Study of the Characteristics of the Turbulent Boundary Layer by the PIV-Method under Laboratory Modelling of the Flow above the Seabed

This article is devoted to the development and application system for Particle Image Velocimetry (PIV) measurements of velocity fields in a turbulent boundary layer within experiments carrying out on the modified hydrodynamic flume of the Atlantic Branch of the Shirshov Institute of Oceanology of the Russian Academy of Sciences. This study is concerned with the preparation of laboratory setup for investigations on the initiation of motion of microplastic particles from the bottom covered with natural bottom sediment. The conditions of three types of bottom material (coarse sand (1–1.5 mm), amber grains (3–4 mm), pebbles (1–2 cm)) were investigated. The developed system of PIV-measurements uses a digital camera of medium framerate and continuous wave laser illumination. The experiments were performed in a wide range of flow velocities, up to 2 m/s, controlled by a pump rate and a control valve position. Average velocity profiles were measured, and the slope of their logarithmic part was used to calculate the corresponding friction velocity. The friction velocity and the bottom roughness are the crucial parameters for the assessment of sedimentation/initiation of motion processes. Friction velocities over sand and amber appear to be similar and significantly smaller than that for pebbles. The pebble roughness scale turned out to be the maximum, as expected. The amber simulated the bottom of intermediate roughness. For this case, due to its relatively low density (1.07 g/cm3), we observed the beginning of particle motion at velocities lower than those necessary for the initiation of sand grain movement. There was a good agreement between the observed threshold of motion for amber with available data obtained by A. Shields. The advantage of PIV-measurements for further investigations of the microplastic dynamics is the possibility of simultaneous measurement of almost instant velocity parameters in the boundary layer together with the observation of microplastic particles added at the bottom surface.

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