Flow visualisation around a horizontal cylinder near a plane wall and subject to waves

International audienceThis study is motivated by the description and the understanding of the vortices formation and development near a cylinder subject to waves and under the influence of the bed. The results presents a classification of flow types for different gap-to-diameter ratio and for two cylinder diameters ðD 1 ¼10 cm and D 2 ¼4 cm, 0.8 m long) according to a systematic wave conditions range. The flow visualisations reveal different mechanisms of separation, development and growth of vortices depending on the Keulegan Carpenter number (0.5 , KC , 26) and the influence of the bed proximity. A flow asymmetry was observed between the crest and the trough of the wave and a stronger vortex activity downstream the cylinder especially for the highest gap-diameter ratios. Furthermore the vortices are rotating preferentially in the same sense than the orbital motion. The flow tends to become more similar to a planar oscillating flow when the cylinder is closer to the bed. The emergence of instabilities in the wake of the cylinder for the highest wave amplitude leads us to measure the velocities in the cylinder axis in order to quantify 3D effects