Chaired by: J. Sørensen, S. Ivanell
13:30
Momentum theory of Joukowsky actuator discs with swirl
Prof. Gijs van Kuik | TU Delft | Netherlands
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Prof. Gijs van Kuik | TU Delft | Netherlands
Actuator disc theory is the basis for most rotor design methods, be it with many extensions and engineering rules added to make it a well-established method. However, the off-design condition of a very low rotational speed Ω of the disc is still a topic for scientific discussions. Several authors have presented solutions of the associated momentum theory for actuator discs with a constant circulation, the so-called Joukowsky discs, showing the efficiency Cp→ꝏ for Ω→ 0. The momentum balance is very sensitive to the choice of the vortex core radius as the pressure and velocity gradients become infinite for Ω→ 0. Viscous vortex cores do not show this singular behaviour so an inviscid core model is sought which removes the momentum balance sensitivity to singular flow. A vortex core with a constant diameter does so. Applying this results in Cp→ 0 for Ω→ 0, instead of Cp→ꝏ. The Joukowsky actuator disc theory is confirmed by a very good match with numerically obtained results. It gives higher Cp values than corresponding solutions for discs with a Goldstein-based wake circulation published in literature.
13:50
How does the presence of a body affect the performance of an actuator disk
Gael de Oliveira | TU-Delft | Netherlands
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Authors:
Gael de Oliveira | TU-Delft | Netherlands
Ricardo Balbino Pereira | Netherlands
Danielle Ragni | Netherlands
Francesco Avallone | Netherlands
Gerard van Bussel | Netherlands
The article seeks to unify the treatment of conservative force interactions between axisymmetric bodies and actuators in inviscid flow. Applications include the study of hub interference, diffuser augmented wind turbines and boundary layer ingestion propeller configurations. The conservation equations are integrated over infinitesimal streamtubes to obtain an exact momentum model contemplating the interaction between an actuator and a nearby body. No assumptions on the shape or topology of the body are made besides (axi)symmetry. Laws are derived for the thrust coefficient, power coefficient and propulsive efficiency. The proposed methodology is articulated with previous efforts and validated against the numerical predictions of a planar vorticity equation solver. Very good agreement is obtained between the analytical and numerical methods.
14:10
Extension of Goldstein’s circulation function for optimal rotors with hub
Prof. Wen Zhong Shen | DTU | Denmark
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Authors:
Prof. Wen Zhong Shen | DTU | Denmark
Jens N. Sørensen | Denmark
Okulov Valery | Denmark
The aerodynamic interaction or interference between rotor blades and hub body is usually very complicated, but some useful simplifications can be made by considering the hub with an infinite cylinder. Various attempts to find the optimum distribution of circulation by the lifting vortex lines method have been previously proposed to describe the blade interaction with the hub modeled by the infinity-cylinder. In this case, the ideal distribution of bound circulation on the rotor blades is such that the shed vortex system in the hub-area is a set of helicoidal vortex sheets moving uniformly as if rigid, exactly as in the case where there is no influence of the streamtube deformations by the central hub-body. In the present investigation, we consider a more specific problem of the rotor-hub interaction where the initial flow streamtubes and the rotor slipstream submit strong deformations at the nose-area of the semi-infinite hub.
14:30
Comparison of classical methods for blade design and the influence of tip correction on rotor performance
Prof. Jens Nørkær Sørensen | DTU | Denmark
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Authors:
Prof. Jens Nørkær Sørensen | DTU | Denmark
Valery Okulov | Denmark
Robert Mikkelsen | Denmark
Igor Naumov | Denmark
Ivan Litvinov | Denmark
The paper contains a comparative study of optimum rotor design employing the classical blade-element/momentum (BE/M) method of Glauert and the blade-element/lifting-line (BE/LL) method by Betz. First, expressions are given for the optimum design, including blade plan forms and local pitch distributions. The comparison shows that the differences in geometry mainly exist at the inner part of the blade and for relatively small tip speed ratios (TSR<5). An important conclusion is that an infinity-bladed approach combined with a tip correction results in a geometry which is nearly identical to the geometry generated from a finite-bladed approach. To verify the design results, an experimental investigation is carried out to study the influence on rotor performance between the two models, and in particular the influence of the tip correction. Employing BE/M without the tip correction (Glauert rotor) and BE/LL with Goldstein's circulation (Betz rotor), two different 3-bladed rotors were designed and manufactured. The rotors are investigated experimentally in a water flume and preliminary results show that the Betz rotor yields the best performance.
14:50
Towards an Engineering Model for the Aerodynamic Forces Acting on Wind Turbine Blades in Quasisteady Standstill and Blade Installation Situations
Mac Gaunaa | DTU | Denmark
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Authors:
Mac Gaunaa | DTU | Denmark
Joachim Heinz | Denmark
Witold Skrzypinski | Denmark
The crossflow principle is a key element used in engineering models for prediction of the aerodynamic loads on wind turbine blades in standstill or blade installation situations, where the flow direction relative to the wind turbine blade has a component in the direction of the blade span direction. In this work, the performance of the crossflow principle is assessed on the DTU 10MW reference blade using 3D CFD calculations. Analysis of the CFD results shows that there is only a narrow region where the crossflow principle describes the aerodynamic loading well. In some conditions the deviation of the crossflow predicted loadings can be quite significant, having a large influence on the integral loads that are very important for single blade installation applications. The main features of these deviations have a systematic behavior on all force components, which is employed to formulate the first version of a correction method to the crossflow principle applicable for wind turbine blades. The new correction model improves the agreement with CFD results for the key aerodynamic loads in crossflow situations.
15:10
Impact of flow inclination on downwind turbine loads and power
Dr. Christian Kress | ETH ZÜRICH | Switzerland
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Authors:
Dr. Christian Kress | ETH ZÜRICH | Switzerland
Ndaona Chokani | Switzerland
Reza S Abhari | Switzerland
Takashi Hashimoto | Switzerland
Masatoshi Watanabe | Switzerland
Takahiko Sano | Switzerland
Mitsuru Saeki | Switzerland
Wind turbines frequently operate under situations of pronounced flow inclinations, such as in complex terrain. In the present work the performance and rotor thrust of downwind and upwind turbines in upward and downward flow inclinations are experimentally investigated. In an upward flow inclination of +13°, downwind turbines are shown to have a 29% larger power output than a corresponding upwind turbine, whereas the relative increase in rotor thrust is only 9%. Furthermore, it is also shown that the performance of downwind turbines is less sensitive to changes in the flow inclination, as the upstream nacelle on downwind turbines beneficially redirects and accelerates the flow around the nacelle into the rotor plane.