The Wake Dynamics Behind a Near-Wall Square Cylinder

Particle image velocimetry is used to experimentally study the wake dynamics behind a near-wall square cylinder subjected to a thick oncoming turbulent boundary layer. The turbulent boundary layer thickness was 3.6 times the cylinder height (⁠$h$⁠) while the Reynolds number based on the freestream velocity and the cylinder height was 12,750. The gap distance (⁠$G$⁠) between the bottom face of the cylinder and the wall was varied, resulting in gap ratios (⁠$G/h$⁠) of 0, 0.3, 0.5, 1.0, 2.0, 4.0, and 8.0. The effects of varying the gap ratio on the mean flow, Reynolds stresses, triple velocity correlation, two-point autocorrelation, and the unsteady wake characteristics were examined. The results indicate that as gap ratio decreases, asymmetry in the wake flow becomes more pronounced, and the size of the mean separation bubbles increases. The magnitudes of the Reynolds stresses and triple velocity correlations generally decrease with the decreasing gap ratio. Moreover, the size of the large-scale structures increases with decreasing gap ratio, and the critical gap ratio, below which Kármán vortex shedding is suppressed, is found to be 0.3. The dominant Strouhal number in the wake flow expressed in terms of the streamwise mean velocity at the cylinder vertical midpoint increases as gap ratio decreases while that based on the freestream velocity is less sensitive to gap ratio for the offset cases (⁠$G/h >$ 0).