Intrinsic features of flow around two side-by-side square cylinders

The wake of two side-by-side square cylinders is investigated in detail based on flow visualization at a Reynolds number (Re) of 300. The cylinder center-to-center spacing ratio T* (= T/W, W is the cylinder width) is varied from 1.0 to 5.0. The intrinsic features of the wake are explored, including the gap vortices, flow switch, stability, merging of two streets into one, etc. The qualitative information on these features is further complemented by the quantitative information extracted from hotwire data at Re = 4.7 104 using both spectral and cross-wavelet analyses. Four flow regimes are identified: (i) the single bluff body regime (T* < 1.2), (ii) the narrow and wide street regime (1.2 < T* < 2.1), (iii) the transition regime (2.1 < T* < 2.4), and (iv) the coupled-street regime (T* > 2.4). The gap flow is found to switch at two distinct time scales, referred to as macro and micro switches. Macro switch occurs at 1.2 < T* < 2.1, where the gap flow is slim in width and biased for a long duration ranging from several vortex shedding periods in the wide street to several hours. Micro switch is observed at 2.1 < T* < 2.4, where the gap flow is thick in width and switches at a frequency of twice the frequency of vortex shedding from the free-stream side of the cylinders. The two streets formed immediately behind the cylinders merge to a single street at a downstream location $x_c angle $xc* that depends on T*. A larger T* corresponds to a larger $x_c angle $xc*. Regime (iv) is characterized by two streets either inphased or antiphased. The antiphased streets persist longer than the inphased.