The effects of wall proximity on vortex shedding from a square cylinder: Three-dimensional effects

The three-dimensional nature of turbulent vortex shedding from a square cylinder in the vicinity of a solid wall is investigated for a Reynolds number of 18 900 as a function of the gap height, S/D. Spanwise surface pressure measurements on the cylinder faces and on the solid wall are complemented by velocimetry data. It is observed that parallel and oblique shedding modes arise naturally. The number of vortex dislocations is clearly related to the variations in the oblique shedding angle. Dislocations occur with increasing probability as the gap height is decreased to S/Dapproximate to0.7. The dislocations are strongly associated with Type A instabilities and vortex splitting, which contribute significantly to phase-jitter. For gap heights close to that for vortex shedding suppression (0.5<S/D<0.7), dislocations occur less frequently. The vortex formation process is increasingly two-dimensional in this range, resulting in strong spanwise correlations and lower phase-jitter. These changes are observed as more sharply peaked frequency spectra, which have been interpreted as shedding frequency "tuning" in earlier studies. (C) 2002 American Institute of Physics.