skip to main content

Title: Modulation and Annihilation of Aeroelastic Limit-Cycle Oscillations Using a Variable-Frequency Disturbance Generator
Nonlinear aeroelastic limit-cycle oscillations (LCOs) have become an area of interest due to both detrimental effects on flying vehicles and use in renewable energy harvesting. Initial studies on the interaction between aeroelastic systems and incoming flow disturbances have shown that disturbances can have significant effects on LCO amplitude, with some cases resulting in spontaneous annihilation of the LCO. This paper explores this interaction through wind-tunnel experiments using a variable-frequency disturbance generator to produce flow disturbances at frequencies near the inherent LCO frequency of an aeroelastic system with pitching and heaving degrees of freedom. The results show that incoming disturbances produced at frequencies approaching the LCO frequency from below produce a cyclic growth-decay in LCO amplitude that resembles interference between multiple sine waves with slightly varying frequencies. An aeroelastic inverse technique is applied to the results to study the transfer of energy between the pitching and heaving degrees of freedom as well as the aerodynamic power moving into and out of the system. Finally, the growth-decay cycles are shown to both excite LCOs in an initially stationary wing and annihilate preexisting LCOs in the same wing by appropriately timing the initiation and termination of disturbance generator motion.  more » « less
Award ID(s):
Author(s) / Creator(s):
; ;
Date Published:
Journal Name:
AIAA Journal
Page Range / eLocation ID:
1447 to 1461
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Periodic upstream flow disturbances from a bluff body have recently been shown to be able to modulate and annihilate limit cycle oscillations (LCOs) in a downstream aeroelastic wing section under certain conditions. To further investigate these phenomena, we have implemented a controllable wind tunnel disturbance generator to enable quantification of the parameter ranges under which these nonlinear interactions can occur. This disturbance generator, consisting of a pitch-actuated cylinder with an attached splitter plate, can be oscillated to produce a von Karman type wake with vortex shedding frequency equal to the oscillation frequency over a range of frequencies around the natural shedding frequency of the cylinder alone. At vortex shedding frequencies away from the LCO frequency of the wing, forced oscillations were observed in the wing, but the wing did not enter self-sustaining LCOs. However, when disturbances were introduced near the LCO frequency, the initially static downstream wing entered self-sustaining oscillations in the presence of the incoming vortices, and these LCOs persisted when the disturbance generator was stopped. Annihilation of the wing LCOs was also observed disturbance vortices were introduced upstream of the wing in LCO. 
    more » « less
  2. The interaction between upstream flow disturbance generators and downstream aeroelastic structures has been the focus of several recent studies at North Carolina State University. Building on this work, which observed the modulation of limit cycle oscillations (LCOs) in the presence of vortex wakes, this study examines the design and validation of a novel disturbance generator consisting of an oscillating cylinder with an attached splitter plate. Analytical design of the bluff body was performed based on specific flow conditions which produced LCO annihilation in previous studies. Computational fluid dynamics simulations and experimental wind tunnel tests were used to validate the ability of the new disturbance generator to produce the desired wake region. Future work will see the implementation of this novel design in conjunction with aeroelastic structures in an effort to modulate and control LCOs, including the excitation and annihilation thereof. 
    more » « less
  3. The rising global trend to reduce dependence on fossil fuels has provided significant motivation toward the development of alternative energy conversion methods and new technologies to improve their efficiency. Recently, oscillating energy harvesters have shown promise as highly efficient and scalable turbines, which can be implemented in areas where traditional energy extraction and conversion are either unfeasible or cost prohibitive. Although such devices are quickly gaining popularity, there remain a number of hurdles in the understanding of their underlying fluid dynamics phenomena. The ability to achieve high efficiency power output from oscillating airfoil energy harvesters requires exploitation of the complexities of the event of dynamic stall. During dynamic stall, the oncoming flow separates at the leading edge of the airfoil to form leading ledge vortex (LEV) structures. While it is well known that LEVs play a significant role in aerodynamic force generation in unsteady animal flight (e.g. insects and birds), there is still a need to further understand their spatiotemporal evolution in order to design more effective energy harvesting enhancement mechanisms. In this work, we conduct extensive experimental investigations to shed-light on the flow physics of a heaving and pitching airfoil energy harvester operating at reduced frequencies of k = fc=U1 = 0.06-0.18, pitching amplitude of 0 = 75 and heaving amplitude of h0 = 0:6c. The experimental work involves the use of two-component particle image velocimetry (PIV) measurements conducted in a wind tunnel facility at Oregon State University. Velocity fields obtained from the PIV measurements are analyzed qualitatively and quantitatively to provide a description of the dynamics of LEVs and other flow structures that may be present during dynamic stall. Due to the difficulties of accurately measuring aerodynamic forces in highly unsteady flows in wind tunnels, a reduced-order model based on the vortex-impulse theory is proposed for estimating the aerodynamic loadings and power output using flow field data. The reduced-order model is shown to be dominated by two terms that have a clear physical interpretation: (i) the time rate of change of the impulse of vortical structures and (ii) the Kutta-Joukowski force which indirectly represents the history effect of vortex shedding in the far wake. Furthermore, the effects of a bio-inspired flow control mechanism based on deforming airfoil surfaces on the flow dynamics and energy harvesting performance are investigated. The results show that the aerodynamic loadings, and hence power output, are highly dependent on the formation, growth rate, trajectory and detachment of the LEV. It is shown that the energy harvesting efficiency increases with increasing reduced frequency, peaking at 25% when k = 0.14, agreeing very well with published numerical results. At this optimal reduced frequency, the time scales of the LEV evolution and airfoil kinematics are matched, resulting in highly correlated aerodynamic load generation and airfoil motion. When operating at k > 0:14, it is shown that the aerodynamic moment and airfoil pitching motion become negatively correlated and as a result, the energy harvesting performance is deteriorated. Furthermore, by using a deforming airfoil surface at the leading and trailing edges, the peak energy harvesting efficiency is shown to increase by approximately 17% and 25% relative to the rigid airfoil, respectively. The performance enhancement is associated with enhanced aerodynamic forces for both the deforming leading and trailing edges. In addition, The deforming trailing edge airfoil is shown to enhance the correlation between the aerodynamic moment and pitching motion at higher reduced frequencies, resulting in a peak efficiency at k = 0:18 as opposed to k = 0:14 for the rigid airfoil. 
    more » « less
  4. Oscillating foil energy harvesting devices are increasingly being considered as a sustainable energy alternative, especially in rivers and tidal areas. This paper applies CFD to an oscillating foil power generation device in order to explore the effects of pitching amplitude, the ratio of heaving amplitude to chord length, and the reduced frequency to the energy harvesting efficiency. Ansys Fluent 17.2 was used for this study, and the results are compared to experimental results that have been previously documented in the open literature. Configurations examined included pitching amplitudes of 65, 70, 75, and 80 degrees; heaving ratios of 0.4, 0.6, and 0.8; and reduced frequencies of 0.1, 0.12, 0.14, and 0.16. Results seems to indicate that the optimal reduced frequency is related to the heaving ratio, with the pitching amplitude only creating slight variations in the power produced by the foil. In the data, configurations with a heaving ratio of 0.4 have highest efficiency at reduced frequencies of either 0.14 or 0.16, but efficiency remains high at both points, which indicates the possibility of a peak in between the two points. Configurations with heaving ratio of 0.6 peak at reduced frequency 0.14 with a significant drop off at reduced frequency of 0.16. Finally, configurations with a heaving ratio of 0.8 show a peak at 0.12 reduced frequency and a significant drop at 0.14 and 0.16. These results suggest that OFEH devices can be effectively optimized for different and potentially varying operating conditions that may be encountered during practical implementation of OFEH technology. 
    more » « less
  5. We study analytically the dynamic response of membrane aerofoils subject to arbitrary, small-amplitude chord motions and transverse gusts in a two-dimensional inviscid incompressible flow. The theoretical model assumes linear deformations of an extensible membrane under constant tension, which are coupled aeroelastically to external aerodynamic loads using unsteady thin aerofoil theory. The structural and aerodynamic membrane responses are investigated for harmonic heave oscillations, an instantaneous change in angle of attack, sinusoidal transverse gusts and a sharp-edged gust. The unsteady lift responses for these scenarios produce aeroelastic extensions to the Theodorsen, Wagner, Sears and Küssner functions, respectively, for a membrane aerofoil. These extensions incorporate for the first time membrane fluid–structure interaction into the expressions for the unsteady lift response of a flexible aerofoil. The indicial responses to step changes in the angle of attack or gust profile are characterised by a slower lift response in short times relative to the classical rigid-plate response, while achieving a significantly higher asymptotic lift at long times due to aeroelastic camber. The unsteady lift for harmonic gusts or heaving motions follows closely the rigid plate lift responses at low reduced frequencies but with a reduced lift amplitude and greater phase lag. However, as the reduced frequency approaches the resonance of the fluid-loaded membrane, the lift response amplitude increases abruptly and is followed by a sharp decrease. This behaviour reveals a frequency region, controlled by the membrane tension coefficient, for which membrane aerofoils could possess substantial aerodynamic benefits over rigid aerofoils in unsteady flow conditions. 
    more » « less