Dynamical Behavior of Unsteady Flowfield of an Elastically Mounted Flapping Airfoil
24 Jan 2018-AIAA Journal (American Institute of Aeronautics and Astronautics)-Vol. 56, Iss: 5, pp 2062-2069
About: This article is published in AIAA Journal.The article was published on 2018-01-24. It has received 6 citations till now. The article focuses on the topics: Flapping & Airfoil.
Citations
More filters
TL;DR: In this article, the transition from periodic to chaotic dynamics in the fluid-elastic response of a 2-d flexible pitch-plunge flapping airfoil with chord-wise rigidity is analyzed.
Abstract: This study focuses on numerically analyzing the transition from periodic to chaotic dynamics in the fluid-elastic response of a 2-dof flexibly-mounted airfoil with chord-wise rigidity. The computational framework is composed of a high fidelity Navier–Stokes solver, weakly coupled with a structural model having geometric nonlinearity represented by cubic order stiffness terms. A low Reynolds number flow regime and a very low structure-to-fluid added mass ratio have been considered to simulate the flying conditions of very light-weight unmanned devices. A bifurcation analysis of the system, in the absence of actuation or control forces, is undertaken with the wind velocity as the control parameter. The route to chaos – identified to be the Ruelle–Takens–Newhouse quasi-periodic route – is established for the first time for a flexible pitch–plunge flapping system. Robust nonlinear time series analysis techniques have been implemented to characterize different complex dynamical states present in the system.
11 citations
TL;DR: In this paper, an alternative simulation approach is proposed to simplify the calculations for the ONERA model, in which the circulatory part of the OnERA model is solved analytically by using Duhamel integral method, and the corresponding aerodynamic loads can be directly expressed in the modal space of the wing motion through only introducing two additional variables.
Abstract: The ONERA aerodynamic model is a nonlinear aerodynamic model which includes the effects of dynamic stall. With the strip theory, the ONERA model is usually used in the aeroelastic analysis of slender wings. To the classical approach for the ONERA model, the circulatory and nonlinear parts are all described by using aerodynamic elements and the simulation cost may be very high. In this paper, an alternative simulation approach is proposed to simplify the calculations for the ONERA model, in which the circulatory part of the ONERA model is solved analytically by using Duhamel integral method. In this way, the corresponding aerodynamic loads can be directly expressed in the modal space of the wing motion through only introducing two additional variables. For a slender wing model, the new simulation approach is used to analyze its nonlinear aeroelastic responses. In the simulation, the number of the state variables for the system using the proposed approach is reduced modestly comparing with that using the classical approach. In addition, for a slender wing with a pylon-store system which includes a free-play gap, both the proposed approach and the classical approach with the same number of aerodynamic elements are used to analyze the nonlinear dynamic behaviors of the system. Simulation results are given to show that the pylon-store system with a free-play gap can lead the occurrence of sub-critical Hopf bifurcation for a slender wing. Additionally, some nonlinear dynamic phenomena about the wing-pylon-store system are observed by using the new approach. But these phenomena cannot be predicted by employing the classical approach with the same number of aerodynamic elements used in the new approach. The peak of the post-critical responses obtained from the classical approach are larger than those obtained from the new approach in most range of free-stream velocity. The nonlinear flutter velocity predicted by using the classical approach is lower than that predicted by using the new approach.
5 citations
TL;DR: In this paper , the authors explored the optimal working range and wake characteristics of the propulsive wing, the wake transition and aerodynamic efficiency of the Propulsive Wing in cruise, and showed that the wake structure will change greatly at high angles of attack and lead to changes in the aerodynamic performance.
Abstract: The propulsive wing is a new concept wing of automatic propulsion with high lift coefficients and has great application value in plant protection and forest fire control. The propulsive wing wake is a reverse Bénard–von Kármán (RBvK) vortex street, which is considered a thrust-generating wake. The wake structure will change greatly at high angles of attack and lead to changes in the aerodynamic performance of the propulsive wing. To explore the optimal working range and the wake characteristics of the propulsive wing, the wake transition and aerodynamic efficiency of the propulsive wing in cruise are numerically studied. The results indicate that there are three types of structures for the propulsive wing wake. When α ≤ 20°, the wake transits from the RBvK vortex street to the critical state with the increase in cruise speed, and the Strouhal number approaches 1.9. The critical wake region decreases gradually with the increase in the angle of attack. The maximum propulsive efficiency is 0.17 at a cruise speed of 15 m/s. When α > 20°, the wake transits directly from the RBvK vortex street to the Bénard–von Kármán (BvK) vortex street and the Strouhal number approaches 0.34. The maximum propulsive efficiency appears at a cruise speed of 10 m/s, which is close to the BvK vortex street boundary. Before entering the stall state, the lift efficiency of the propulsive wing increases with the increase in cruise speed and angle of attack, up to 3–5.
1 citations
11 Jan 2021
TL;DR: In this paper, the fluid-structure interaction (FSI) framework is composed of an incompressible Navier-Stokes solver weakly coupled with a two degree-of-freedom (dof) nonlinear structural model.
Abstract: This paper investigates the transitional flow dynamics behind a passively flapping airfoil supported by nonlinear springs in the presence of gusty inflow. The fluid-structure interaction (FSI) framework is composed of an incompressible Navier-Stokes solver weakly coupled with a two degree-of-freedom (dof) nonlinear structural model. The fluid-elastic system shows a rich bifurcation behavior in terms of successive Hopf bifurcations in uniform flow condition as the mean wind speed is increased. Presence of gusty fluctuations in the inflow makes the dynamics more complex through transitional states that we refer to as ‘intermittency’ between different dynamical states. A regular intermittent state between quasi-periodic dynamics and low amplitude aperiodic response has been observed when the FSI system is subjected to a time harmonic gust in terms of sinusoidal fluctuation. A parametric study has been carried out for various amplitudes and frequencies of the sinusoidal fluctuation to demarcate the transitional regimes. Thereafter, the system is subjected to random gusts modeled as Ornstein-Uhlenbeck process and ‘on-off’ and ‘burst’ type intermittent dynamics have been observed for long time-scale and short time-scale input fluctuations respectively. The intermittent states have been characterized through time series analyses tools and the corresponding flow-field dynamics has been investigated in detail.
1 citations
TL;DR: In this paper , the effect of low intensity additive noise on a subcritical pitch-plunge aeroelastic system through the lens of complex networks was investigated, and the results have shown that a careful consideration of the stochastic perturbations is needed in the design and operations of such systems, and that complex networks based methods are successful in providing novel perspectives on the topology of the phase-space.
References
More filters
TL;DR: In this article, the vortical flow patterns in the wake of a NACA 0012 airfoil pitching at small amplitudes were studied in a low speed water channel, and it was shown that a great deal of control can be exercised on the structure of the wake by the control of the frequency, amplitude and also the shape of the oscillation waveform.
Abstract: The vortical flow patterns in the wake of a NACA 0012 airfoil pitching at small amplitudes are studied in a low speed water channel. it is shown that a great deal of control can be exercised on the structure of the wake by the control of the frequency, amplitude and also the shape of the oscillation waveform. An important observation in this study has been the existence of an axial flow along the cores of the wake vortices. Estimates of the magnitude of the axial flow suggest a linear dependence on the oscillation frequency and amplitude.
672 citations
TL;DR: In this article, the equations of motion of a two-dimensional airfoil oscillating in pitch and plunge are derived for a structural nonlinearity using subsonic aerodynamics theory.
421 citations
TL;DR: In this article, the ability of a sinusoidally plunging airfoil to produce thrust, known as the Knoller-Betz or Katzmayr effect, was investigated experimentally and numerically.
Abstract: The ability of a sinusoidally plunging airfoil to produce thrust, known as the Knoller-Betz or Katzmayr effect, is investigated experimentally and numerically. Water-tunnel experiments are performed providing flow visualization and laser Doppler velocimetry data of the unsteady wakes formed by the plunging foils. Vortical structures and time-averaged velocity profiles in the wake are compared with numerical computations from a previously developed inviscid, unsteady panel code that utilizes a nonlinear wake model
408 citations
TL;DR: In this paper, a numerical model for two-dimensional flow around an airfoil undergoing prescribed heaving motions in a viscous flow is presented, and the model is used to examine the flow characteristics and power coefficients of a symmetric aerodynamic model.
Abstract: A numerical model for two-dimensional flow around an airfoil undergoing prescribed heaving motions in a viscous flow is presented. The model is used to examine the flow characteristics and power coefficients of a symmetric airfoil heaving sinusoidally over a range of frequencies and amplitudes. Both periodic and aperiodic solutions are found. Additionally, some flows are asymmetric in that the upstroke is not a mirror image of the downstroke. For a given Strouhal number – defined as the product of dimensionless frequency and heave amplitude – the maximum efficiency occurs at an intermediate heaving frequency. This is in contrast to ideal flow models, in which efficiency increases monotonically as frequency decreases. In accordance with Wang (2000), the separation of the leading-edge vortices at low heaving frequencies leads to diminished thrust and efficiency. At high frequencies, the efficiency decreases similarly to inviscid theory. Interactions between leading- and trailing-edge vortices are categorized, and the effects of this interaction on efficiency are discussed. Additionally, the efficiency is related to the proximity of the heaving frequency to the frequency of the most spatially unstable mode of the average velocity profile of the wake; the greatest efficiency occurs when the two frequencies are nearly identical. The importance of viscous effects for low-Reynolds-number flapping flight is discussed.
361 citations
TL;DR: Water-tunnel tests of a NACA 0012 airfoil that was oscillated sinusoidally in plunge are described in this article, where dye flow visualization and single-component laser Doppler velocimetry (LDV) measurements for a range of freestream speeds, frequencies, and amplitudes of oscillation are explored.
Abstract: Water-tunnel tests of a NACA 0012 airfoil that was oscillated sinusoidally in plunge are described. The flowered downstream of the airfoil was explored by dye flow visualization and single-component laser Doppler velocimetry (LDV) measurements for a range of freestream speeds, frequencies, and amplitudes of oscillation. The dye visualizations show that the vortex patterns generated by the plunging airfoil change from drag-producing wake flows to thrust-producing jet flows as soon as the ratio of maximum plunge velocity to freestream speed, i.e., the nondimensional plunge velocity, exceeds approximately 0.4. The LDV measurements show that the nondimensional plunge velocity is the appropriate parameter to collapse the maximum streamwise velocity data covering a nondimensional plunge velocity range from 0.18 to 9.3
326 citations