Spanwise correlation of unsteady lift due to vortex shedding on the square cylinder: Perspectives provided by a time–frequency analysis

Abstract

Spanwise correlation of unsteady lift due to vortex shedding is of particular interest for slender structures. In this paper, Hilbert transform and Wavelet transform were applied as time–frequency analysis tools to the sectional lift of a square cylinder. For Wavelet transform, the complex Morlet wavelet and the generalized Morse wavelet were considered as the mother wavelets, and the synchrosqueezing technique was used for post-processing. At high Reynolds number and in smooth flow, Hilbert transform and Wavelet transform mutually confirm that vortex shedding is characterized by an instantaneous shedding frequency fluctuating in real-time. As an example, at Reynolds number 3.9×104, the results of Wavelet transform suggest a standard deviation of the instantaneous shedding frequency of at least 4% about the conventional Strouhal frequency. The extracted instantaneous frequency is found stochastic, showing reasonably good Gaussian properties for its probability density function. The instantaneous frequencies simultaneously extracted at two cross sections of a spanwise separation are correlated, being the correlation coefficient lower than that calculated with fluctuating lifts. The shape of the coherence of the instantaneous frequency is found quite similar to that of turbulence, and the magnitude overall decreases with the increase of spanwise separation. The fluctuation of the instantaneous vortex shedding frequency, especially the low-frequency part (lower than about one-half of the Strouhal frequency), is believed critical in determining the spanwise correlation of unsteady lift, which was confirmed reconstructable via the extracted instantaneous shedding frequency. Finally, attention in this paper has also been paid to a reliable extraction and quantification of the concerning time–frequency characteristics. For instance, tests of the used time–frequency tools first on the purposely prepared artificial signal suggest, that the high-frequency fluctuation in the instantaneous vortex shedding frequency (higher than the Strouhal frequency) can actually not be reliably extracted.