Analysis of mean velocity and turbulence measurements with ADCPs

• Published in: Advances in water resources 2015-07, Vol.81, p.172-185
• Main Authors: De Serio, Francesca, Mossa, Michele
• Format: Article
• Language: English
• Subjects: Bottom drag coefficient; Computational fluid dynamics; Fluid dynamics; Fluid flow; Log law velocity profile; Marine; Mathematical models; Reynolds stress; Shear stress; Stations; Turbulence; Turbulent flow; Turbulent kinetic energy; Variance method; Wind drag coefficient
• ISSN: 0309-1708; 1872-9657
• DOI: 10.1016/j.advwatres.2014.11.006

The present study examines the vertical structure of the coastal current in the inner part of the Gulf of Taranto, located in the Ionian Sea (Southern Italy), including both the Mar Grande and Mar Piccolo basins. To this aim, different measuring stations investigated by both a Vessel Mounted Acoustic Doppler Current Profiler (VM-ADCP) and a bottom fixed ADCP were taken into consideration. Two surveys were carried out in the target area on 29.12.2006 and on 11.06.2007 by the research unit of the Technical University of Bari (DICATECh Department), using a VM-ADCP to acquire the three velocity components along the water column in selected stationing points. The measurements were taken in shallow waters, under non-breaking wave conditions, offshore the surf zone. Due to the recording frequency of the instrument time-averaged vertical velocity profiles could be evaluated in these measuring stations. Water temperature and salinity were also measured at the same time and locations by means of a CTD recorder. A rigidly mounted ADCP, located on the seabed in the North-Eastern area of the Mar Grande basin, provided current data relative to the period 10–20 February 2014. Set to acquire the three velocity components with higher frequency with respect to the VM-ADCP, it allowed us to estimate the turbulent quantities such as Reynolds stresses and turbulent kinetic energy by means of the variance method. Therefore, the present research is made up of two parts. The first part examines the current pattern measured by the VM-ADCP and verifies that, for each station, the classical log law reproduces well the vertical profile of the experimental streamwise velocities extending beyond its typical limit of validity up to the surface i.e. reaching great heights above the sea bed. This behavior is quite new and not always to be expected, being generally limited to boundary layers. It has been convincingly observed in only few limited experimental works. In the present study this occurred when two conditions were met: (i) the flow was mainly unidirectional along the vertical; (ii) the interested layer was non-stratified. The second part of the research studies the turbulent statistics derived from the beam signals of the fixed ADCP by means of the variance method. This technique had the advantage of being able to measure the time evolution of the turbulent mixing throughout the entire water column, thus making it possible to perform a detailed study on momentum transfer and turb