Experiments were carried out with two bluff cylinders, i. e., a circular
cylinder and a trapezoidal cylinder, of aspect ratios ranging from 3 to 12.5,
at Reynolds numbers of 104. For each cylinder, measurements were made
with two hot-wire probes situated in the near wake region and an array of
MEMS thermal film sensors situated spanwisely on the cylinder surface.
The three-dimensional characteristics of vortex shedding were thus
examined by spanwise correlation, Wavelet analysis and 2-D Fourier
transformation of the signals measured. Results obtained show that more
events of three-dimensional vortex shedding could be identified at larger
aspect ratios, as indicated by a probability value defined as the time
durations of these events versus the total time sampled. It also found that
the probability values of the circular cylinder were higher than those of the
trapezoidal cylinder. 2-D FFT analysis of the signal traces of the multiple
MEMS sensors indicates that vortex shedding around a circular cylinder had
spanwise wave lengths greater than 1.5 times the diameter of the cylinder.
In contrast, the vortex shedding structures associated with the trapezoidal
cylinder appeared to be more two-dimensional, but the vortex shedding
frequencies tended to be spread over a wider range.