Francesco Lalli

Full Field Measurements in a River Mouth by Means of Particle Tracking Velocimetry

The measurement of marine streams is a difficult task even for well tested velocimetry methods based on advanced image analysis. The wide size of the measurement domain (from hundreds to thousands meters), the extended range of flow velocities (from mm/s to several m/s), the long time-scales (from minutes to weeks) and the related length and time scales of vortical structures involved in marine dynamics do not allow a straightforward application of Particle Image Velocimetry (PIV) and related methods. Due to the previous requirements, a natural choice is to consider Lagrangian “tracers” to be introduced in the fluid as candidates to sample the flow field in time. Therefore, the most useful measurement technique which enables to derive global velocity data from Lagrangian “tracers” is Particle Tracking Velocimetry, PTV (Tropea & Foss, 2007). As will be outlined in section 3, a large variety of Lagrangian buoys have been designed in the past to probe large-scale marine motion (larger than 50 km), whereas a lot of work is still necessary regarding the measurements at smaller scales. This aspect must be also considered within the debate on the effective role of mesoscales (around 10 km) and small-scales (100 m) in generating the “local” geophysical turbulence field (submesoscales) in the sea and how this dynamics is also dependent on the large-scales (Kanarska et al. 2007, Ozgokmen 2011). Therefore, on-site measurements of flow at submesoscale level by using effective Lagrangian buoys are needed (Griffa et al. 2007, Haza et al. 2010, Wang 2010).