This study examines the use of a passively actuated trailing edge of a thin wing during oscillation motion. The integration of a flexible trailing edge with an oscillating wing has the ability to alter the transient lift and drag characteristics, as well as the time averaged values. The results are obtained for a chord-length based Reynolds number of 0 and 40,000, and at oscillation frequencies of 0.5 and 1 Hz. The non-dimensional heaving amplitude is fixed at 0.25 and the pitching is 20°. The flexibility of the trailing edge is controlled by a torsion rod between the main wing and the trailing edge. Three conditions are evaluated: a very stiff rod (essentially non-flexible trailing edge), a moderately flexible rod and a very flexible rod. Results obtained indicate that lift and drag have a shift in the time averaged values, where the drag and lift both decrease as the trailing edge flexibility increases. These findings have application to both enhanced propulsion and energy harvesting.
An experimental study was conducted to explore the effect of surface flexibility at the leading and trailing edges on the near-wake flow dynamics of a sinusoidal heaving foil. Midspan particle image velocimetry (PIV) measurements were taken in a closed-loop wind tunnel at a Reynolds number of 25,000 and at a range of reduced frequencies (k = fc/U) from 0.09 to 0.20. Time-resolved and phase-locked measurements are used to describe the mean flow characteristics and phase-averaged vortex structures and their evolution. Large-eddy scale (LES) decomposition and swirling strength analysis are used to quantify the vortical structures. The results demonstrate that trailing edge flexibility has minimal influence on the mean flow characteristics. The mean velocity deficit for the flexible trailing edge and rigid foils remains constant for all reduced frequencies tested. However, the trailing edge flexibility increases the swirling strength of the small-scale structures, resulting in enhanced cross-stream dispersion. Flexibility at the leading edge is shown to generate a large-scale leading edge vortex (LEV) for k ≥ 0.18. This results in a reduction in the swirling strength due to vortex interactions when compared to the flexible trailing edge and rigid foils. Furthermore, it is shown that the large-scale LEV is responsible for extracting a significant portion of energy from the mean flow, reducing the mean flow momentum in the wake. The kinetic energy loss in the wake is shown to scale with the energy content of the LEV.
An experimental study was conducted to explore the effect of surface flexibility at the leading and trailing edges on the near-wake flow dynamics of a sinusoidal heaving foil. Mid-span particle image velocimetry measurements were taken in a closed loop wind tunnel at a Reynolds number of 25,000 and at a range of reduced frequencies (k = fc/U) from 0.09–0.20. Time resolved and phase locked measurements were used to describe the mean flow characteristics and phase averaged vortex structures and their evolution throughout the oscillation cycle. Large eddy scale decomposition and swirl strength analysis were used to quantify the effect of flexibility on the vortical structures. The results demonstrate that flexibility at the trailing edge has a minimal influence on the mean flow characteristics when compared to the purely rigid foil. The mean velocity deficit for the flexible trailing edge and rigid foils is shown to remain constant for all reduced frequencies tested. However, the trailing edge flexibility increases the swirl strength of the small scale structures, which results in enhanced cross stream dispersion of the mean velocity profile. Flexibility at the leading edge is shown to generate a large scale leading edge vortex for k ≥ 0.18. This results in a reduction in the swirl strength due to the complex vortex interactions when compared to the flexible trailing edge and rigid foils. Furthermore, it is shown that the large scale leading edge vortex is responsible for extracting a significant portion of the energy from the mean flow, resulting in a substantial reduction of mean flow momentum in the wake. The kinetic energy loss in the wake is shown to scale well with the energy content of the leading edge vortex.
The aerodynamic performance of an oscillating pitching and plunging foil operating in the energy harvesting mode is experimentally investigated. Experiments are conducted in a closed-loop recirculating wind tunnel at Re of 24,000 to 48,000, and reduced frequencies (k) of 0.04 to 0.08. Foil kinematics are varied through the following parameter space: heaving amplitude of 0.3c, pitching amplitudes of θ0 = 45° to 75°, as well as phase lag between sinusoidal pitching and heaving motions of Φ = 30° to 120°. Aerodynamic force measurements are collected to show the energy extraction performance (power coefficient and efficiency) of the foil. Coupled with the force measurements, flow fields are collected using particle image velocimetry. The flow field characteristics are used to supplement the force results, shedding light into flow features that contribute to increased energy extraction at these k values. In addition, inertia-induced passive chord-wise flexibility at the leading edge (LE) of the foil is investigated in order to assess its feasibility in this application. Results indicate that favorable performance occurs near θ0 = 45°, Φ = 90° and k = 0.08. When k is decreased (through increased U∞) to 0.04, overall extraction performance becomes insensitive to θ0 and Φ. This is supported by the flow field measurements, which show premature leading edge vortex (LEV) evolution and detachment from the foil surface. Although overall performance was reduced with the passive LE flexibility, these results indicate that a proper tuning of the LE may result in a delay of the LEV detachment time, yielding increased energy harvesting at this otherwise inefficient operating parameter space.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.