Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session R13: Aerodynamics: Unsteady Aerodynamics II: Flapping and Flexible Wings |
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Chair: Mingjun Wei, New Mexico State University Room: 201 |
Tuesday, November 24, 2015 12:50PM - 1:03PM |
R13.00001: Adjoint-based optimization for the understanding of the aerodynamics of a flapping plate Mingjun Wei, Min Xu An adjoint-based optimization is applied on a rigid flapping plate and a flexible flapping plate for drag reduction and for propulsive efficiency. Non-cylindrical calculus is introduced to handle the moving boundary. The rigid plate has a combined plunging and pitching motion with incoming flow, the control parameter is the phase delay which is considered first as a constant then as an arbitrary time-varying function. The optimal controls with different cost functions provide different strategies to reach maximum drag reduction or propulsive efficiency. The flexible plate has plunging, pitching, and deformation which is defined by the first two natural modes. With the same optimization goals, the control is instead the amplitude and phase delay of the pitching, the first eigen mode, and the second eigen mode. Similar analyses are taken to understand the conditions for drag reduction and propulsive efficiency when flexibility is involved. It is also shown that the flexibility plays a more important role at lower Reynolds number. [Preview Abstract] |
Tuesday, November 24, 2015 1:03PM - 1:16PM |
R13.00002: Wing-Fixed PIV and force measurements of a large transverse gust encounter Gino Perrotta The unsteady aerodynamics of an aspect ratio 4 flat plate wing encountering a large-amplitude transverse gust were investigated using PIV in the wing-fixed reference frame and direct unsteady force measurements. Using a new experimental facility at the University of Maryland, the wing was towed at Reynolds number 20,000 through a 7m-long tank of nominally quiescent water containing a single cross-stream planar jet with velocity equal to the wing's towed velocity - a transverse gust ratio equal to one. The planar jet was created by pumping water through 30 cylindrical nozzles arranged in a single row. PIV confirms that the individual jets converge into a single, narrow, planar gust with a streamwise velocity profile resembling a canonical cosine-squared gust. Forces and fluid velocities of this wing-gust interaction will be presented for two pre-gust conditions: attached flow on the wing and stalled flow over the wing. In both cases, the gust encounter results in a momentary spike in lift coefficient. The peak lift coefficient was measured between 3 and 6 and varies with angle of attack. At low angle of attack, the attached flow wing produces less lift before the gust and much more (non-circulatory) lift during the gust than the stalled wing. Although the flow over the wing at low angle of attack separates during the gust and reattaches afterwards, the recovery time is similar to that of the high angle case, on the order of 10 chord lengths travelled. [Preview Abstract] |
Tuesday, November 24, 2015 1:16PM - 1:29PM |
R13.00003: Proper Orthogonal Decomposition of Flow-Field in Non-Stationary Geometry Victor Troshin, Avraham Seifert, David Sidilkover, Gilead Tadmor This work presents a proper orthogonal decomposition (POD) methodology for a flow field in a domain with moving boundaries. A relatively simple volume preserving mapping which transforms a deforming to stationary domain is described. This mapping was created by combining a transfinite interpolation and volume adjustment algorithm. The algorithm is based on iterative solution of the Laplace equation with respect to the displacement potential of the grid points. The transformed domain is suitable for proper orthogonal decomposition procedure. The presented mapping can be applied to a wide variety of flow problems which contain single or in some cases multiple deforming boundaries. Currently, this method is presented for 2D geometries, however, it can be expanded to 3D cases. This approach can assist in creation of low order models for complex aero-elastic systems which to date could not be analysed by existing POD approaches. Finally, the method is demonstrated on CFD results of pitching and plunging ellipse in still fluid. [Preview Abstract] |
Tuesday, November 24, 2015 1:29PM - 1:42PM |
R13.00004: On the correlation between force production and the flow field around a flapping flat-plate wing S\"oren \"Oz, Swathi Krishna, Karen Mulleners One of the several sophisticated flight skills that insects exhibit is hovering, which is accomplished largely by modulating the wing kinematics and thereby the flow field around the wings. Along with the prolonged attachment of the leading edge vortex, the wing reversal mechanisms form the basis by which insects regulate the magnitude and direction of forces produced. The duration and starting point of these directional flips are studied in the current experimental investigation. Particle image velocimetry is conducted to evaluate the flow features inherent to changes in wing reversal during the stroke of a flat plate, which is modelled based on hoverfly characteristics. The duration of rotation is one-third of the total time period. A +10\% phase shift is used for delayed rotation, a -10\% phase shift for advanced rotation. Phase-averaged data is analysed to understand the influence of a delayed or advanced rotation on the formation and evolution of large and small scale structures, their interactions with the wing, and disintegration. Additionally, force data is used to quantify the effects of phase-shift in terms of lift and drag variation and is correlated with the vortex dynamics. [Preview Abstract] |
Tuesday, November 24, 2015 1:42PM - 1:55PM |
R13.00005: Effect of advanced and delayed rotation on the dominant flow pattern and its temporal evolution Esra Uksul, Swathi Krishna, Karen Mulleners During a flapping cycle of an insect, complex time dependent flows are produced as the wing reciprocates, producing a maximum lift at the stroke reversals. By flipping the wing rapidly at the end of each stroke, the insect modulates the flow around the wing and hence the aerodynamic forces necessary to hover. The duration and starting point of the flip play an important role in determining the amount of lift produced. To understand and tailor the effect of wing kinematics on the aerodynamic performance we focussed on the vortex dynamics of the flow field. Phase-averaged data from particle image velocimetry was used to evaluate the flow features inherent to changes in rotation during a stroke of a flat plate, which is modelled based on hoverfly characteristics. The period of rotation is one-third of the total time period. A +10\% phase shift is used for delayed rotation, a -10\% phase shift for advanced rotation. Vortex detection methods like the $\lambda_2$ and $\Gamma_2$ criteria are used to determine the effect of a delay or early rotation on the trajectories, size, shape and location of the prominent vortical structures. Proper orthogonal decomposition is used to study the influence of the phase-shifts on the dominant mode structure and the related time-scales. [Preview Abstract] |
Tuesday, November 24, 2015 1:55PM - 2:08PM |
R13.00006: A Lagrangian approach to study flow topology around a flapping flat-plate wing Swathi Krishna, Karen Mulleners, Melissa Green The incredible flight performance of insects can be attributed in part to the generation and maintenance of stable regions of vorticity, which is achieved by manipulating the wing kinematics. Along with the prolonged attachment of the leading edge vortex during translation of the wing, the rotational motion at the end of the stroke is critical as it generates large amounts of lift required for the insect to remain air-borne while hovering. The wing reversal entails a change in the flow-field around the wing which is closely tied to variations in force production. Based on phase-averaged particle image velocimetry data we analyze the effect of a shift in the rotational phase of a flapping wing on the flow characteristics. A topological study is conducted using Lagrangian vortex detection techniques in order to characterize the shear layer formation, vortex interactions and flow separation. The Lagrangian analysis includes the calculation of Finite Time Lyapunov Exponents based on particle trajectories. An objective approach is employed to trace the location of separation or attachment points as an indication for changes in the strength, stability and shedding frequencies of vortices. These trajectories are correlated with fluctuations in aerodynamic force coefficients. [Preview Abstract] |
Tuesday, November 24, 2015 2:08PM - 2:21PM |
R13.00007: Vortical Flow Structures in the Near-Wake of a Heaving Airfoil with Passively Actuated Leading and Trailing Flaps. Firas Siala, Alexander Totpal, James Liburdy The flow physics of flying animals has recently received significant attention, mostly in the context of developing bio-inspired micro air vehicles and oscillating flow energy harvesters. Of particular interest is the understanding of the impact of airfoil flexibility on the flow physics. Research efforts showed that some degree of surface flexibility enhanced the strength and size of the leading edge vortex. In this study, the influence of flexibility on the near-wake dynamics and flow structures is investigated using 2D PIV measurements. The experiments are conducted in a wind tunnel at a Reynolds number of 30,000 and a range of reduced frequencies from 0.09 to 0.2. The flexibility is attained using a torsion rod forming a hinge between the flap and the main wing. Vortex flow structures are visualized using large eddy scale decomposition technique and quantified using swirling strength analysis. It is found that trailing edge flexibility increases the vortex swirling strength compared to a rigid airfoil, whereas leading edge flexibility decreases the swirling strength. Furthermore, the integral length scale determined from the autocorrelation of the velocity fluctuations is found to be approximately equal to the actual vortex size. The vortex convective velocity is shown to be independent of flexibility and oscillation frequency, and it is represented by a trimodal distribution, with peak values at 0.8, 0.95 and 1 times the free stream velocity. [Preview Abstract] |
Tuesday, November 24, 2015 2:21PM - 2:34PM |
R13.00008: Chord-wise Tip Actuation on Flexible Flapping Plates Nathan Martin, Morteza Gharib The aerodynamic characteristics of low aspect ratio flapping plates are strongly influenced by the interaction between tip and edge vortices. This has led to the development of tip actuation mechanisms which bend the tip towards the root of the plate in the span-wise direction during oscillation to investigate its impact. In our current work, a tip actuation mechanism to bend a flat plate's two free corners towards one another in the chord-wise direction is developed using a shape memory alloy. The aerodynamic forces and resulting flow field are investigated from dynamically altering the tip chord-wise curvature while flapping. The frequency of oscillation, stroke angle, flexibility, and tip actuation timing are independently varied to determine their individual effects. These results will further the fundamental understanding of flapping wing aerodynamics. [Preview Abstract] |
Tuesday, November 24, 2015 2:34PM - 2:47PM |
R13.00009: Efficient passive pitching motion caused by elastic deformation in flexible flapping wing MAVs Trong Nguyen, Tien Truong, Khoon Seng Yeo, Tee Tai Lim Computational and experimental models which mimic Hawkmoth wings were constructed to investigate the effects of wing flexibility. The wing actuation mechanism is minimal with only one degree of freedom in sweeping motion with neither active pitching nor elevation. Despite the simplicity of the imparted motion, the wing models in both computations and experiments delivered convincing deformation features such as wing twisting and camber which closely resembles the ones observed in real Hawkmoth wings. The generated aerodynamic forces are remarkable both in magnitude and efficiency. The study hence reveals that a complicated actuation mechanism might not be required to produce the sophisticated and efficient motion of insect wings, which in fact could be the result of collective elastic deformation thanks to their highly optimized structure mainly comprised of well-organized veins and membranes. [Preview Abstract] |
Tuesday, November 24, 2015 2:47PM - 3:00PM |
R13.00010: On the thrust performance of a 2D flapping foil in a forward flight condition Sunil Manohar Dash, Kim Boon Lua, Tee Tai Lim Past studies have shown that the thrust performance of a 2D airfoil undergoing simple harmonic motion in both pitch and heave in a forward flight condition is dependent on maximum effective angle of attack ($\alpha _{\mathrm{o}})$ and Strouhal number ($S_{T})$. For a given $\alpha _{\mathrm{o}}$, it is found that the thrust coefficient ($C_{T})$ increases with $S_{T}$ until it reaches a peak value at the critical Strouhal number ($S_{Tc})$; beyond which $C_{T}$ deteriorates considerably. In order to extend $S_{Tc}$ and therefore increase the max.$C_{T}$, the airfoil must oscillate at a higher $\alpha_{\mathrm{o}}$. Further, it is found that, regardless of $\alpha_{\mathrm{o}}$ thrust degeneration is accompanied by cessation of the induced effective angle of attack profile ($\alpha $(t)) to exhibit simple harmonic function of time. As to why non simple harmonic function of $\alpha $(t) is detrimental to thrust generation is not fully understood. In an attempt to better understand this phenomenon, both numerical simulations and comparative experiments are performed on a 2D flapping elliptic foil at Re of 5000. Our results show that the proximity of the leading edge vortex from the previous stroke to the oscillating foil plays a crucial role in the thrust generation. Detailed results will be discussed in the presentation. [Preview Abstract] |
Tuesday, November 24, 2015 3:00PM - 3:13PM |
R13.00011: Locomotion of a flapping flexible plate in ground effect Xi-Yun Lu, Chao Tang Locomotion of a three-dimensional flapping flexible plate in ground effect is studied numerically by the coupled solution of the fluid flow and the plate motion. When the leading-edge of the flexible plate is forced to take a vertical oscillation near a ground, the plate moves freely due to the fluid-structure interaction. Mechanisms underlying the dynamics of the plate near the ground are elucidated. The ground effect can enhance propulsive speed and improve propulsive efficiency, especially in the medium bending stiffness regime. The analysis of unsteady dynamics and deformation of plate indicates that the ground effect becomes weaker for more flexible plate. Therefore it is found that a suitable degree of flexibility can improve the propulsive performance in ground effect. The vortical structure and pressure distribution around the plate and their connection with the dynamics of the plate are also investigated. [Preview Abstract] |
Tuesday, November 24, 2015 3:13PM - 3:26PM |
R13.00012: Thrust and Lift generation of heaving and pitching oscillating foil propulsion in ground effect Amin Mivehchi, Jason M. Dahl, Stephen Licht Experimental results are presented for the thrust and lift generation on a NACA0012 airfoil undergoing heave and pitch oscillation near a solid boundary. For ground effect in the steady flow over a lifting surface, lift and drag forces are altered by an enhanced spanwise flow around the tip of the lifting surface, resulting in a strong low pressure region on the upper part of the wing and increased lift in the presence of a boundary. In the present study, this effect is investigated for an inherently unsteady flow, a propulsive flapping foil. It is found that ground effect has a significant effect on the instantaneous and average lift and thrust forces generated by the oscillating foil with heave and pitch motion. It is found that the forces on a flapping foil in the presence of the ground is not only dependent on the aspect ratio but shows high dependency on the kinematics of motion such as maximum angle of attack, frequency of flapping, and the distance from the ground. The relation between these parameters and their effect on the cycle averaged thrust, lift, propulsive efficiency, and instantaneous force over the airfoil is shown. It is hypothesized that ground effect may be used as a proxy sensor for identifying solid boundaries with biomimetic underwater vehicles. Keywords: Ground effect, Flapping foil propulsion, flow-structure interaction [Preview Abstract] |
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