Influence of wall proximity on vortex shedding from a square cylinder

Mean and fluctuating surface pressure data are presented for a square cylinder of side length D placed near a solid wall at Re(D)=18,900. One oncoming boundary layer thickness, delta=0.5 D was used. Measurements were made for cylinder to wall gap heights, S, from S/D=0.07 to 1.6. Four gap-dependent flow regimes were found. For S/D > 0.9, the flow and the vortex shedding strength are similar to the no-wall case. Below the critical gap height of 0.3D, periodic activity is fully suppressed in the near wake region. In between, for 0.3 < S/D < 0.9, the wall exerts a greater influence on the flow. For 0.6 < S/D < 0.9, the mean drag and the strength of the shed vortices decrease as the gap is reduced, while the mean lift towards the wall increases. Evidence is presented that for S/D > 0.6 the influence of the viscous wall flow in the gap is not dominant and that, consequently, inviscid flow theory can describe changes in the mean lift as S/D decreases. For 0.3 < S/D < 0.6, the flow reattaches intermittently on the bottom face of the cylinder and viscous effects become important. Below the gap height of 0.4D, periodic activity cannot be observed on the cylinder. (Author)