Strouhal number is an essential dimensionless quantity for characterizing the vortex shedding frequency of the flow around a bluff body. In this study, the Strouhal number for a near–wall circular cylinder fully immerged in a shear flow was experimentally investigated within the subcritical Reynolds number range. The examined gap–to–diameter ratios (e/D) are in the range of 0.10∼2.0. The velocity fluctuations of the leewake and the flow visualization were measured with an Acoustic Doppler Velocimeter and the Particle Image Velocimetry system, respectively. The wall–proximity effects on the vortex shedding and the corresponding Strouhal number (St) were investigated experimentally. Although the regular Kármán–like vortex–shedding can be suppressed for e/D≤0.30, the power spectra for the velocity fluctuations in the leewake indicate that the characteristic frequency can still be well identified. The experimental results indicate that St increases with the decrease of e/D to a maximum value at e/D=0.40; and the maximum of St is approximately 15 percent larger than that under wall–free condition. Broadband spectrum phenomenon was observed under the very–small e/D conditions (i.e., e/D≤0.20). The shear parameter (K) of the shear flow is proposed, and its explicit expression is derived for characterizing the wall–proximity. The relationship between the Strouhal number St and e/D (or K) is finally established for 0.10≤e/D≤2.0.
When the fluid flows toward the leading edge of a circular cylinder in the absence of a plane wall, a regular pattern of vortices can be formed in the wake (Blevins, 1990). Nevertheless, for a near–wall cylinder, the wake vortex will be significantly asymmetric and the corresponding structural responses will be changed (see Daneshvar et al., 2020). The flow–structure interactions for such a near–wall cylinder have been intensively investigated by numerous researchers with wide engineering applications, e.g., the on–bottom stability (see Gao, 2017), the local scour (see Freds∅e et al., 1987; Gao et al., 2006), and the vortex–induced vibrations of submarine pipelines (see Yang et al., 2008; Wang et al., 2013; Liu and Gao, 2019).
