Wavelet based detection of vortex shedding around a cylinder oscillating in still water

Document Type : Original Article

Authors

1 Assistant Professor, Road, Housing & Urban Development Research Center;

2 Associate professor, School of Civil Engineering, University of Tehran

3 Assistant Professor, School of Civil Engineering, University of Tehran, Tehran, Iran

Abstract

The present paper aims to study the lift forces acting on a cylinder oscillating in still water from spectral point of view. In the previous researches, vortex shedding frequency has been related to the fundamental lift frequency. Here, the wavelet analysis is used as a relatively new concept in spectral analyses. The simultaneous time-frequency representations of the lift forces are investigated to localize the flow induced transitory characteristics. The peaks in the wavelet coefficients attributed to vortex shedding are studied. The abrupt changes in the lift forces have also been studied by discrete wavelet decomposition. Small spikes have been observed in the results due to vortex shedding. Wavelet analysis is considered as an efficient alternative method to predict vortex shedding for larger Keulegan-Carpenter numbers (KC). Two different gap-to-diameter ratios, 0.1 and 1.0, are considered to account for the effect of bed proximity. Regular vortex shedding is suppressed for lower gap ratios; this fact is confirmed by wavelet analysis as well. The KC numbers in the present study are in the range of 15 to 40. The flow is in the subcritical regime with Reynolds number in the range of 9500-26000. The cylinder and the plane bed are both smooth.

Keywords