Experiments on the flow past a square cylinder placed near a wall

Experiments are reported on the flow past a square cylinder placed near a wall at Re = 22,000 (based on the cylinder diameter, D). Visualization studies were carried out for various widths G of the gap between cylinder and wall. Over the range of the dimensionless gap width G/D = 0.35-0.5, the fraction of time with periodic shedding motion increases from zero to one. Below this range, the shedding motion is completely suppressed and, above it, regular shedding occurs at all times. Detailed two-component laser-Doppler velocimeter measurements were carried out for a gap width of G/D = 0.75. Phase-averaged statistics were obtained with a special signal-processing procedure; the phase relation was determined from a pressure signal measured on the cylinder side wall. In addition to time-averaged mean-velocity and stress components, phase-averaged components are presented. A detailed comparison is made with the results obtained previously for the flow past a free-standing cylinder away from walls. The Strouhal number is roughly the same and so are many flow features, particularly in the base region near the cylinder, but further downstream the flow with a nearby wall develops a distinct asymmetry, with the vortex trajectories moving away from the wall and the wake developing an oblique character. Differences are noted between the inner and the outer vortices, and the greatest differences in the free-standing cylinder case are the considerably larger values of the u fluctuations in the near wake. The flow topology was found to be similar to that in the flow past two side-by-side cylinders. (C) Elsevier Science Inc., 1996