Wind Energy

212 H. Guo et al.
Flow
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(a) Sketch of side view (b) Sketch of front view (c) Torque resisting system
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4-1
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Fig. 38.1. The test setup: 1 – airfoil, 2 – side plates, 3 – airfoil supporters, 4 –
torque resisting system, 5 – setup base, 6 – connection pole, 7 – force sensor system,
8 – water tunnel, 9 – cover plate, and 4-1 – U shaped connectors, 4-2 – circular
plate, 4-3 – pins around which the connectors can turn freely
chord length of c =0.1 m is selected as the test model, and the test Reynolds
number in terms of the airfoil chord-length is Re =5× 10 3 . The test angle of
attack of the airfoil ranges from 0 ◦ to 80 ◦ .
A 3-component force sensor, 9251A, made by Kistler Company, was used
to measure the instantaneous lift and drag simultaneously. The test setup
is shown in Fig. 38.1. The force sensor had to be positioned out of the test
section due to its non-water resistant. A unique torque resisting system, shown
in Fig. 38.1c, was designed to successfully avoid the influence of any torque.
Two side-plates are used to prevent 3-D flow effects.
Particle image velocimetry (PIV) and laser sheet visualization were also
used to obtain quantitative information and detailed flow phenomena.
38.3 Experimental Results and Discussions
The typical power spectrum density (PSD) results of the lift at α = 20 ◦
and 40 ◦ and the corresponding PIV results are presented in Fig. 38.2a,b,
respectively, where γ is the circulation Γ around the PIV view field divided
by its area. The PSD results of lift will reflect its response to the flow field
unsteady properties which are shown by the temporal changes of γ. Fromthe
comparison, following findings are summarized:
1. At lower angles of attack, α ≤10 ◦ , there is no evident peak in the PSD
curve of lift except at f ∗ =1.2, the self-vibration frequency of model.
2. At α = 20 ◦ , a peak appears at f ∗ = 0.16 Hz in the PSD curve of
lift measurements, and grows to its maximum with the angle of attack
increasing to α =40 ◦ , when γ nearly has the same primary frequency
of f ∗ = 0.18. Such synchronization implies that the second peak lift
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