Detecting wave-induced flow velocities around a smooth circular cylinder, fixed horizontally near the bed of a wave channel, represents an important part in physical modeling and in the knowledge of the complex hydrodynamic phenomena under such structures. The experimental investigation consists of the measurement of the velocity field with a Laser Doppler Anemometer (LDA) system, especially on the wave boundary layer around the cylinder. The Laser head is located inside the structure and the beams are focussed on the flow to permit the velocity measurement near the cylinder surface with a high spatial resolution.
In prototype scale, the bed proximity of pipe-lines and the separation effects, driven by the boundary layer characteristics, plays a significant role for the hydrodynamic loads. The wave induced flow add a complexity combination such as the acceleration of the flow, the orbital motion and also the boundary layer thickness of the incoming wave. In order to study the wall proximity effect, three different gap ratio e/D are selected (e/D equal to 0.02, 0.08 and 0.5); Different regimes of the Keulegan Carpenter number enable to investigate the wake formation varying from a creeping flow type (KC = 1.5) to a separated flow (KC = 8.8) through attached vortices cases (KC = 2.8 and KC = 4.8). The first part of the results is concerned with the wave velocity profiles and also the wave boundary layer characteristics (thickness, time dependent features, separation on the bed). The analysis of the velocities distribution around the cylinder provides a quantification of the boundary layer thickness and its dynamic evolution in connection with the water surface elevation. For the smallest gap ratio, the high velocities illustrate the effect of a jet-like flow between the bed and the cylinder.
