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 S28: Wing Flexibility |
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Chair: Haoxiang Luo, Vanderbilt University Room: Ballroom II |
Tuesday, November 22, 2011 3:05PM - 3:18PM |
S28.00001: On passive wing response in flapping flight Albert Medina, Jeff D. Eldredge The aerodynamic role of passive wing mechanics in biological flight remains poorly understood. This computational and experimental study focuses on the effects of flexibility on flapping foils. Our approach recognizes that deflections of insect wings primarily occur along major flexion lines, and therefore focuses on simplified wings with discrete rigid structures. In two-dimensional numerical simulations a wing consisting of rigid elliptical bodies connected by torsion springs is subjected to various acceleration profiles. Both the early-time behavior and sustained vortex shedding are investigated, and compared with an analogous rigid wing. It is found that the vortex shedding period is reduced with decreasing spring stiffness. These results are presented in the general context of vortex-induced vibrations. Our experiments focus on a dynamically scaled three-dimensional fruitfly wing in which we isolate deflection to flexion lines running largely spanwise or chordwise. We observe that previously-observed phenomena of the leading-edge vortex, such as vortex bursting, are affected by the passive response of the wing structure. Chordwise flexion has a significantly greater impact on this behaviour compared to spanwise flexion. [Preview Abstract] |
Tuesday, November 22, 2011 3:18PM - 3:31PM |
S28.00002: Flexibility and inertia of flapping wings in forward flight Fang-Bao Tian, Haoxiang Luo, Xi-Yun Lu Insect wings typically deform passively in flight under the combined aerodynamic force and inertia of the wing. To study the effect of the wing flexibility on the aerodynamic performance, a two-dimensional numerical study is employed to simulate the fluid-structure interaction of an elastic plate performing forward flight. The leading edge of the plate is clamped, while the rest of the chord is free to deform, leading to passive pitching and a dynamic camber. The wing stiffness and mass ratio are varied, and their effects on the lift, thrust, and aerodynamic power are investigated. The results shows that the moderate chordwise deformation can improve both lift and thrust performance significantly. The instantaneous passive pitching angle and consequently the forces are largely affected by the mass ratio that determines whether the deformation is caused by the wing inertia or the aerodynamic force. The high mass ratio wings, whose deformation is due to the wing inertia, can produce more thrust than the low mass ratio wing at the same amount of deformation. However, the high thrust is gained at a price of more power requirement. [Preview Abstract] |
Tuesday, November 22, 2011 3:31PM - 3:44PM |
S28.00003: Role of Wing/Body Flexibility in Insect Maneuver Chengyu Li, Haibo Dong, Samane Zeyghami It's widely thought insects are able to accomplish fast maneuver via adjustment of wing kinematics. However, it's still unclear how wing flexibility plays roles in this process. In this work, an integrated study combining high-speed photogrammetry and direct numerical simulation (DNS), for a freely flying dragonfly (\textit{Erythemis Simplicicollis}) in 110 degree turn, is used to reveal both aerodynamic and dynamic roles of its body and wings. Quantitative measurements have shown the significant difference of deformation between all wings as well as up to 18 degree bending of the tail. Unsteady 3D vortex formation and associated aerodynamic forces calculated from high-fidelity simulations are used to illustrate how the turn is accomplished within three dragonfly wing beats. This work is supported by NSF CBET-1055949. [Preview Abstract] |
Tuesday, November 22, 2011 3:44PM - 3:57PM |
S28.00004: Understanding the Role of Chord-wise Flexibility in Flapping Wing Flight Zachary Gaston, Haibo Dong, Hui Wan, Michael Ol Aerodynamic performance of flapping hinged plates is numerically studied to explore the effects of chord-wise flexibility in flapping wing flight. The plate with chord-wise flexibility is modeled as a two-link mechanism with a torsional spring hinge in between. The upper-link of the plate is controlled by prescribed motion and the rest of body is subjected to passive deflection due to fluid-body interaction. The effect of forced to natural frequency ratio is studied first for a flapping hinged-plate, on which prescribed hovering motion is actively applied. The effects of torsional stiffness and chord-wise flexibility are further explored for pitching and plunging plates, observing the flow phenomena and lift production as a result of this change. Comparisons between rigid plates, free-to-pivot hinged plates, and the torsional spring hinged plates are made, identifying a more optimal model for promoting lift production in flapping plates. [Preview Abstract] |
Tuesday, November 22, 2011 3:57PM - 4:10PM |
S28.00005: Hovering performance of a two dimensional skeleton-reinforced flexible wing Kourosh Shoele, Qiang Zhu Skeleton-reinforced membrane is a typical biological design. An important application of these systems is in biolocomotion apparatus, most notably the wings of insects. The structural characteristics of the wing, a composite structure including a soft membrane reinforced by embedded skeleton(venation) is an important factor in performance of a insect flexible wing during hovering flight. To study how the structural anisotropy affects the aerodynamic performance of the deformable wing, a two-dimensional numerical study is applied to simulate the flow-structure interaction of a wing during hovering flight. In this two-dimensional rendition, the underlying veins are modeled as springs, and the membrane is modeled as a flexible plate. The effect of the wing anisotropy on lift production and power expenditure is studied for a range of veins rigidity, Reynolds number and wing inertia. It is shown that with flexible veins and the leading edge strengthening, the lift production can be significantly increased. In addition, the detailed distribution of veins stiffness in the wing has a significant effect on the unsteady flow pattern around the wing. [Preview Abstract] |
Tuesday, November 22, 2011 4:10PM - 4:23PM |
S28.00006: Flow and forces of flexible wings under a plunging motion Diego Campos, Lawrence Ukeiley The effects of flexibility on the flow fields and production of aerodynamic forces of Zimmerman planform wings with different stiffness are investigated. The wings are subjected to a symmetric sinusoidal plunging motion under forward flight conditions. Particle Image Velocimetry (PIV) is used to measure the flow at different spanwise locations and Laser Doppler Vibrometry (LDV) is utilized to obtain deformation characteristics from the wings. The results from the PIV and LDV analysis are phase averaged discrete points throughout the plunging cycle, resulting in three component flow field data coupled with wing twist obtained from the LDV, thus allowing for a better understanding of the fluid-structure interactions. The forces are then calculated through a momentum balance technique to better understand the effects of different stiffness. The Q criterion is used to identify and analyze the vortical structures that form around the wing. Results show the strongest leading edge vortex formation between 70 and 80 percent span. The effect of the deformation on the production and evolution of the vortical structures will be related to the generation of aerodynamic forces. [Preview Abstract] |
Tuesday, November 22, 2011 4:23PM - 4:36PM |
S28.00007: Dynamics of Freely Swimming Flexible Foils Silas Alben, Charles Witt, T. Vernon Baker, Erik Anderson, George Lauder We use experiments, simulations, and modeling to study thin foils which are oscillated at the leading edge and are free to move unidirectionally under the resulting fluid forces. We find resonant-like peaks in the swimming speed as a function of foil length and rigidity. We find good agreement between the inviscid model and the experiment in the foil motions (particularly the wavelengths of their shapes), the dependences of their swimming speeds on foil length and rigidity, and the corresponding flows. The model predicts that the foil speed is proportional to foil length to the -1/3 power and foil rigidity to the 2/15 power. These scalings give a good collapse of the experimental data. [Preview Abstract] |
Tuesday, November 22, 2011 4:36PM - 4:49PM |
S28.00008: Shape and Swimming of Flexible Pitching Foils Anders Andersen, Bjarne Bach, Teis Schnipper We present an experimental study using flexible metal foils which are driven with simple harmonic pitching oscillations in water. We vary the frequency and amplitude of the oscillations and explore the foil response at frequencies both below and above the lowest resonance frequency. We compare our observations with theoretical predictions taking into account the elastic properties of the foil and using a simple model of the fluid forces. Finally, we make use of an experimental setup in which the pitching foils are mounted on a simple carousel that allows them to rotate (swim) freely and compare our observations on the rotation (swimming) velocity with that of rigid foils performing similar pitching oscillations. [Preview Abstract] |
Tuesday, November 22, 2011 4:49PM - 5:02PM |
S28.00009: Bistable property of a flexible ring in a uniform flow Bo Young Kim, Soo Jai Shin, Hyung Jin Sung An improved version of the immersed boundary (IB) method is developed for simulating a flexible ring clamped at one point in a uniform flow. The boundary of the ring consists of a flexible filament with tension and bending stiffness, which can be modeled as a linear spring with spring constant k and initially unstretched length. In our simulation, we observe bistable states, one stationary stable and the other self-sustained periodically flapping, that coexist over a range of flow velocities depending on the initial inclination angle. The bistable property of the initially elliptic flexible ring is observed in the simulations. The Reynolds number range of the bistability region and the flapping amplitude are specified for various aspect ratios (a/b). It is found that for a/b=0.5, the bistability region is most postponed and the flapping amplitude at the self-sustained flapping state is minimized. A new bistability phenomenon is observed that with certain aspect ratio two periodically flapping states coexist with different amplitudes in a Reynolds number range, instead of the stationary stable and periodically flapping states. [Preview Abstract] |
Tuesday, November 22, 2011 5:02PM - 5:15PM |
S28.00010: Scaling in Flexible Flapping Wings Chang-kwon Kang, Hikaru Aono, Wei Shyy The role of flexibility on the aerodynamic performance of a flapping wing is investigated. We consider chordwise, spanwise, and isotropic flexibility. Overall, the aerodynamic force is determined by the Reynolds number, reduced frequency ($k$), and Strouhal number ($St$). In particular, at the Reynolds number regime of O($10^3$-$10^4$) and the reduced frequency of O($1$), the added mass force, related to the acceleration of the wing, is important. Based on the order of magnitude and energy balance arguments, a relationship between the propulsive force and the maximum relative wing tip deformation parameter is established. The parameter depends on the density ratio, $St$, $k$, natural and flapping frequency ratio, and flapping amplitude. It seems that the maximum propulsive force is obtained when flapping near the resonance, whereas the optimal propulsive efficiency is reached when flapping at about half of the natural frequency; both are supported by the reported studies. [Preview Abstract] |
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