Drag crisis of a circular cylinder near a plane boundary

The pressure distribution and the hydrodynamic forces on a circular cylinder placed near a plane boundary are investigated experimentally over a range of Reynolds numbers (Re) of 1.1 × 105-4.3 × 105 and gap (G) to cylinder diameter (D) ratio (G/D) of 0-1.0. The objective of the study is to quantify the influence of G/D on the force coefficients when the boundary layer on the cylinder surface transits from laminar to turbulent. The hydrodynamic forces acting on the cylinder are obtained by integrating the measured pressure around the cylinder surface. A significant drag reduction from about 0.9 to 0.35 is observed for G/D≥0.5 in the range of Re = 1.9 × 105-2.7 × 105. At smaller G/D values of 0.25 and 0.1, the drag coefficient shows much less reduction than those observed at larger G/D values. No obvious drag reduction is found at G/D  = 0.01 and 0. Based on the observed features of pressure distributions and force coefficients, the boundary layer transition from laminar to turbulent is inferred for all the gap ratios (G/D = 0 ∼ ∞).