Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session G6: Aerodynamics: Flapping Wings |
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Chair: Karen Mulleners, Ecole Polytechnique Federale de Lausanne Room: B114 |
Monday, November 21, 2016 8:00AM - 8:13AM |
G6.00001: Effect of Oscillatory Plunging Motion on Airfoil Boundary Layer and Wake Behavior Mark Agate, Jesse Little, Andreas Gross, Hermann Fasel The effects of small amplitude ($0.030 < A/c < 0.048$) high frequency ($0.61 < \pi fc/U_{\infty} < 0.70$) plunging motion of the X-56A airfoil are examined at Re=200,000 for three angles of attack. Two angles of attack were chosen at pre-stall conditions and one angle of attack was selected to study post-stall effects. Static stall of the airfoil is $12.25^{\circ}$ and the examined angles are $10^{\circ}$, $12^{\circ}$, and $14^{\circ}$. The purpose of this research is to examine the aerodynamic influence of structural motion when the wing is vibrating close to its eigenfrequency near static stall. The aerodynamic characteristics generated by the plunging motion are considered with specific focus on the laminar separation bubble near the leading edge. For the cases examined, the static lift is greatly exceeded. At the plunging case of $10^{\circ}$ angle of attack, experimental results are very similar to those obtained from Theodorsen's Theory. For the $12^{\circ}$ plunging case, lift exceeds that predicted by Theodorsen's Theory and the leading edge bubble bursts during the oscillation cycle. At the static stall condition of $14^{\circ}$, plunging periodically reattaches the flow and the bubble bursting is much more significant. [Preview Abstract] |
Monday, November 21, 2016 8:13AM - 8:26AM |
G6.00002: Circulation Produced by a Flapping Wing During Stroke Reversal Matthew Burge, Matthew Ringuette We investigate the circulation behavior of the 3D flow structures formed during the stroke-reversal of a 2-degree-of-freedom flapping wing in hover. Previous work has related circulation peaks to the unsteady wing kinematics and forces. However, information from experiments detailing contributions from the multiple, 3D flow structures is lacking. The objective of this work is to quantitatively study the spanwise circulation as well as the spanwise flow which advects vorticity in the complex loop topology of a flapping wing during stroke reversal. We analyze the flow features of a scaled wing model using multi-plane stereo digital particle image velocimetry in a glycerin-water mixture. Data plane locations along the wing span are inspired by the time-resolved behavior of the 3D vortex structures observed in our earlier flow visualization studies. As with our prior work, we vary dimensionless parameters such as the pitching reduced frequency to understand their effect on the circulation. This research provides insight into the vortex dynamics produced by the coupled rotational and pitching wing motions during stroke reversal, when lift generation is challenging. [Preview Abstract] |
Monday, November 21, 2016 8:26AM - 8:39AM |
G6.00003: Decomposing the aerodynamic forces of low-Reynolds flapping airfoils Manuel Moriche, Manuel Garcia-Villalba, Oscar Flores We present direct numerical simulations of flow around flapping NACA0012 airfoils at relatively small Reynolds numbers, $Re = 1000$. The simulations are carried out with TUCAN, an in-house code that solves the Navier-Stokes equations for an incompressible flow with an immersed boundary method to model the presence of the airfoil. The motion of the airfoil is composed of a vertical translation, heaving, and a rotation about the quarter of the chord, pitching. Both motions are prescribed by sinusoidal laws, with a reduced frequency of $k=1.41$, a pitching amplitude of 30deg and a heaving amplitude of one chord. Both, the mean pitch angle and the phase shift between pitching and heaving motions are varied, to build a database with 18 configurations. Four of these cases are analysed in detail using the force decomposition algorithm of Chang (1992) and Mart\'{\i}n Alc\'antara et al.(2015). This method decomposes the total aerodynamic force into added-mass (translation and rotation of the airfoil), a volumetric contribution from the vorticity (circulatory effects) and a surface contribution proportional to viscosity. In particular we will focus on the second, analysing the contribution of the leading and trailing edge vortices that typically appear in these flows. [Preview Abstract] |
Monday, November 21, 2016 8:39AM - 8:52AM |
G6.00004: Modelling forces and flow features in flapping wings: a POD based approach. Marco Raiola, Stefano Discetti, Andrea Ianiro A novel POD-based approach to decompose the aerodynamic forces acting on a flapping wing along with the most relevant flow features is proposed. The method is applied to experimental data including PIV and force measurements at $Re=3600$ and $St=0.2$. An actuated 2D flapping wing with a NACA 0012 airfoil is designed to produce independent heaving and pitching motion. The wing is equipped with a 6 Degrees-Of-Freedom balance, providing aerodynamic force measurements. Planar PIV measurements are carried out to obtain a phase-locked flow features description in the wing near field. The PIV phase-averaged flow fields are transformed into flow fields in the reference frame fixed with respect to the moving wing. The POD performed on the vorticity field provides a time basis, constituted by the vorticity time coefficients, on which it is possible to project both the flow fields and the forces in order to assess the force contribution of each POD mode. The force generation is mostly ascribed to the first 4 modes. A satisfactory description of the measured forces is achieved through a truncation to the first 6 modes. A more detailed analysis of the flow field projections is useful to determine the force generation mechanism. [Preview Abstract] |
Monday, November 21, 2016 8:52AM - 9:05AM |
G6.00005: Numerical study of tandem flapping wings hovering near ground Srinidhi N G, Vengadesan S The ground effect on tandem elliptical foils hovering in an inclined stroke plane is studied using immersed boundary projection method. The computations are carried out at a low Reynolds number, $Re=100$, in a quiescent fluid at different heights from the ground. The effect of phase relationship, $\psi$, between the fore- and hindwings on force variation is studied. Flow induced by the rebound vortices changes the effective angle of attack (AoA) of the wings and influences the force generation. In some cases, the shed vortices merge with the rebound vortices and create a sustained recirculating vortex which has a significant effect on the force generation of the forewing. In counter-stroking ($\psi=180^o$) and in-phase stroking ($\psi=0^o$), the rebound vortices increase the effective AoA of the forewing and increase the lift coefficients; interestingly, for $\psi=90^o$, such an increase in forces is not observed. Except for the cases with $\psi=90^o$, time-averaged vertical force coefficient of the forewing is always greater than the hindwing. For selected cases, backward in time finite-time Lyapunov exponent (FTLE) ridges are used in conjunction with vorticity contours to gain more insight into the vorticity dynamics. [Preview Abstract] |
Monday, November 21, 2016 9:05AM - 9:18AM |
G6.00006: Linearized propulsion theory of flapping airfoils revisited Ramon Fernandez-Feria A vortical impulse theory is used to compute the thrust of a plunging and pitching airfoil in forward flight within the framework of linear potential flow theory. The result is significantly different from the classical one of Garrick that considered the leading-edge suction and the projection in the flight direction of the pressure force. By taking into account the complete vorticity distribution on the airfoil and the wake the mean thrust coefficient contains a new term that generalizes the leading-edge suction term and depends on Theodorsen function $C(k)$ and on a new complex function $C_1(k)$ of the reduced frequency $k$. The main qualitative difference with Garrick's theory is that the propulsive efficiency tends to zero as the reduced frequency increases to infinity (as $1/k$), in contrast to Garrick's efficiency that tends to a constant ($1/2$). Consequently, for pure pitching and combined pitching and plunging motions, the maximum of the propulsive efficiency is not reached as $k \rightarrow \infty$ like in Garrick's theory, but at a finite value of the reduced frequency that depends on the remaining non-dimensional parameters. The present analytical results are in good agreement with experimental data and numerical results for small amplitude oscillations. [Preview Abstract] |
Monday, November 21, 2016 9:18AM - 9:31AM |
G6.00007: Mean, coherent and stochastic flow structure interactions in the near-wake of an oscillatory foil Firas Siala, James Liburdy Particle image velocimetry measurements are conducted to investigate the transport mechanism of flow kinetic energy in the near-wake of an oscillating foil at a reduced frequency of 0.18-0.2. Velocity triple decomposition is used to decompose the flow into mean, coherent and stochastic fields, and the kinetic energy evolution equations are utilized to study the energy exchange between the three components of the flow fields. The results show that the leading edge vortex (LEV) is responsible in extracting the majority of the free stream kinetic energy via the coherent shear strain. Furthermore, a scale-based model that characterizes the energy content of the LEV is developed. It is shown that coherent kinetic energy produced by the mean rate of strain scales remarkably well with the LEV energy content estimated by the model. [Preview Abstract] |
Monday, November 21, 2016 9:31AM - 9:44AM |
G6.00008: Effect of Mean Angle of Attack Modulation on Dynamic Stall Kyle Heintz, Thomas Corke Wind tunnel experiments at $M=0.2$ were conducted on a cambered airfoil instrumented with surface pressure transducers that was oscillated with two independent frequencies. The primary input, $f_1$, corresponds to a range of reduced frequencies, while the slower, secondary input, $f_2$, drives the modulation of the mean angle of attack, thus varying the stall-penetration angle, $\alpha_{pen}$. Various combinations transitioned different regimes of dynamic stall from ``light" to ``deep". Results suggest that when $\alpha_{pen}$ is falling between consecutive cycles, the aerodynamic loads do not fully recover to the values seen when $\alpha_{pen}$ is rising, even though the airfoil recedes to $\alpha_{pen}<0$ during each oscillation. The experimental data is presented in terms of load coefficients, aerodynamic damping, and their phase relationships to pitch angle. [Preview Abstract] |
Monday, November 21, 2016 9:44AM - 9:57AM |
G6.00009: The role of wing kinematics of freely flying birds downstream the wake of flapping wings Krishnamoorthy Krishnan, Roi Gurka Avian aerodynamics has been a topic of research for centuries. Avian flight features such as flapping, morphing and maneuvering make bird aerodynamics a complex system to study, analyze and understand. Aerodynamic performance of the flapping wings can be quantified by measuring the vortex structures present in the downstream wake. Still, the direct correlation between the flapping wing kinematics and the evolution of wake features need to be established. In this present study, near wake of three bird species (western sandpiper, European starling and American robin) have been measured experimentally. Long duration, time-resolved, particle image velocimetry technique has been used to capture the wake properties. Simultaneously, the bird kinematics have been captured using high speed camera. Wake structures are reconstructed from the collected PIV images for long chord distances downstream. Wake vorticities and circulation are expressed in the wake composites. Comparison of the wake features of the three birds shows similarities and some key differences are also found. Wing tip motions of the birds are extracted for four continuous wing beat cycle to analyze the wing kinematics. Kinematic parameters of all the three birds are compared to each other and similar trends exhibited by all the birds have been observed. A correlation between the wake evolutions with the wing motion is presented. It was found that the wings' motion generates unique flow patterns at the near wake, especially at the transition phases. At these locations, a drastic change in the circulation was observed. [Preview Abstract] |
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