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 G30: Aerodynamics: Flapping and Flexible Wings |
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Chair: Matthew Ringuette, State University of New York, Buffalo Room: 2016 |
Monday, November 24, 2014 8:00AM - 8:13AM |
G30.00001: Influence of Ground Effect on Low Aspect Ratio Membrane Wings Robert Bleischwitz, Roeland de Kat, Bharathram Ganapathisubramani Inspired by the current interest of membrane wings for Micro Air Vehicles ($MAVs$) and hard limits in aerodynamic performance for wings in moderate Reynolds number regimes, an experimental wind tunnel study is conducted at a Reynolds number of approximately 65,000 to determine the aeromechanics of flexible, low aspect ratio ($AR$) membrane wings ($AR$ $\leq$ 2) in the vicinity of the ground. Pitch angle $\alpha$ and height over ground $h/c$ is varied with a traverse system. Flexible membrane wings are compared with rigid flat plates. A rolling road is used to impose the ground effect and the boundary layer leading up to the road is removed using a suction system. Time-averaged lift, drag and pitch moment changes are captured with a 6-axis force transducer and its effects are interpreted in terms of the membrane motions obtained using Direct-Image-Correlation ($DIC$) technique. Flow-structure-ground interactions are examined and the membrane dynamics are compared to results obtained outside of ground effect. Ultimately, understanding the ground effect on flexible membrane wings at moderate Reynolds numbers could help to design Wing-in-Ground $MAVs$ with extended range and reduced energy consumption. [Preview Abstract] |
Monday, November 24, 2014 8:13AM - 8:26AM |
G30.00002: Lift on Flexible and Rigid Cambered Wings at High Incidence Anya Jones, Peter Mancini, Kenneth Granlund, Michael Ol The effects of camber and camber change due to elastic deflection of a membrane wing were investigated for wings in rectilinear translation with parameter variations in wing incidence and acceleration. Direct force and moment measurements were performed on a rigid flat plate wing, rigid cambered wings, and a membrane wing. Features in the force histories were further examined via flow visualization by planar laser illumination of fluorescent dye. Below 10 degrees of incidence, Wagner's approximation accurately predicts the time-evolution of lift for the rigid wings. At higher incidence, flow separation results in force transients, and the effect of wing camber is no longer additive. Both the rigid flat plate and rigid cambered wings reach peak lift at a 35 degree angle of attack, whereas the flexible wing experiences stall delay and reaches peak lift at 50 degrees. Due to the aeroelasticity of the flexible membrane, flow over the suction surface remains attached for much higher incidence angles than for the rigid wings. For incidence angles less than 30 degrees, the peak lift of the flexible wing is lower than that of its rigid counterparts. Beyond 30 degrees, the flexible wing experiences an aeroelastically induced stall delay that allows lift to exceed the rigid analogs [Preview Abstract] |
Monday, November 24, 2014 8:26AM - 8:39AM |
G30.00003: Characteristics of the flow around tandem flapping wings Luke Muscutt, Bharathram Ganapathisubramani, Gabriel Weymouth Vortex recapture is a fundamental fluid mechanics phenomenon which is important to many fields. Any large scale vorticity contained within a freestream flow may affect the aerodynamic properties of a downstream body. In the case of tandem flapping wings, the front wing generates strong large scale vorticity which impinges on the hind wing. The characteristics of this interaction are greatly affected by the spacing, and the phase of flapping between the front and rear wings. The interaction of the vorticity of the rear wing with the shed vorticity of the front wing may be constructive or destructive, increasing thrust or efficiency of the hind wing when compared to a wing operating in isolation. Knowledge of the parameter space where the maximum increases in these are obtained is important for the development of tandem wing unmanned air and underwater vehicles, commercial aerospace and renewable energy applications. This question is addressed with a combined computational and experimental approach, and a discussion of these is presented. [Preview Abstract] |
Monday, November 24, 2014 8:39AM - 8:52AM |
G30.00004: Aerodynamics of flapping insect wing in inclined stroke plane hovering with ground effect Krishne Gowda V, S. Vengadesan This work presents the time-varying aerodynamic forces and the unsteady flow structures of flapping insect wing in inclined stroke plane hovering with ground effect. Two-dimensional dragonfly model wing is chosen and the incompressible Navier-Stokes equations are solved numerically by using immersed boundary method. The main objective of the present work is to analyze the ground effect on the unsteady forces and vortical structures for the inclined stroke plane motions. We also investigate the influences of kinematics parameters such as Reynolds number (Re), stroke amplitude, wing rotational timing, for various distances between the airfoil and the ground. The effects of aforementioned parameters together with ground effect, on the stroke averaged force coefficients and regimes of force behavior are similar in both normal (horizontal) and inclined stroke plane motions. However, the evolution of the vortex structures which produces the effects are entirely different. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G30.00005: Flapping flight: effect of asymmetric kinematics Nakul Pande, Siddharth Krithivasan, Sreenivas K.R. Flapping flight has received considerable attention in the past with its relevance in the design of micro-air vehicles. In this regard, asymmetric flapping of wings offers simple kinematics. Nevertheless, it leads to symmetry-breaking in the flow field and generation of sustained lift. It has been observed previously with flow visualization experiments and Discrete Vortex Method (DVM) simulations that if the down-stroke time period is lesser than the up-stroke time, there is a net downward momentum imparted to the fluid. This is seen as a switching the flow field from a four-jet (symmetric) to a two-jet (asymmetric) configuration when the stroke-time ratio is progressively varied. This symmetry breaking has been studied experimentally using Particle Image Velocimetry (PIV) across a range of Reynolds Numbers and asymmetry ratios. Results are also corroborated with results from 3-D numerical simulations. Study helps in shedding light on the effectiveness of asymmetric kinematics as a lift generation mechanism. [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G30.00006: Effect of wing flexibility on phasing of tandem wings in forward flight Vishal Naidu, John Young, Joseph Lai The dragonfly with two pairs of wings in tandem uses different phases between the wing pairs to suit the needs of the flight. Previous studies to understand the effect of phasing in forward flight are based on rigid wings. This is in contrast to the highly flexible dragonfly wings, with varying spanwise and chordwise flexibility. Here, we study flexible flapping wing simulations using Fluid Structure Interaction (FSI) in forward flight, at an advance ratio of 0.3 and Reynolds number of approximately 1300. The FSI simulations are carried out for phase 90$^{\circ}$ (hindwing leading), 0$^{\circ}$ (in-phase) and 180$^{\circ}$ (anti-phase). The performance of flexible wings will be compared with that of the rigid wings and the effect of flexibility will be discussed. [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G30.00007: High amplitude surging and plunging motions at low Reynolds number Jeesoon Choi, Tim Colonius, David Williams Aerodynamic forces and flow structures associated with high amplitude oscillations of an airfoil in the streamwise (surging) and transverse (plunging) direction are investigated in two-dimensional simulations at low Reynolds number (Re$=$10$^{2}$ $\sim$ 10$^{3}$). While the unsteady aerodynamic forces for low-amplitude motions were mainly affected by the leading-edge vortex (LEV) acting in- or out-of phase with the quasi-component of velocity, large-amplitude motions involve complex vortex interactions of LEVs and trailing-edge vortices (TEVs) with the moving body. For high-amplitude surging, the TEV, instead of the LEV, induces low-pressure regions above the airfoil during the retreating portion of the cycle near the reduced frequency, k$=$0.5, and enhances the time-average forces. The time required for the LEV to convect along the chord becomes an intrinsic time scale, and for plunging motions, there is a sudden change of flow structure when the period of the motion is not long enough for the LEV to convect through the whole chord. [Preview Abstract] |
Monday, November 24, 2014 9:31AM - 9:44AM |
G30.00008: Using adjoint-based optimization to study wing flexibility in flapping flight Mingjun Wei, Min Xu, Haibo Dong In the study of flapping-wing flight of birds and insects, it is important to understand the impact of wing flexibility/deformation on aerodynamic performance. However, the large control space from the complexity of wing deformation and kinematics makes usual parametric study very difficult or sometimes impossible. Since the adjoint-based approach for sensitivity study and optimization strategy is a process with its cost independent of the number of input parameters, it becomes an attractive approach in our study. Traditionally, adjoint equation and sensitivity are derived in a fluid domain with fixed solid boundaries. Moving boundary is only allowed when its motion is not part of control effort. Otherwise, the derivation becomes either problematic or too complex to be feasible. Using non-cylindrical calculus to deal with boundary deformation solves this problem in a very simple and still mathematically rigorous manner. Thus, it allows to apply adjoint-based optimization in the study of flapping wing flexibility. We applied the ``improved" adjoint-based method to study the flexibility of both two-dimensional and three-dimensional flapping wings, where the flapping trajectory and deformation are described by either model functions or real data from the flight of dragonflies. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G30.00009: Flow Structure and Force Variation with Aspect Ratio for a Two-Degree-of-Freedom Flapping Wing Matthew Burge, James Favale, Matthew Ringuette We investigate experimentally the effect of aspect ratio (AR) on the flow structure and forces of a two-degree-of-freedom flapping wing. Flapping wings are known to produce complex and unsteady vortex loop structures, and the objective is to characterize their variation with AR and how this influences the lift force. Previous results on rotating wings demonstrated that changes in AR significantly affect the three-dimensional flow structure and lift coefficient. This is primarily due to the relatively greater influence of the tip vortex for lower AR. At Reynolds number of order O(10$^3$) we test wings of AR = 2-4, values typically found in nature, with simplified planform shapes. The lift force is measured using a submersible transducer at the base of the wing in a glycerin-water mixture. The qualitative, three-dimensional vortex loop structure for different ARs is obtained using multi-color dye flow visualization. Guided by this, quantitative three-component flow information, namely vorticity, the Q-criterion, and circulation, is acquired from stereoscopic particle image velocimetry in key planes. Of interest is how these parameters and the vortex loop topology vary with AR, and their connection to features in the unsteady force signal. [Preview Abstract] |
Monday, November 24, 2014 9:57AM - 10:10AM |
G30.00010: Modeling unsteady forces and pressures on a rapidly pitching airfoil Nicole K. Schiavone, Scott T.M. Dawson, Clarence W. Rowley, David R. Williams This work develops models to quantify and understand the unsteady aerodynamic forces arising from rapid pitching motion of a NACA0012 airfoil at a Reynolds number of 50 000. The system identification procedure applies a generalized DMD-type algorithm to time-resolved wind tunnel measurements of the lift and drag forces, as well as the pressure at six locations on the suction surface of the airfoil. Models are identified for 5-degree pitch-up and pitch-down maneuvers within the overall range of 0--20 degrees. The identified models can accurately capture the effects of flow separation and leading-edge vortex formation and convection. We demonstrate that switching between different linear models can give accurate prediction of the nonlinear behavior that is present in high-amplitude maneuvers. The models are accurate for a wide-range of motions, including pitch-and-hold, sinusoidal, and~pseudo-random pitching maneuvers. Providing the models access to a subset of the measured data channels can allow for improved estimates of the remaining states via the use of a Kalman filter, suggesting that the modeling framework could be useful for aerodynamic control applications. [Preview Abstract] |
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