INTERFERENCE BETWEEN A CIRCULAR-CYLINDER AND A PLANE WALL UNDER WAVES

The how around a circular cylinder placed close to a plane boundary un der water waves and the correlated hydrodynamic forces have been inves tigated. The Keulegan-Carpenter number was fixed at 4.9 and the Reynol ds number at 1800. The study comprises laser Doppler velocity measurem ents in the vicinity of the cylinder for two typical gap-to-diameter r atios, e/D. In the first case, the gap between the cylinder and the pl ane boundary was lower than the thickness of the oscillatory boundary layer on the wall. In the second case, the cylinder was placed far fro m the wall boundary layer. Distinct vortex flow regimes have been obse rved and different mechanisms of wall effect have been pointed out in the two cases. The hydrodynamic forces exerted on the cylinder were me asured in a range of e/D from 0.04 to 1.5. Force measurements and flow regimes were correlated and the variation of the force coefficients w as interpreted by using a parameter involving the thickness of the osc illatory wall boundary layer. Force coefficient variations obtained in purely oscillatory flow studies may be similarly interpreted.