Comparison between spanwise characteristics of vortex shedding from circular and trapezoidal cylinders

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.