Turbulence structure in intermediate wake of a circular cylinder

The large-scale and small-scale structures in the intermediate wake of a circular cylinder and their interrelations are studied by a phase-averaging method at a Reynolds number of 16,000. It is found that the transverse velocity fluctuation at each measurement point can serve as a proper reference signal to phase-average the large-scale structures. Thus, this reference signal can reduce the phase jitter. The vorticity of the large-scale structures attains a maximum at their centers, with contours of constant vorticity approximating concentric circles. A mechanism is proposed for the entrainment by large-scale structures of the irrotational fluid outside the wake and for the diffusion of their vorticity. An irrotational fluid is shown to exist between two successive large-scale structures.