C. Jin, I. Potts, D. C. Swailes, M. W. Reeks 16 correlation which covers the upper bound of sub-critical Reynolds number range. C ′ L does not display much variation when the Reynolds number remains below the critical value. Richter and Naudascher[42]’s data , which are extrapolated from their experimental observations performed at a smooth circular cylinder in a wind-tunnel with a blockage ratio of 1 / 4, are included for further comparisons. It can observed that, when interpreting C ′ L in terms of u$_{o}$ , the values of C ′ L for downstream cylinders are significantly higher than that of an unconfined circular cylinder and do not fall in the scatter region of an confined circular cylinder ([42]). This indicates that the vortex shedding from downstream cylinders may be augmented further by the feature of approaching wake turbulence from upstream cylinders. Consequently, it yields considerably higher values of C ′ L on downstream cylinders as presented in figure 9 in which C ′ L is interpreted with the free stream velocity u$_{o}$ . Moreover, two interesting points can be derived from figure 8 and 9. First, as far as the fluctuating magnitudes of the two variables are concerned, it can be observed that the first column displays smaller values than downstream cylinders. The second observation is that the maximum fluctuation of C ′ L is achieved on the second or the third column cylinder. A similar phenomena was reported by Liang and Papadakis. [21]. Figure 10 presents close-up views of an instantaneous velocity vector map in the middle plane of the flow domain around the four cylinders C1, C2, C3, C4. In accordance with the results of previous researchers ([14]), in the sub-critical regimes, the separating shear layers becomes turbulent. In figure 10a, it can be observed that small-scale vortexes are being formed in the shear layers behind C1. Nevertheless, such small vortexes appear not to be formed behind the downstream cylinders as shown in figure 10b c d. This can be explained that the approaching stream for the downstream cylinders, the wake of upstream cylinders, is of particularly high turbulence level, so that there is no transition that is closely connected with the vortex formation in the shear layer. 3.4 Shear-layer instability and vortex shedding C ′ L = 0 . 52 − 0 . 06 × [ log ( Re/ 1600)] − 2 . 6 (5 . 4 × 10 3 < Re < 2 . 2 × 10 $^{5}$) (28)