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The Science of Making Torque from Wind (TORQUE 2018)

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Room: BL.28 Carassa e Dadda
Chaired by:
Flemming Rasmussen | DTU
Topic: AA. Aerodynamics and Aeroacoustics
Form of presentation: Oral
Duration: 90 minutes

Authors:
Peter McKeich Jamieson, J. Michael R. Graham, Edward Hart, Alexander Giles

Abstract:
The general momentum theory of Glauert has given rise to many different simplified or approximate models of blade element momentum theory. The main obstacle to formulating the general momentum theory as a closed system of equations rigorously consistent with axisymmetric potential flow through an actuator disc with a rotating wake lies in obtaining a valid analytic form of the axial force balance. A valid integral form of axial force balance on the whole streamtube bounding the actuator disc has long been established for propellors and wind turbines but the most obvious differential form was shown to be incorrect by Goorjian in 1972. A new analytic differential form is derived from first principles and, in conjunction with an analytic form derived for the far wake pressure, this enables formulation of the general momentum equations in terms of five key variables, the far wake radius and the axial and tangential induction factors at the plane of the actuator disc (rotor) and in the far wake.

Authors:
Gijs van Kuik

Abstract:
Actuator disc theory is the basis for rotor design and analysis, valid for discs representing wind turbine rotors as well as propellers. In Froude’s momentum theory swirl is absent, in Joukowsky’s momentum theory this is included. The momentum theory including swirl, developed in WES, 2:307-316,2017, as well as potential flow calculations have been expanded to propeller discs. For infinite tip speed ratio the classical Froude results are recovered. For low values of the tip speed ratio the propeller discs show an expanding instead of contracting wake, like wind turbine discs. Both flow regimes show a complete blockage of the flow for a non-zero minimum tip speed ratio. For all wind turbine discs, so irrespective of the tip speed ratio, the velocity in the meridian plane is constant at the disc, for all propeller discs this is not constant.

Authors:
Emil Krog Kruse, Niels N. Sørensen, Christian Bak

Abstract:
Leading Edge Roughness (LER) has become a critical challenge for wind turbine owners, as it can reduce the energy production of their turbines considerably. LER has yet to be systematically categorized, and this makes it problematic to predict its impact. A common method for emulating LER is to equip a test blade section with zigzag tape and examine its effect in wind tunnel tests. This study focuses on a geometrical representation of zigzag tape in both 2D and 3D CFD simulations, with wind tunnel testing on a NACA 63-418 airfoil to evaluate the changes in aerodynamic characteristics. As 3D CFD requires an immense amount of computing power, the study also explores the possibility of limiting simulations to 2D.

Authors:
Gael de Oliveir

Abstract:
Wind turbine performance depends on the quality of aerodynamic profiles and the ability to optimize airfoils depends on the accuracy of polar prediction codes. The present contribution unveils a machine-learning method for improving the accuracy of viscous-inviscid interaction codes like Xfoil or Rfoil. New turbulent closure relations are inferred from publicly available polar data to obtain an improved version of the Rfoil code. The closure relations are parametrized with a custom class-shape transformation approach and a database of airfoil polar data is used to define a formal measure of code inaccuracy. Code inaccuracy metrics are then minimized with an interior point gradient algorithm. The resulting version of the Rfoil code provides enhanced predictions of maximum lift and glide-ratio, and is compatible with airfoil optimization usages.

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