The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform

Fritz-olaf Lehmann - One of the best experts on this subject based on the ideXlab platform.

  • three dimensional wing structure attenuates Aerodynamic Efficiency in flapping fly wings
    Journal of the Royal Society Interface, 2020
    Co-Authors: Henjaniniane Wehmann, Thomas Engels, Fritz-olaf Lehmann
    Abstract:

    The aerial performance of flying insects ultimately depends on how flapping wings interact with the surrounding air. It has previously been suggested that the wing's three-dimensional camber and corrugation help to stiffen the wing against Aerodynamic and inertial loading during flapping motion. Their contribution to Aerodynamic force production, however, is under debate. Here, we investigated the potential benefit of three-dimensional wing shape in three different-sized species of flies using models of micro-computed tomography-scanned natural wings and models in which we removed either the wing's camber, corrugation, or both properties. Forces and Aerodynamic power requirements during root flapping were derived from three-dimensional computational fluid dynamics modelling. Our data show that three-dimensional camber has no benefit for lift production and attenuates Rankine-Froude flight Efficiency by up to approximately 12% compared to a flat wing. Moreover, we did not find evidence for lift-enhancing trapped vortices in corrugation valleys at Reynolds numbers between 137 and 1623. We found, however, that in all tested insect species, Aerodynamic pressure distribution during flapping is closely aligned to the wing's venation pattern. Altogether, our study strongly supports the assumption that the wing's three-dimensional structure provides mechanical support against external forces rather than improving lift or saving energetic costs associated with active wing flapping.

  • Phasing of dragonfly wings can improve Aerodynamic Efficiency by removing swirl
    Journal of the Royal Society Interface, 2008
    Co-Authors: James R. Usherwood, Fritz-olaf Lehmann
    Abstract:

    Dragonflies are dramatic, successful aerial predators, notable for their flight agility and endurance. Further, they are highly capable of low-speed, hovering and even backwards flight. While insects have repeatedly modified or reduced one pair of wings, or mechanically coupled their fore and hind wings, dragonflies and damselflies have maintained their distinctive, independently controllable, four-winged form for over 300 Myr. Despite efforts at understanding the implications of flapping flight with two pairs of wings, previous studies have generally painted a rather disappointing picture: interaction between fore and hind wings reduces the lift compared with two pairs of wings operating in isolation. Here, we demonstrate with a mechanical model dragonfly that, despite presenting no advantage in terms of lift, flying with two pairs of wings can be highly effective at improving Aerodynamic Efficiency. This is achieved by recovering energy from the wake wasted as swirl in a manner analogous to coaxial contra-rotating helicopter rotors. With the appropriate fore–hind wing phasing, Aerodynamic power requirements can be reduced up to 22 per cent compared with a single pair of wings, indicating one advantage of four-winged flying that may apply to both dragonflies and, in the future, biomimetic micro air vehicles.

Sunil K Agrawal - One of the best experts on this subject based on the ideXlab platform.

Niels N Sorensen - One of the best experts on this subject based on the ideXlab platform.

  • design of a wind turbine rotor for maximum Aerodynamic Efficiency
    Wind Energy, 2009
    Co-Authors: Jeppe Johansen, Helge Aagaard Madsen, Mac Gaunaa, Christian Bak, Niels N Sorensen
    Abstract:

    The design of a three-bladed wind turbine rotor is described, where the main focus has been highest possible mechanical power coefficient, CP, at a single operational condition. Structural, as well as off-design, issues are not considered, leading to a purely theoretical design for investigating maximum Aerodynamic Efficiency. The rotor is designed assuming constant induction for most of the blade span, but near the tip region, a constant load is assumed instead. The rotor design is obtained using an actuator disc model, and is subsequently verified using both a free-wake lifting line method and a full three-dimensional Navier–Stokes solver. Excellent agreement is obtained using the three models. Global CP reaches a value of slightly above 0.51, while global thrust coefficient CT is 0.87. The local power coefficient Cp increases to slightly above the Betz limit on the inner part of the rotor; the local thrust coefficient Ct increases to a value above 1.1. This agrees well with the theory of de Vries, which states that including the effect of the low pressure behind the centre of the rotor stemming from the increased rotation, both Cp and Ct will increase towards the root. Towards the tip, both Cp and Ct decrease due to tip corrections as well as drag. Copyright © 2008 John Wiley & Sons, Ltd.

  • 3D Navier-Stokes Simulations of a rotor designed for Maximum Aerodynamic Efficiency
    45th AIAA Aerospace Sciences Meeting and Exhibit, 2007
    Co-Authors: Jeppe Johansen, Mac Gaunaa, Christian Bak, Niels N Sorensen
    Abstract:

    The present paper describes the design of a three-bladed wind turbine rotor taking into account maximum Aerodynamic Efficiency only and not considering structural as well as offdesign issues. The rotor was designed assuming constant induction for most of the blade span, but near the tip region a constant load was assumed. The rotor design was obtained using an Actuator Disc model and was subsequently verified using both a free wake Lifting Line method and a full 3D Navier-Stokes solver. Excellent agreement was obtained using the three models. Global mechanical power coefficient, CP, reached a value of slightly above 0.51, while global thrust coefficient, CT, was 0.87. The local power coefficient, Cp, increased to slightly above the Betz limit on the inner part of the rotor as well as the local thrust coefficient, Ct, increased to a value above 1.1. This agrees well with the theory of de Vries which states that including the effect of the low pressure behind the centre of the rotor stemming from the increased rotation both Cp and Ct will increase towards the root. Towards the tip both Cp and Ct decrease due to tip corrections as well as drag.

Jeppe Johansen - One of the best experts on this subject based on the ideXlab platform.

  • design of a wind turbine rotor for maximum Aerodynamic Efficiency
    Wind Energy, 2009
    Co-Authors: Jeppe Johansen, Helge Aagaard Madsen, Mac Gaunaa, Christian Bak, Niels N Sorensen
    Abstract:

    The design of a three-bladed wind turbine rotor is described, where the main focus has been highest possible mechanical power coefficient, CP, at a single operational condition. Structural, as well as off-design, issues are not considered, leading to a purely theoretical design for investigating maximum Aerodynamic Efficiency. The rotor is designed assuming constant induction for most of the blade span, but near the tip region, a constant load is assumed instead. The rotor design is obtained using an actuator disc model, and is subsequently verified using both a free-wake lifting line method and a full three-dimensional Navier–Stokes solver. Excellent agreement is obtained using the three models. Global CP reaches a value of slightly above 0.51, while global thrust coefficient CT is 0.87. The local power coefficient Cp increases to slightly above the Betz limit on the inner part of the rotor; the local thrust coefficient Ct increases to a value above 1.1. This agrees well with the theory of de Vries, which states that including the effect of the low pressure behind the centre of the rotor stemming from the increased rotation, both Cp and Ct will increase towards the root. Towards the tip, both Cp and Ct decrease due to tip corrections as well as drag. Copyright © 2008 John Wiley & Sons, Ltd.

  • Determination of the Maximum Aerodynamic Efficiency of Wind Turbine Rotors with Winglets
    Journal of Physics: Conference Series, 2007
    Co-Authors: Mac Gaunaa, Jeppe Johansen
    Abstract:

    The present work contains theoretical considerations and computational results on the nature of using winglets on wind turbines. The theoretical results presented show that the power augmentation obtainable with winglets is due to a reduction of tip-effects, and is not, as believed up to now, caused by the downwind vorticity shift due to downwind winglets. The numerical work includes optimization of the power coefficient for a given tip speed ratio and geometry of the span using a newly developed free wake lifting line code, which takes into account also viscous effects and self induced forces. Validation of the new code with CFD results for a rotor without winglets showed very good agreement. Results from the new code with winglets indicate that downwind winglets are superior to upwind ones with respect to optimization of Cp, and that the increase in power production is less than what may be obtained by a simple extension of the wing in the radial direction. The computations also show that shorter downwind winglets (>2%) come close to the increase in Cp obtained by a radial extension of the wing. Lastly, the results from the code are used to design a rotor with a 2% downwind winglet, which is computed using the Navier-Stokes solver EllipSys3D. These computations show that further work is needed to validate the FWLL code for cases where the rotor is equipped with winglets.

  • 3D Navier-Stokes Simulations of a rotor designed for Maximum Aerodynamic Efficiency
    45th AIAA Aerospace Sciences Meeting and Exhibit, 2007
    Co-Authors: Jeppe Johansen, Mac Gaunaa, Christian Bak, Niels N Sorensen
    Abstract:

    The present paper describes the design of a three-bladed wind turbine rotor taking into account maximum Aerodynamic Efficiency only and not considering structural as well as offdesign issues. The rotor was designed assuming constant induction for most of the blade span, but near the tip region a constant load was assumed. The rotor design was obtained using an Actuator Disc model and was subsequently verified using both a free wake Lifting Line method and a full 3D Navier-Stokes solver. Excellent agreement was obtained using the three models. Global mechanical power coefficient, CP, reached a value of slightly above 0.51, while global thrust coefficient, CT, was 0.87. The local power coefficient, Cp, increased to slightly above the Betz limit on the inner part of the rotor as well as the local thrust coefficient, Ct, increased to a value above 1.1. This agrees well with the theory of de Vries which states that including the effect of the low pressure behind the centre of the rotor stemming from the increased rotation both Cp and Ct will increase towards the root. Towards the tip both Cp and Ct decrease due to tip corrections as well as drag.

Zaeem A Khan - One of the best experts on this subject based on the ideXlab platform.