting swimming speed and locomotion gait. Swimmers are shown to take advantage of resonance phenomena to enhance speed and efficiency. Furthermore, we demonstrate how a minimal proprioceptive model, in which the local muscle activation is function of body curvature, is sufficient to exploit hydro-mechanic properties and drive elastic instabilities associated with thrust production. Our results quantitatively agree with live fish experiments and provide a mechanistic basis for the relation U/L $\sim $ f between the swimmer's speed U, length L and tail beat frequency f determined empirically by Bainbridge more than half a century ago. [Preview Abstract] |
Sunday, November 23, 2014 9:05AM - 9:18AM |
A5.00006: The effect of input perturbations on swimming performance Andrea M. Lehn, Patrick J.M. Thornycroft, George V. Lauder, Megan C. Leftwich The influence of flexibility and fluid characteristics on the hydrodynamics of swimming has been investigated for a range of experimental systems. One investigative method is to use reduced-order physical models---pitching and heaving hydrofoils. Typically, a smooth, periodic, input signal is used to control foil motion in experiments that explore fundamental factors (aspect ratio, shape, etc.) in swimming performance. However, the significance of non-smooth input signals in undulating swimmers is non-trivial. Instead of varying external properties, we study the impact of perturbed input motions on swimming performance. A smooth sinusoid is overlaid with high frequency, low amplitude perturbations as the input signal for a heaving panel in a closed loop flow tank. Specifically, 1 cm heave amplitude base sinusoids are added to 0.1 cm heave perturbations with frequencies ranging from 0.5 to 13 Hz. Two thin foils with different stiffness are flapped with the combined input signals in addition to the individual high heave and low heave signals that were added to create the combined inputs. Results demonstrate that perturbations can increase thrust and that adding the perturbed signal to a base frequency alters wake structure. [Preview Abstract] |
Sunday, November 23, 2014 9:18AM - 9:31AM |
A5.00007: A numerical study of vortex-induced drag of elastic swimmer models Thomas Engels, Dmitry Kolomenskiy, Kai Schneider, Joern Sesterhenn Swimming organisms exploit bending waves to produce propulsive force. The achievable cruising speed, depends on the drag force, which balances the propulsive force. Predicting the cruising velocity at intermediate Reynolds numbers thus requires accurately predicting the drag force. In addition to the friction drag, the vortex induced drag, which may play a significant role, has only recently gained the attention of experimentalists. Based on observations obtained using simplyfied mechanical swimmers, which consist of flexible plates with driven pitching motion, Raspa et al. (PoF 26, 2014), established a basic model to explain the influence of the finite aspect ratio by the formation of trailing longitudinal tip-vortices. Here, these generic swimmers are simulated numerically. We vary the aspect ratio in order to assess the influence of coherent vortices on the drag force. The solid model is based on chordwise flexible foils that undergo large, non-linear deformations. They are actively coupled with a 3D Navier-Stokes solver, based on Fourier transforms and the volume penalization to impose the no-slip boundary conditions. The numerical approach allows to access the entire 3D instantaneous flow field and yields thus new insights into the vortex-induced drag. [Preview Abstract] |
Sunday, November 23, 2014 9:31AM - 9:44AM |
A5.00008: Strongly Coupled Fluid-Body Dynamics in the Immersed Boundary Projection Method Chengjie Wang, Jeff D. Eldredge A computational algorithm is developed to simulate dynamically coupled interaction between fluid and rigid bodies. The basic computational framework is built upon a multi-domain immersed boundary method library, whirl, developed in previous work. In this library, the Navier-Stokes equations for incompressible flow are solved on a uniform Cartesian grid by the vorticity-based immersed boundary projection method of Colonius and Taira. A solver for the dynamics of rigid-body systems is also included. The fluid and rigid-body solvers are strongly coupled with an iterative approach based on the block Gauss-Seidel method. Interfacial force, with its intimate connection with the Lagrange multipliers used in the fluid solver, is used as the primary iteration variable. Relaxation, developed from a stability analysis of the iterative scheme, is used to achieve convergence in only 2-4 iterations per time step. Several two- and three-dimensional numerical tests are conducted to validate and demonstrate the method, including flapping of flexible wings, self-excited oscillations of a system of linked plates and three-dimensional propulsion of flexible fluked tail. [Preview Abstract] |
Sunday, November 23, 2014 9:44AM - 9:57AM |
A5.00009: Vortex-induced drag and the role of aspect ratio in undulatory swimmers Ramiro Godoy-Diana, Veronica Raspa, Sophie Ramananarivo, Benjamin Thiria During cruising, the thrust produced by a self-propelled swimmer is balanced by a global drag force. For a given object shape, this drag can involve skin friction or form drag, both being well-documented mechanisms. However, for swimmers whose shape is changing in time, the question of drag is not yet clearly established. We address this problem by investigating experimentally the swimming dynamics of undulating thin flexible foils. Measurements of the propulsive performance together with full recording of the elastic wave kinematics are used to discuss the general problem of drag in undulatory swimming. We show that a major part of the total drag comes from the trailing longitudinal vortices that roll-up on the lateral edges of the foils. This result gives a comparative advantage to swimming foils of larger span thus bringing new insight to the role of aspect ratio for undulatory swimmers. Ref: Physics of Fluids, Vol. 26, 041701 (2014). [Preview Abstract] |
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