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標題: Lock-on characteristics behind two side-by-side cylinders of diameter ratio two at small gap ratio
作者: Petry, Y.R.Lee
Lu, Wei-Ting
Chou, Shy-Tsin
Kuo, Cheng-Hsiung
Project: Experiments in Fluids, Volume 53, Issue 4, Page(s) 891-908.
The lock-on characteristics, the detailed interactions
and downstream evolutions of the wakes behind
side-by-side cylinders of unequal diameter (D/d = 2),
spaced by a gap ratio 0.75 (G/D = 0.75), are investigated
at Reynolds number 600 by the dye flow visualization,
laser Doppler anemometry (LDA) and particle image
velocimeter (PIV) velocity measurements. The lock-on
frequency bands are studied by LDA and PIV at Reynolds
number 2,000. The D, d and G are the diameters of the
large, the small cylinders and the net gap between two
cylinders, respectively. Periodic excitations, in form of
rotary oscillation about the cylinder center, are applied to
the large cylinder with the same amplitude. It is found that
while the large cylinder is excited, two lock-on frequency
bands of the wake behind the large cylinder are detected.
These two lock-on frequency bands correspond to the
primary and the one-third sub-harmonic lock-on of the
wake behind large cylinder, respectively. These two lockon
frequency bands distribute symmetrically about the
fundamental and the third superharmonic of the natural
shedding frequency behind a single cylinder at the same
Reynolds number. The left-shifted frequency band
(1.8 B fe/fos B 2.0) is not considered as a locked-on frequency
band because the phase difference between two
excitation frequencies across fe/fos = 2.0 vary significantly.
While the wake behind the large cylinder is locked-on at
fe/3 (or fos), the gap flow becomes unbiased and the frequency
of the wake behind small cylinder remains around
the natural shedding frequency. Thus, the frequency band
of 3.0 B fe/fos B 3.22 is also not locked-on because the
phase difference in the narrow wake excited at fe/fos = 2.93
and 3.07 changes significantly. Note fe and fos denote the
excitation frequency and the natural shedding frequency
behind a single large cylinder, respectively.
ISSN: 0723-4864
DOI: 10.1007/s00348-012-1335-z
Appears in Collections:機械工程學系所

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