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Abstract: Flow characteristics in the near wake of a circular cylinder located close to a fully developed turbulent boundary layer are investigated experimentally using particle image velocimetry (PIV). The Reynolds number based on the cylinder diameter (D) is 1.2×104 and the incident boundary layer thickness (δ) is 0.4D. Detailed velocity and vorticity fields in the wake region (0

Abstract: The flow patterns in the near wake of a cylinder (either circular or square in shape, D=25mm) placed in the proximity of a fully developed turbulent boundary layer (thickness δ=0.4D) are investigated experimentally using particle image velocimetry (PIV). The effects of changing the gap height (S) between the cylinder bottom and the wall surface, over the gap ratio range S/D=0.1–1.0, have been investigated. The results show that both the ensemble-averaged and instantaneous flow fields are strongly dependent on S/D. The flow patterns for the two types of cylinders share many similarities with respect to the change in S/D, such as the reduced recirculation length and increased velocity fluctuation in the near wake with increasing S/D, as well as the trend of suppression of vortex shedding at small S/D and onset of vortex shedding at large S/D. However, developments of the shear layers, in terms of wake width, flow curvature, etc., are considerably different for these two types of cylinders. In general, the wake development and momentum exchange for the square cylinder are slower those for the circular cylinder at the same gap ratio. Correspondingly, it is shown that the periodic vortex shedding is delayed and weakened in the case of square cylinder, as compared to that of the circular cylinder at the same S/D. [Copyright &y& Elsevier]

This paper presents an experimental study of the structural dynamics and the wake vortex modes of a lightly damped, elastically mounted cylinder undergoing vortex-induced vibration (VIV) in the transverse direction. The cylinder is neutrally buoyant with the mass ratio m * = 1.0, and has a low damping coefficient ζ = 0.0173. The influence of variation in the free-stream velocity (corresponding to the reduced velocity range 1.53 ≤ U * ≤ 6.62, or the Reynolds number range 3000 ≤ Re ≤ 13,000) on the structural dynamics of the cylinder is examined, in conjunction with detailed flow measurements around the cylinder using digital Particle Image Velocimetry (PIV). The cylinder is stationary at U * ≤ 2.55, and begins to vibrate at U * ≥ 3.05 (or in synchronization regime). The vibrating cylinder exhibits the so-called “soft” synchronization phenomenon that the vortex shedding frequency is synchronous with the structural vibration frequency (or response frequency) of the cylinder, rather than with the natural frequency of the system. The measured data for the cylinder undergoing VIV fall within the initial branch and the upper branches, and agree well with published data on free vibrating cylinders at similar low mass and damping. The transition from initial- to upper-branch is characterized by a switch of vortex formation mode from the classical 2S mode to the newly-discovered 2P O mode by Morse and Williamson (2009, Journal of Fluid Mechanics, Vol. 634, pp. 5–39) based on a forced vibrating cylinder. The distinction between the 2S and 2P O modes is vivid in terms of length- and velocity-scales of the shed vortices, and is also reflected by quantitative characterization of near-wake flow statistics, including the mean and turbulence properties. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to investigate the flow around two tandem cylinders placed near and parallel to a plane wall. The Reynolds number based on the cylinder diameter ( D ) was 6300. The cylinder centre-to-centre spacing ratio ( L ⁎ = L / D ) was varied from 1.5 to 6, and the gap-height-to-cylinder-diameter ratio ( G ⁎ = G / D ) from 0.15 to 2. The flow fields were measured using Particle Image Velocimetry (PIV), in conjunction with measurements of fluid dynamic forces (drag and lift) on the downstream cylinder using load cell. The flow strongly depends on the combined value of G ⁎ and L ⁎ . With reference to G ⁎ , the flow could be classified as vortex-shedding suppression regime ( G ⁎ <0.3), intermediate-gap regime (0.3< G ⁎ <1) where vortex shedding occurs but is influenced by wall proximity, and large-gap regime ( G ⁎ >1) where the wall influence becomes negligible. Similarly, three categories can be identified as a function of L ⁎ , namely, extended-body regime 1< L ⁎ <2, reattachment regime at 2< L ⁎ <4, and impinging regime at L ⁎ >4. Variations of dynamic drag and lift coefficients, spectra, Strouhal numbers, and Reynolds shear stress are also presented to characterize the different flow regimes in the G ⁎ – L ⁎ plane. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of the flow around four circular cylinders arranged in a square configuration. The Reynolds number was fixed at Re=8000, the pitch-to-diameter ratio between adjacent cylinders was varied from P/D=2 to 5 and the incidence angle was changed from α=0° (in-line square configuration) to 45° (diamond configuration) at an interval of 7.5°. The flow field was measured using digital Particle Image Velocimetry (PIV) to examine the vortex shedding characteristics of the cylinders, together with direct measurement of fluid dynamic forces (lift and drag) on each cylinder using a piezoelectric load cell. Depending on the pitch ratio, the flow could be broadly classified as shielding regime (P/D≤2), shear layer reattachment regime (2.5≤P/D≤3.5) and vortex impinging regime (P/D≥4). However, this classification is valid only in the case that the cylinder array is arranged nearly in-line with the free stream (α≈0°), because the flow is also sensitive to α. As α increases from 0° to 45°, each cylinder experiences a transition of vortex shedding pattern from a one-frequency mode to a two-frequency mode. The flow interference among the cylinders is complicated, which could be non-synchronous, quasi-periodic or synchronized with a definite phase relationship with other cylinders depending on the combined value of α and P/D. The change in vortex pattern is also reflected by some integral parameters of the flow such as force coefficients, power spectra and Strouhal numbers. [ABSTRACT FROM AUTHOR]