Reynolds Number Dependence of the Flow Structure Behind Two Side-by-Side Cylinders

The wake structure of two side-by-side cylinders was experimentally investigated using flow visualization and hotwire 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 water and wind tunnels at a Reynolds number (Re) range of 150 – 14300. It has been found that, as Re increases, the flow structure behind the cylinders would 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 ƒ₀^* = ƒ₀d/U_∞ ≈ 0.09, where ƒ₀ is the dominant frequency and U_∞ is the free-stream velocity. On the other hand, two frequencies, ƒ₀^* ≈ 0.3 and 0.09, characterized the two-street flow structure. These are associated with the narrow and wide street frequency, respectively. It is further observed that the critical Re, at which transition from single to two streets occurs, increases as T/d decreases. The present finding help clarify previous scattered reports for 1.2 < T/d < 1.5: detection of one dominant frequency by some but two by others.