Flow characteristics of circular cylinders arranged side-by-side in shallow water

Abstract

The flow around two and three side-by-side circular cylinders of equal diameter in shallow water was investigated experimentally using the particle image velocimetry (PIV) technique, over a transverse gap ratio in the range of G/ D=1.0–3.0 with an increment of 0.25. The experiments were conducted in a closed-loop water channel with Reynolds number 5000 based on the cylinder diameter. In addition to the PIV measurements, flow visualization studies were also carried out. The PIV technique provided instantaneous and averaged patterns of velocity, vorticity and Reynolds stress contours and streamline topology. For the two side-by-side cylinder case, it was found that the flow structure behind the cylinders is asymmetrical at small gap ratios as a result of jet-like flow between the cylinders. The jet-like flow tends to deflect toward the narrow wake region that has a higher vortex shedding frequency. In the case of three cylinders, both an asymmetrical flow structure at small gap ratio ( G/ D=1.25) and a symmetrical flow structure at intermediate gap ratios (1.5≤ G/ D≤2.0) were observed. Bistable wake regions were obtained for the asymmetrical cases. The Reynolds stress ( u′ v′) downstream of the upper cylinder was significantly attenuated for G/ D=1.25 where the jet-like flow is more effective for both two and three circular cylinder arrangements.