Reynolds number effects on the flow structure behind two side-by-side cylinders

Abstract

The wake structure of two side-by-side cylinders was experimentally investigated using the laser-induced fluorescence flow visualization, particle image velocimetry and hot-wire techniques. The investigation was focused on the asymmetrical flow regime, i.e., T/d=1.2–1.6, where T is the center-to-center cylinder spacing and d is the cylinder diameter. Experiments were conducted in both the water tunnel and the wind tunnel at a Reynolds number (Re) range of 150–14 300. It has been found that, as Re increases, the flow structure behind the cylinders may change from one single vortex street to two streets with one narrow and one wide for the same T/d. The one-street flow structure is dominated by one frequency f0*=f0d/U∞≈0.09, where f0 is the dominant frequency and U∞ is the free-stream velocity. On the other hand, two frequencies, f0*≈0.3 and 0.09, characterized the two-street flow structure. These are associated with the narrow and wide street, respectively. It is further observed that the critical Re, at which the transition from single to two streets occurs, increases as T/d decreases. The present finding clarifies previous scattered reports for 1.2<T/d<1.5: the detection of one dominant frequency by some but two by others.