On the bistable phenomenon of a finite length square cylinder wake with POD method

he near wake of a 2D square cylinder and that of a finite length square cylinder with the aspect ratio of 7 was measured by PIV. The Reynolds number based on free stream oncoming flow and cylinder width is 9300. Proper orthogonal decomposition (POD) was performed to analysis the contributions of each mode on turbulence kinetic energy (TKE). Instantaneous flow properties were compared between the near wake of finite length and 2D square cylinders, utilizing the coefficients of POD modes for flow reconstruction. It was found that, in 2D cylinder wake, the first 2 POD modes, which represent the Karman vortex shedding, dominated the near wake and contributed 73.6% of total TKE; while in the finite length square cylinder wake, the TKE contribution from the first two modes was only 45.9%. There were two typical instantaneous flow status occur in the finite length cylinder wake: one was the status of Karman vortex shedding dominated by the first two POD modes, similar to that in a 2D cylinder wake; the other was symmetrical flow status which was dominated by the third and/or other higher POD modes.