Characteristics of square cylinder wake transition flows

The wake transition regime in square cylinder flow is investigated experimentally. Mode A and B transitions similar to those captured in circular cylinder flows are found to exist in this regime and by using dye- and laser-induced fluorescence visualization, the spanwise and streamwise vortex structures are captured. Upon comparison with their corresponding modes in circular cylinder flows, some differences are noted. Most notably, the critical Reynolds numbers at which Mode A and B occur for square cylinder flows were estimated to be approximately 160 and 200, respectively, and are lower than those found in circular cylinder flows, which are generally agreed to be approximately 188-190 and 230-260, respectively. Also, the spanwise wavelengths for the two modes in square cylinder flows (5.2 D(s) and 1.2 D(s) for Modes A and B, respectively) are longer than their counterparts in circular cylinder flows (3-4D(o) and 0.8-1D(o), respectively). Furthermore, by using a hot wire to measure the shedding frequency, it was found that the Mode A transition here does not exhibit a hysteresis phenomenon, which is unlike the case in circular cylinder flows. The Mode A to B transition here is also not associated with a sudden increase in Strouhal number, which is again unlike the corresponding situation in circular cylinder flows. However, the vortical structures are noted to be similar between the corresponding modes for the two bluff body shapes, and hence the formation mechanism can be deemed to be similar. (C) 2003 American Institute of Physics.