Downstream Development of the Wakes behind Cylinders

Wake development behind circular cylinders and flat plates was investigated in the research water tanks. The aluminium dust method was used to observe the flow patterns. At the intermediate Reynolds number range the Kármán vortex streets are formed in the wakes behind cylindrical obstacles. But these primary Kármán vortex streets are not stable. They are more and more deformed as the distance from the obstacle is increased, and finally break down. Thereafter, however, the wake in most cases begins to rearrange itself again into a configuration of the Kármán vortex street. The dimension of the secondary Kármán vortex street thus produced is much larger than that of the primary one. For the circular cylinder, when Reynolds number is lower than about 150, the ratio of the wave length of the secondary vortex street to that of the primary one is about 1.8 to 3.6, while it is about 10 when Reynolds number is higher than about 150. Sometimes the secondary Kármán vortex street simply decays by viscous diffusion as it moves downstream, but in most cases it rearranges itself again to the next Kármán vortex street.