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Ramon Fernandez-feria - One of the best experts on this subject based on the ideXlab platform.

  • Note on optimum propulsion of heaving and pitching airfoils from linear potential theory
    Journal of Fluid Mechanics, 2017
    Co-Authors: Ramon Fernandez-feria
    Abstract:

    The conditions that maximize the propulsive efficiency of a heaving and pitching airfoil are analysed using a novel formulation for the thrust force within the linear potential theory. Stemming from the vortical impulse theory, which correctly predicts the decay of the thrust efficiency as the inverse of reduced frequency $k$ for large $k$ (Fernandez-Feria, Phys. Rev. Fluids , vol. 1, 2016, 084502), the formulation is corrected here at low frequencies by adding a constant representing the viscous drag. It is shown first that the thrust coefficient and propulsive efficiency thus computed agree quite well with several sets of available experimental data, even for not so small flapping amplitudes. For a pure pitching motion, it is found that the maximum propulsion efficiency is reached for the airfoil pitching close to the three-quarter chord Point from the leading edge with a relatively large reduced frequency, corresponding to a relatively low thrust coefficient. According to the theory, this efficiency peak may approach unity. For smaller $k$ , other less pronounced local maxima of the propulsive efficiency are attained for pitching Points ahead of the leading edge, with larger thrust coefficients. The linear theory also predicts that no thrust is generated at all for a pitching axis located between the three-quarter chord Point and the trailing edge. These findings contrast with the results obtained from the classical linear thrust by Garrick, with the addition of the same quasi-static thrust, which are also computed in the paper. For a combined heaving and pitching motion, the behaviour of the propulsive efficiency in relation to the pitching axis is qualitatively similar to that found for a pure pitching motion, for given fixed values of the feathering parameter (ratio between pitching and heaving amplitudes) and of the phase shift between the pitching and heaving motions. The peak propulsive efficiency predicted by the linear theory is for an airfoil with a pitching axis close to, but ahead of, the three-quarter chord Point, with a relatively large reduced frequency, a feathering parameter of approximately $0.9$ and a phase shift slightly smaller than  $90^{\circ }$ .

Guy Dumas - One of the best experts on this subject based on the ideXlab platform.

  • Free-pitching flapping-foil turbines with imposed sinusoidal heave
    Journal of Fluids and Structures, 2019
    Co-Authors: Matthieu Boudreau, Kevin Gunther, Guy Dumas
    Abstract:

    Abstract This work investigates the dynamics of a semi-passive flapping-foil with a prescribed sinusoidal heave motion and a passive pitch motion with the objective of extracting energy from an oncoming fluid flow. This implies that the heave motion is mechanically driven while the foil is elastically supported in pitch. The pitch motion therefore results from the interaction of the foil with the flow and its elastic supports, namely springs and dampers. Numerical simulations have been conducted at a Reynolds number of 3 . 9 × 1 0 6 based on the chord length. Positive efficiencies and periodic pitch motions of large amplitude are obtained when the frequency of the pitch motion synchronizes itself to the frequency of the prescribed heave motion. The conditions under which it happens are explored. The results of this study demonstrate that an optimal power-generation performance can be maintained over large variations of the moment of inertia and pitch stiffness, provided that they are properly scaled. This is achieved by combining these two structural parameters into a single effective parameter: the effective pitch stiffness coefficient. Moreover, four different positions of the pitch axis are considered, ranging from the leading edge to the three-Quarter-Chord Point. By adjusting the governing structural parameters adequately, efficiencies exceeding 40% can be achieved with all four positions of the pitch axis, with a maximum of 46.0% obtained when the pitch axis is located at the Quarter-Chord Point. It is found that a phase lag near 90 ° between the heave and the pitch motions is only optimal with this specific position of the pitch axis. It needs to be larger than 90 ° when the pitch axis is located upstream of the Quarter-Chord Point and smaller than 90 ° when it is located downstream of this position.

  • Investigation of the energy-extraction regime of a novel semi-passive flapping-foil turbine concept with a prescribed heave motion and a passive pitch motion
    Journal of Fluids and Structures, 2019
    Co-Authors: Matthieu Boudreau, Kevin Gunther, Guy Dumas
    Abstract:

    Abstract Due to the inherent complexity of the mechanisms needed to prescribe the heave and the pitch motions of optimal flapping-foil turbines, several research groups are now investigating the potential of using unconstrained motions. The amplitude, the phase and the frequency of such passive motions are resulting from the interaction of the foil with the flow and its elastic supports, namely springs and dampers. More specifically, this work proposes an innovative semi-passive flapping-foil turbine concept with a prescribed sinusoidal heave motion and a passive pitch motion. Two-dimensional numerical simulations have been carried out at a Reynolds number of 3 . 9 × 1 0 6 based on the chord length with a foil having its pitch axis located at the Quarter-Chord Point. A parametric study has been conducted by varying the value of the static moment, which involves the distance between the center of mass and the pitch axis, and the frequency of the prescribed heave motion. Different responses of the foil have been observed and one of them corresponds to an energy-extraction regime that is characterized by periodic limit-cycle oscillations of large amplitudes with a phase lag between the heave and the pitch motions ranging between 90 ° and 105 ° . A maximum efficiency of 45.4% has been reached, hence confirming the great potential of this turbine concept. This works shows that such good performance is achieved when the center of mass of the foil is located downstream of the pitch axis and when no leading-edge vortices are formed.

Nathaniel Osterberg - One of the best experts on this subject based on the ideXlab platform.

  • Experimental Investigation of Dynamic Stall on Pliant Wings for Micro Air Vehicles
    54th AIAA Aerospace Sciences Meeting, 2016
    Co-Authors: Nathaniel Osterberg
    Abstract:

    This work presents the experimental characterization of an elastic membrane wing in dynamic stall conditions. In this experiment, a 2:1 aspect ratio pre-tensioned membrane wing was pitched about its quarter chord Point from 10 – 20 degrees of angle of attack at reduced frequencies of 0.05 and 0.1 and Reynolds numbers of 50,000 and 66,000. The relationship between time-dependent lift and membrane shape was studied by measuring the elastic displacements of the membrane and the wing’s aerodynamic loads, using digital image correlation software and a six-axis sting balance respectively. A direct correlation between membrane normal force and displacement is observed, as well as an increase in overall lift performance for the membrane wing in dynamic pitching conditions. The increase in lift performance was greatest where reduced frequency was 0.1 and Reynolds number was 66,000. In this test condition the pitching membrane wing experienced a maximum coefficient of lift 15% greater than the same wing in a static angle of attack condition, and 40% greater than the equivalent rigid wing in static pitch condition.

Matthieu Boudreau - One of the best experts on this subject based on the ideXlab platform.

  • Free-pitching flapping-foil turbines with imposed sinusoidal heave
    Journal of Fluids and Structures, 2019
    Co-Authors: Matthieu Boudreau, Kevin Gunther, Guy Dumas
    Abstract:

    Abstract This work investigates the dynamics of a semi-passive flapping-foil with a prescribed sinusoidal heave motion and a passive pitch motion with the objective of extracting energy from an oncoming fluid flow. This implies that the heave motion is mechanically driven while the foil is elastically supported in pitch. The pitch motion therefore results from the interaction of the foil with the flow and its elastic supports, namely springs and dampers. Numerical simulations have been conducted at a Reynolds number of 3 . 9 × 1 0 6 based on the chord length. Positive efficiencies and periodic pitch motions of large amplitude are obtained when the frequency of the pitch motion synchronizes itself to the frequency of the prescribed heave motion. The conditions under which it happens are explored. The results of this study demonstrate that an optimal power-generation performance can be maintained over large variations of the moment of inertia and pitch stiffness, provided that they are properly scaled. This is achieved by combining these two structural parameters into a single effective parameter: the effective pitch stiffness coefficient. Moreover, four different positions of the pitch axis are considered, ranging from the leading edge to the three-Quarter-Chord Point. By adjusting the governing structural parameters adequately, efficiencies exceeding 40% can be achieved with all four positions of the pitch axis, with a maximum of 46.0% obtained when the pitch axis is located at the Quarter-Chord Point. It is found that a phase lag near 90 ° between the heave and the pitch motions is only optimal with this specific position of the pitch axis. It needs to be larger than 90 ° when the pitch axis is located upstream of the Quarter-Chord Point and smaller than 90 ° when it is located downstream of this position.

  • Investigation of the energy-extraction regime of a novel semi-passive flapping-foil turbine concept with a prescribed heave motion and a passive pitch motion
    Journal of Fluids and Structures, 2019
    Co-Authors: Matthieu Boudreau, Kevin Gunther, Guy Dumas
    Abstract:

    Abstract Due to the inherent complexity of the mechanisms needed to prescribe the heave and the pitch motions of optimal flapping-foil turbines, several research groups are now investigating the potential of using unconstrained motions. The amplitude, the phase and the frequency of such passive motions are resulting from the interaction of the foil with the flow and its elastic supports, namely springs and dampers. More specifically, this work proposes an innovative semi-passive flapping-foil turbine concept with a prescribed sinusoidal heave motion and a passive pitch motion. Two-dimensional numerical simulations have been carried out at a Reynolds number of 3 . 9 × 1 0 6 based on the chord length with a foil having its pitch axis located at the Quarter-Chord Point. A parametric study has been conducted by varying the value of the static moment, which involves the distance between the center of mass and the pitch axis, and the frequency of the prescribed heave motion. Different responses of the foil have been observed and one of them corresponds to an energy-extraction regime that is characterized by periodic limit-cycle oscillations of large amplitudes with a phase lag between the heave and the pitch motions ranging between 90 ° and 105 ° . A maximum efficiency of 45.4% has been reached, hence confirming the great potential of this turbine concept. This works shows that such good performance is achieved when the center of mass of the foil is located downstream of the pitch axis and when no leading-edge vortices are formed.

Kevin Gunther - One of the best experts on this subject based on the ideXlab platform.

  • Free-pitching flapping-foil turbines with imposed sinusoidal heave
    Journal of Fluids and Structures, 2019
    Co-Authors: Matthieu Boudreau, Kevin Gunther, Guy Dumas
    Abstract:

    Abstract This work investigates the dynamics of a semi-passive flapping-foil with a prescribed sinusoidal heave motion and a passive pitch motion with the objective of extracting energy from an oncoming fluid flow. This implies that the heave motion is mechanically driven while the foil is elastically supported in pitch. The pitch motion therefore results from the interaction of the foil with the flow and its elastic supports, namely springs and dampers. Numerical simulations have been conducted at a Reynolds number of 3 . 9 × 1 0 6 based on the chord length. Positive efficiencies and periodic pitch motions of large amplitude are obtained when the frequency of the pitch motion synchronizes itself to the frequency of the prescribed heave motion. The conditions under which it happens are explored. The results of this study demonstrate that an optimal power-generation performance can be maintained over large variations of the moment of inertia and pitch stiffness, provided that they are properly scaled. This is achieved by combining these two structural parameters into a single effective parameter: the effective pitch stiffness coefficient. Moreover, four different positions of the pitch axis are considered, ranging from the leading edge to the three-Quarter-Chord Point. By adjusting the governing structural parameters adequately, efficiencies exceeding 40% can be achieved with all four positions of the pitch axis, with a maximum of 46.0% obtained when the pitch axis is located at the Quarter-Chord Point. It is found that a phase lag near 90 ° between the heave and the pitch motions is only optimal with this specific position of the pitch axis. It needs to be larger than 90 ° when the pitch axis is located upstream of the Quarter-Chord Point and smaller than 90 ° when it is located downstream of this position.

  • Investigation of the energy-extraction regime of a novel semi-passive flapping-foil turbine concept with a prescribed heave motion and a passive pitch motion
    Journal of Fluids and Structures, 2019
    Co-Authors: Matthieu Boudreau, Kevin Gunther, Guy Dumas
    Abstract:

    Abstract Due to the inherent complexity of the mechanisms needed to prescribe the heave and the pitch motions of optimal flapping-foil turbines, several research groups are now investigating the potential of using unconstrained motions. The amplitude, the phase and the frequency of such passive motions are resulting from the interaction of the foil with the flow and its elastic supports, namely springs and dampers. More specifically, this work proposes an innovative semi-passive flapping-foil turbine concept with a prescribed sinusoidal heave motion and a passive pitch motion. Two-dimensional numerical simulations have been carried out at a Reynolds number of 3 . 9 × 1 0 6 based on the chord length with a foil having its pitch axis located at the Quarter-Chord Point. A parametric study has been conducted by varying the value of the static moment, which involves the distance between the center of mass and the pitch axis, and the frequency of the prescribed heave motion. Different responses of the foil have been observed and one of them corresponds to an energy-extraction regime that is characterized by periodic limit-cycle oscillations of large amplitudes with a phase lag between the heave and the pitch motions ranging between 90 ° and 105 ° . A maximum efficiency of 45.4% has been reached, hence confirming the great potential of this turbine concept. This works shows that such good performance is achieved when the center of mass of the foil is located downstream of the pitch axis and when no leading-edge vortices are formed.