Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session BH: Biofluids II: Flying |
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Chair: Jun Zhang, New York University Room: Salt Palace Convention Center 250 B |
Sunday, November 18, 2007 10:34AM - 10:47AM |
BH.00001: Flexible wings in flapping flight Lionel Moret, Benjamin Thiria, Jun Zhang We study the effect of passive pitching and flexible deflection of wings on the forward flapping flight. The wings are flapped vertically in water and are allowed to move freely horizontally. The forward speed is chosen by the flapping wing itself by balance of drag and thrust. We show, that by allowing the wing to passively pitch or by adding a flexible extension at its trailing edge, the forward speed is significantly increased. Detailed measurements of wing deflection and passive pitching, together with flow visualization, are used to explain our observations. The advantage of having a wing with finite rigidity/flexibility is discussed as we compare the current results with our biological inspirations such as birds and fish. [Preview Abstract] |
Sunday, November 18, 2007 10:47AM - 11:00AM |
BH.00002: Experimental Investigation of Unsteady Stall Characteristics in Flapping Flight Nathan Lunsford, Jamey Jacob This study examines the effects of wing rigidity on the aerodynamics of flapping wings, particularly in regard to unsteady separation. A flapping wing model was constructed and tested using a variety of diagnostics including force balance, PIV, and flow visualization. Three different wing flexibilities relative to each other were used: flexible, semi-flexible, and rigid. The setups were tested at a range of Reynolds numbers and flapping frequencies in addition to static measurements. The more flexible wings show more peaks and instability in the unsteady lift results at higher frequencies, possibly resulting from movement in the flexible trailing edge. At lower frequencies, the lift and drag curves correlate with each other in a sinusoidal pattern. This pattern's peaks align with the theoretical values from the frequency. It is seen that the flexible wing has lower energy content at lower frequencies and higher content at higher frequencies. This is a result of energy content being distributed from the dominant flapping frequency to higher modes on the wing as flexibility increases, which could be used to excite natural frequencies found in the wing. [Preview Abstract] |
Sunday, November 18, 2007 11:00AM - 11:13AM |
BH.00003: At the scale of insects flapping flight can be more efficient than fixed wing flight Umberto Pesavento, Z. Jane Wang A typical fruitfly supports a weight of 1 mg by flapping a pair of wings of radius $r \approx 0.2$ cm, and mean chord $\bar{c} \approx 6.8 \times 10^{-2}$ cm. The flow around its flapping wings has a Reynolds number of about 100. Recent studies showed that at such Reynolds number an insect can take advantage of unsteady effects to enhance lift production. Here we find that it can also take advantage of these unsteady effects to be more efficient than a classical fixed-wing supporting the same weight. [Preview Abstract] |
Sunday, November 18, 2007 11:13AM - 11:26AM |
BH.00004: Comparing flight strategies in species of fruit flies Itai Cohen, Leif Ristroph, Gordon Berman, Z. Jane Wang Observing different species of fruit flies offers an opportunity to compare flight strategies for insects of varying size but of nearly identical body and wing architecture. Using automated three-dimensional high-speed videography, we have captured many free-flight sequences of flies. We extract complete body and wing kinematics and determine the fluid forces acting on the wings using custom-written tracking and analysis software. We find that, in addition to lift, drag plays an important role in providing the vertical force needed for these insects to stay aloft. In this presentation, we will describe how this strategy of using drag varies among different species of Drosophila. [Preview Abstract] |
Sunday, November 18, 2007 11:26AM - 11:39AM |
BH.00005: An Interspecific Comparison of Fruitfly Flight Gordon Berman, Z. Jane Wang There has been much interest in studying how aspects of animal locomotion are affected by variation in organism size and morphology. In the case of insect flight, we are particularly interested in how the mechanisms of lift production and the energy efficiency of flight are affected by size and wing-structure. Here, we analyze 3D free-flight data of fruitflies to study differences in flight performance between species of varying morphology in both wild-types and mutants. [Preview Abstract] |
Sunday, November 18, 2007 11:39AM - 11:52AM |
BH.00006: Passive Wing Pitching in Insect Flight Attila Bergou, Z. Jane Wang We recently found that the wings of a flying insect can be passively oriented by aerodynamic forces and the inertia of the wing itself. Building upon this result, we investigate the role that the gross morphological parameters of the wing (e.g. the position of the torsion axis) play in the passive orientation of the wing. In addition, we explore the consequences that passive pitching has on the wing kinematics of an insect. [Preview Abstract] |
Sunday, November 18, 2007 11:52AM - 12:05PM |
BH.00007: Three-dimensional vortical structures around a flapping wing in hovering motion Jihoon Kweon, Haecheon Choi In this study, we investigate three-dimensional vortical structures around a flapping wing in hovering motion using numerical simulation. The three-dimensional wing shape and kinematics are based on the data of the Drosophila model by Dickinson et al. (Science 1999) and realized using an immersed boundary method (Kim \& Choi, JCP 2006). The Reynolds number is 136 based on the maximum translational velocity and mean chord length. During the translational motion, a strong wing-tip vortex is generated and stays in the wake. In the following stroke, the wing passes through a region of high momentum fluids induced by this wing-tip vortex, and has a high-pressure region on the pressure surface. On the other hand, leading- and trailing-edge vortices are generated during the translational motion and shed at the stroke reversal. These shed vortices interact with the wing in the following stroke. This interaction process is similar to that discussed in two-dimensional mechanism of hovering motion (Birch \& Dickinson, JEB 2003). However, the interaction between the wing and wing-tip vortex is completely three-dimensional. To further investigate the issue on 2D and 3D mechanisms, we vary the Reynolds number. The results of these computations will be discussed in the presentation. [Preview Abstract] |
Sunday, November 18, 2007 12:05PM - 12:18PM |
BH.00008: Flow field characteristics of an ornithopter Alfredo Juarez, James Allen This paper details phase locked PIV measurements from a model Ornithopther flying in a wind tunnel at representative flight conditions. Testing over a range of Strouhal numbers, 0.1-0.3, shows that the unsteady wake is composed of coherent vortical structures that resemble vortex rings. A single ring is formed in the wake of each wing during one wing beat. Momentum balance from velocity field measurements are used to estimate the lift and drag of the ornithopter. [Preview Abstract] |
Sunday, November 18, 2007 12:18PM - 12:31PM |
BH.00009: Force generation in transient deployment of square or triangular flat panels in the presence of a wall Alexis Pierides, Yiannis Andreopoulos We investigate the force generated by square or tringular flaps hinged at the wall beneath a flow during their transient deployment with an angular velocity between 10 and 100 rad/s. The objective of this study is to understand the mechanisms of unsteady flapping-wings motion and the system of vortices generated. The transient flow field has been simulated experimentally in a low speed wind tunnel and computationally by using CFD with moving boundaries capabilities. The results indicated that all lift and drag force coefficients during the transient deployment are different than the corresponding coefficients under stationary conditions at the same deployment angle after adjusting for inertial effects. These dynamic effects depend on the Strouhal number which can be considered as the ratio the Stokes to Reynolds number of the flow. It was found that these effects are augmented with increasing Strouhal number and decrease with increasing boundary layer thickness. Reasonable agreement has been found between computational and experimental data. [Preview Abstract] |
Sunday, November 18, 2007 12:31PM - 12:44PM |
BH.00010: Volume penalization to model falling leaves Dmitry Kolomenskiy, Kai Schneider Numerical modeling of solid bodies moving through viscous incompressible fluid is considered. The 2D Navier-Stokes equations, written in the vorticity-streamfunction formulation, are discretized using a Fourier pseudo-spectral scheme with adaptive time-stepping. Solid obstacles of arbitrary shape are taken into account using the volume penalization method. Time- dependent penalization is implemented, making the method capable of solving problems where the obstacle follows an arbitrary motion. Numerical simulations of falling leaves are performed, using the above model supplemented by the discretized ODEs describing the motion of a solid body subjected to external forces and moments. Various regimes of the free fall are explored, depending on the physical parameters and initial conditions. The influence of the Reynolds number on the transition between fluttering and tumbling is investigated, showing the stabilizing effect of viscosity. [Preview Abstract] |
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