Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session LV: Propulsion Interactions |
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Chair: Haecheon Choi, Seoul National University Room: 205A-D |
Monday, November 23, 2009 3:35PM - 3:48PM |
LV.00001: Interaction between the fore- and hind-wings in hovering flight of modelled dragonfly Jihoon Kweon, Haecheon Choi In the present study, we investigate the interaction between the fore- and hind-wings in hovering flight of modelled dragonfly using 3D numerical simulation. The three-dimensional wing shape is based on that of {\it Aeschna juncea} (Norberg 1972) and numerically realized using an immersed boundary method (Kim et al. 2001). The wing flapping motion is modelled using a sinusoidal function and the stroke plane angle is 60$^{\circ}$. We consider 12 different phase differences between the fore- and hind-wings ($\phi=0^{\circ} \sim ~$330$^{\circ}$). The Reynolds number is 1,000 based on the maximum translational velocity and mean chord length. In counter stroke ($\phi=180^{\circ}$), the wing-tip vortices from both wings are connected, generating an entangled wing-tip vortex (e-WTV). A strong downward motion induced by this vortex decreases the vertical force in the following stroke (Kweon \& Choi 2008). In parallel stroke ($\phi=0^{\circ}$), both wings meet e-WTV during the upstroke and thus the decrease of vertical force is small. At $\phi=270^{\circ}$, although e-WTV is generated on a relatively narrow region, the hind-wing moves downward along with e-WTV, resulting in a significant reduction of vertical force on the hind-wing. Therefore, the sum of vertical forces on both wings is maximum with parallel stroke and minimum at $\phi=270^{\circ}$. The power required has a similar trend to the vertical force and thus the efficiency does not show a large variation with the phase difference. [Preview Abstract] |
Monday, November 23, 2009 3:48PM - 4:01PM |
LV.00002: Wing-Wake Interactions between Ipsilateral Wings in Dragonfly Flight Haibo Dong, Zongxian Liang Bilateral and ipsilateral wing-wing interactions can be commonly observed in insect flights. As a representative example of ipsilateral wing-wing interaction, dragonflies in flight have been widely studied. An important fact is that the flow over their hindwings is affected by the presence of the forewings. Wake capture and phase-change play very important role on aerodynamic performance of the hindwings We present a direct numerical simulation of a modeled dragonfly (Aeshna juncea) in slow flight as studied in Azuma et al (JEB 1985). Realistic morphologies of wing, body, and kinematics are used for maximum including wing and body features of a dragonfly. This work aims to study the relations between wake-topology and aerodynamic performance due to wing-wing and wing-wake interactions of dragonfly ipsilateral wings. DNS results are also compared with Local Momentum Theory (Azuma et al). [Preview Abstract] |
Monday, November 23, 2009 4:01PM - 4:14PM |
LV.00003: Unsteady aerodynamics of dragonfly using a wing-wing model from the perspective of a force decomposition Chin-Chou Chu, Chien C. Chang, Chen-Ta Hsieh The lift and thrust associated with insect flight strongly depend on the complex wake patterns produced by wing-wing and wing-wake interactions. We propose to investigate the aerodynamics of dragonfly using a simplified wing-wing model from the perspective of many-body force decomposition (JFM 600, p95) and the associated force elements. The aerodynamic force, lift or thrust, of the wing-wing system is analyzed in terms of its four constituent components, each of which is directly related to a physical effect. These force components for each individual wing include two potential contributions credited to the wing motion itself, contribution from the vorticity within the flow, and contributions from the surface vorticity on its and other wing's surfaces. The potential contribution due to added-mass effect is often non-negligible. Nevertheless, the major contribution to the forces comes from the vorticity within the flow. The relative importance of these components relies heavily on the motions of the two wings such as the respective angles of attack, the amplitude and speed of translational motions, and the amplitude and speed of wing rotations. In addition to the dynamic stall vortex, several important mechanisms of high lift or thrust are also identified. [Preview Abstract] |
Monday, November 23, 2009 4:14PM - 4:27PM |
LV.00004: The interaction of two bodies falling in tandem Nicolas Brosse, Patricia Ern We have investigated experimentally the interaction of two identical bodies falling in a fluid at rest at intermediate Reynolds numbers ($100 < Re < 300$). The bodies are disks of various diameter-to-thickness ratios ($2 < d/t < 10$) and of density close to the fluid one. They are released either consecutively or simultaneously at two different locations. The path of the bodies when they fall separately is either rectilinear or a periodic zigzag depending on the corresponding values of $d/t$ and $Re$. We will focus here on the case of two bodies released consecutively and exhibiting a rectilinear path when they fall separately. The motion of the bodies was recorded by two travelling cameras. While the forebody (the first body released) follows a rectilinear path, the aftbody (the second body released) accelerates thanks to the forebody's wake and oscillates when $Re$ is close to the critical value of appearance of the zigzag motion. Though the forebody also accelerates, the aftbody eventually catches up the forebody. Afterwards, thick bodies ($d/t = 3$) separate, whereas thinner bodies ($d/t > 5$) continue their fall together. We will describe quantitatively the characteristics of the motion of the two bodies before and after grouping. [Preview Abstract] |
Monday, November 23, 2009 4:27PM - 4:40PM |
LV.00005: Hydrodynamic interactions between flagella Pieter Janssen, Michael Graham Many bacteria, such as \emph{E. coli}, use several rotating flagella to propel themselves at low-Reynolds numbers. If the flagella are all rotating counter-clockwise, they bundle up, and the cell moves at great speed. However, if one flagellum starts to rotate clockwise, it disentangles from the bundle, and the cell starts to rotate randomly. After a while, the rotation of all flagella becomes counter-clockwise again, and the cell starts moving again, now in a different direction. The bundling and disentangling is poorly understood from a fluid mechanics point of view. We investigate the hydrodynamic interactions between flagella that may lead to the bundling. Flagella are modeled as series of spheres connected through hinges with bending and twisting resistance. Hydrodynamic interaction between the spheres is incorporated through standard expressions. The cell body is described with a boundary-integral method. Synchronization between the flagella is shown, and we investigate the effect of stiffness, pitch and length of the flagella, and of the hook connecting the flagellum to the cell. Furthermore, we show the effect on the orientation, rotation and speed of the cell body under the influence of multiple flagella. [Preview Abstract] |
Monday, November 23, 2009 4:40PM - 4:53PM |
LV.00006: Synchronization of Swimming Microorganisms Gwynn Elfring, Eric Lauga Flagellated eukaryotic cells (such as spermatozoa) have been observed to synchronize their flagella when swimming in close proximity. Using a 2D model, we find that hydrodynamic interactions alone can lead to synchronization if the waveforms of the flagella display front-back asymmetry. Depending on the nature of the asymmetry, the phase-locked conformation can minimize or maximize the energy dissipated by the co-swimming cells. We show that due to kinematic reversibility, this front-back asymmetry is necessary for synchronization in a Newtonian fluid, and discuss the differences in a non-Newtonian fluid. [Preview Abstract] |
Monday, November 23, 2009 4:53PM - 5:06PM |
LV.00007: Sychronization of flagella and cilia due to viscous interactions David Gagnon, Bian Qian, Hongyuan Jiang, Thomas Powers, Kenneth Breuer Motivated by the observed coordination of nearby beating cilia and rotating bacterial flagella, we use a scaled model experiment to show that hydrodynamic interactions can cause synchronization between rotating paddles driven at constant torque in a very viscous fluid. Systems with two and three paddles are explored, and interactions between symmetric and asymmetric paddles are tested. For two-paddle systems, synchronization is only observed when the shafts supporting the paddles have some flexibility, and the phase difference in the synchronized state depends on the symmetry of the paddles. Calculations using the method of regularized stokeslets and simple analytic theory match the experimental observations well. [Preview Abstract] |
Monday, November 23, 2009 5:06PM - 5:19PM |
LV.00008: Wake-mediated synchronization and drafting in coupled flags Silas Alben A recent experiment has shown ``inverted drafting'' in flags: the drag force on one flag is increased by excitation from the wake of another. Here we use vortex sheet simulations to show that inverted drafting occurs when the flag wakes add coherently to form strong vortices. By contrast, normal drafting occurs for higher-frequency oscillations, when the vortex wake becomes more complex and mixed on the scale of the flag. The types of drafting and dynamics (synchronization and erratic flapping) depend on the separation distance between the flags. For both tandem and side-by-side flags in synchronized flapping, the phase difference depends nearly monotonically on separation distance. These results provide a framework for how bodies interact through their wakes, and may be used to identify optimal rigidities and separation distances for bodies in collective locomotion. [Preview Abstract] |
Monday, November 23, 2009 5:19PM - 5:32PM |
LV.00009: Fish schooling as a basis for wind farm design Robert Whittlesey, John Dabiri It is known that horizontal axis wind turbines (HAWT) suffer from reduced aerodynamic efficiency when in close proximity to neighboring turbines. In contrast, recent work has shown that closely spaced vertical axis wind turbines (VAWT) may benefit from enhanced performance, reducing the associated land use for VAWT wind farm installations. A potential flow model of VAWT interactions is developed to determine configurations that optimize the power output of the array. A geometric arrangement based on fish schooling has been shown to significantly increase the array performance as measured by an Array Power Coefficient, which compares the average performance of turbines in the array to an isolated turbine. The results suggest that significant gains may be obtained through careful arrangement of VAWTs, showing up to a two order of magnitude decrease in land use (equivalently, a two order of magnitude increase in power density) compared to HAWTs. [Preview Abstract] |
Monday, November 23, 2009 5:32PM - 5:45PM |
LV.00010: Role of Flexibility in Thrust Production of a Mechanical Swimming Lamprey Megan Leftwich, Alexander Smits To develop a comprehensive model of lamprey locomotion, we use a robotic lamprey as a means of investigating the wake structure during swimming with an anatomically designed tail of varying degrees of flexibility. A programmable microcomputer actuates 11 servomotors that produce a traveling wave along the length of the lamprey body. The waveform is based on kinematic studies of living lamprey. The shape of the tail is taken from CT scan data of the silver lamprey, and it is constructed of flexible PVC gel. Plastic inserts allow the the degree of flexibility to be changed. PIV measurements in the wake behind the most flexible tail show a 2P wake structure that quickly looses coherence as it is convected downstream. This is in contrast to the strongly coherent and symmetrical 2P wake seen in previous experiments using a rigid, rectangular tail. The project is supported by NIH CNRS Grant 1R01NS054271. [Preview Abstract] |
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