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Research Papers

Stall in Yawed Flow Conditions: A Correlation of Blade Element Momentum Predictions With Experiments

[+] Author and Article Information
Wouter Haans, Gijs van Kuik, Gerard van Bussel

Faculty of Aerospace Engineering, Kluyverweg 1, Delft University of Technology, Delft, 2629 HS, The Netherlands

Tonio Sant

Faculty of Aerospace Engineering, Kluyverweg 1, Delft University of Technology, Delft, 2629 HS, The Netherlands and  University of Malta, Msida, MDS 07, Malta

For more information on XFOIL and the software itself, available under the GNU General Public License, see http://raphael.mit.edu/xfoil/

J. Sol. Energy Eng. 128(4), 472-480 (Jul 16, 2006) (9 pages) doi:10.1115/1.2349545 History: Received April 21, 2006; Revised July 16, 2006

Yawed flow conditions introduce unsteady loads in a wind turbine that affect generated power quality and fatigue life. An unsteady phenomenon of special concern is dynamic stall, due to the significant load fluctuations associated with it. Although the assumptions underlying blade element momentum (BEM) models are totally inadequate in yawed flow conditions, these models, modified with engineering models, are still widely used in industry. It is therefore relevant to assess the capabilities of BEM models in predicting the location of dynamic stall on the blade for a rotor in yawed flow conditions. A rotor model is placed in an open jet wind tunnel and tested in yawed flow conditions. The locations of dynamic stall on the blade of a rotor model as a function of the blade position are observed. Two experimental techniques are used; tufts glued to the blade and hot-film anemometry in the near wake. The results from the two techniques are compared and possible causes for differences are identified. Furthermore, the rotor model in yaw is modeled with a simple BEM model, utilizing a Gormont dynamic stall model. The regions of dynamic stall on the blades predicted by the BEM model are compared with the experimental results. The BEM model seems capable of a crude prediction of the dynamic stall locations found for the rotor model in yawed flow conditions.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of the setup. The rotor model is set to a yaw angle Ψ, with ym the yaw axis. The traversing rig, used for hot-film anemometry in the wake, is shown too. (xm,ym,zm) is a Cartesian coordinate system, fixed to the rotor support, (r,θ,z) is a cylindrical coordinate system, fixed to the rotor support as well. zm and z both are in the direction normal to the rotor plane. The blade azimuth angle is defined as θb.

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Figure 2

W¯∕Wjet-exit¯ measured a plane coinciding with the rotor plane when Ψ=0degr, normalized with the maximum value of W¯∕Wjet-exit¯, recorded in that plane. (x,y)=(0,0) corresponds to the hub center.

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Figure 3

Rotor blade equipped with two rows of tufts, from r∕Rt=0.37 to 0.97. The trip strip at 0.1c can clearly be seen. Flow condition: Ψ=45deg, λ=8, photo taken: θb=180deg.

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Figure 4

Single-film hot-film probe of the “normal” type, with probe coordinate system (xp,yp,zp). For the wake measurements, the probe is aligned with the rotor z axis, the sensor is oriented vertically; yp is parallel to ym.

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Figure 5

Ensemble average ⟨Veff⟩ and standard deviation sVeff of 54 Veff samples per blade azimuth angle θb for (r∕Rt,θ,z∕c)=(0.8,135deg,0.44). Blade passage: θb≈135deg,315deg, wake passage: θb≈0deg,180deg. Two setups are plotted, one with two clean blades and one where trip strips are glued to the two blades, at 0.1c. The symmetry between the two sets of blade and wake passages, one for each blade, is significantly improved for the trip strip setup.

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Figure 6

Tuft recordings at the θb=60deg blade position, for λ=8 (top photo) and 5.5 (bottom photo). As both conditions have identical rotational frequency f, the differences in tuft orientations are due to changes in aerodynamic loads on the tufts only. For the λ=5.5 condition, stall is identified on the first 5 and 4 inboard tufts for the leading edge and trailing edge rows of tufts, respectively.

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Figure 7

Ensemble mean ⟨Veff⟩ and standard deviation sVeff at two locations 0.44c downstream of the rotor plane: (r∕Rt,θ)=(0.6,240deg) left and (r∕Rt,θ)=(0.6,60deg) right. Blade passage: θb≈60deg,240deg. The left plot (typical signal I) is associated with (mostly) attached local blade flow, the right plot (typical signal II) with local stall on the blades.

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Figure 8

Schematic of velocity and force decompositions a blade element, both in cylindrical (θ,z) coordinates and in Cartesian (η,ζ) coordinates, parallel and normal to the local blade chord

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Figure 9

cl versus α for two-dimensional wind tunnel measurement on a clean airfoil at Rec=1.5∙105, see Vermeer (17) and from XFOIL computations, with transition fixed on the upper surface at 0.1c. Approximation from steady thin airfoil theory is included.

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Figure 10

BEM model prediction of cl versus α on blade stations r∕Rt=0.4(★) and r∕Rt=0.8(엯). λ=5.5. At r∕Rt=0.4, dynamic stall is predicted

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Figure 11

Normalized α and cl versus θb for the r=0.4Rt-blade station, λ=5.5. The α distribution resembles a 1P harmonic. The cl distribution shows the point of stall and reattachment.

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Figure 12

Boundary of the stall region on the blades for a full cycle, observed with two experimental techniques: tufts (엯) and hot-film anemometry (★). (a) λ=5.5, (b) λ=8.

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Figure 13

Boundary of the stall region on the blades for a full cycle, observed with tufts (엯), hot-film anemometry (★), and predicted with a BEM model (◇). (a) λ=5.5, (b) λ=8.

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Figure 14

Boundary of the stall region on the blades for a full cycle predicted with a BEM model (◇). A criterium is imposed; at a given blade station αmax≥αstall,2d. Tuft (엯) and hot-film anemometry (★) measurements are included as well.

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