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Three-dimensional (3-D) wake transitions of a steady flow past two side-by-side circular cylinders are investigated through Floquet analysis and direct numerical simulations (DNS) over the gap-to-diameter ratio up to 3.5 and Reynolds number up to 400. When the flows behind two cylinders form in-phase and anti-phase wakes at large, the wake transition is similar to the isolated cylinder counterpart, with the critical for the onset of 3-D transition happens at around 180. At small, 3-D transition becomes interestingly complex due to the distinct characteristics formed in base flows. The suddenly drops to around 60-100 and forms distinct variation trends with. Precisely, the of the single symmetric wake (SS,) is more than half of the isolated cylinder counterpart due to the large length scale of the SS wake. Only mode A is detected in SS. In the asymmetric single wake (ASS,) and flip-flop wake (FF,), the 3-D transition develops at and 75-60, respectively. The decrease in with increasing is because of the increased level of wake asymmetry in ASS and irregular vortex shedding in FF. Floquet analysis predicts two new unstable modes, namely mode A and subharmonic mode C, of ASS. Both modes are transient features in 3-D DNS and the flow eventually saturates into a new 3-D mode, mode ASS. The gap flow of mode ASS is distinctly characterised by its time-independent spanwise waviness structure that is deflected towards different transverse directions with a long wavelength of about cylinder diameters. The 3-D mode of the FF is irregular both temporally and spatially. Variations of with, the characteristics and the physical mechanisms of each 3-D mode are discussed in this study.
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