M. Ben Meftah

Scour holes downstream of bed sills in low-gradient channels

An experimental study on long local scouring downstream of bed sills in a monogranular sand bed was carried out in the hydraulic laboratory flume at the Mediterranean Agronomic Institute of Bari (Italy). The main objectives of this study were to determine scour hole dimension, with its maximum scour depth as a function of time and at the equilibrium stage, the scour hole shapes and the investigation of the influence of sills on the distribution of the three-velocity components through the scour hole at the same stage. Four experimental configurations were tested, the main difference between them being the distance between sills. Based on experimental data, the classical dimensional analysis of the variables that influence the development of the scour hole has been carried out in order to obtain two empirical formulas predicting the maximum scour depth and the length of the scour hole at the equilibrium stage. Moreover, it was observed that the distance between sills influences the scour hole dimension and shape. The three-velocity components of the flow, measured with an acoustic Doppler velocimeter, show that in the scour hole, at the equilibrium stage, the three components of the flow turbulence intensities are very high. Near-bed flow vortexes in addition to secondary currents are also observed.

Prediction of channel flow characteristics through square arrays of emergent cylinders

This paper describes a theoretical approach of turbulent flow in an infinite square array of emergent cylinders distributed uniformly along a channel bottom. Laboratory experiments were carried out to investigate the flow velocity characteristics in order to confirm this theoretical approach. A model array of vertical, rigid, circular, and threaded steel cylinders, of regular distribution, was mounted on the bed of a recirculating hydraulic channel. Measurements of the three-dimensional flow velocity components were carried out using a 3D acoustic Doppler velocimeter. At a certain distance downstream of a lateral row of cylinders, the transversal profiles of velocities and turbulence parameters are expected to be periodic functions in the lateral position whose period is the centre-to-centre spacing of two side-by-side cylinders. Some empirical expressions for predicting the turbulent flow features have been established. The formulation proposed is consistent with the experimental data over a certain rang...

Vertical dense jet in flowing current

The discharge of brackish water, as a dense jet in a natural water body, by the osmotic power plants, undergoes complex mixing processes and has significant environmental impacts. This paper focuses on the mixing processes that develop when a dense round jet outfall perpendicularly enters a shallow flowing current. Extensive experimental measurements of both the salinity and the velocity flow fields were conducted to investigate the hydrodynamic jet behavior within the ambient current. Experiments were carried out in a closed circuit flume at the Coastal Engineering Laboratory (LIC) of the Technical University of Bari (Italy). The salinity concentration and velocity fields were analyzed, providing a more thorough knowledge about the main features of the jet behavior within the ambient flow, such as the jet penetration, spreading, dilution, terminal rise height and its impact point with the flume lower boundary. In this study, special attention is given to understand and confirm the conjecture, not yet experimentally demonstrated, of the development and orientation of the jet vortex structures. Results show that the dense jet is almost characterized by two distinct phases: a rapid ascent phase and a gradually descent phase. The measured flow velocity fields definitely confirm the formation of the counter-rotating vortices pair, within the jet cross-section, during both the ascent and descent phases. Nevertheless, the experimental results show that the counter-rotating vortices pair of both phases (ascent and descent) are of opposite rotational direction.

Physical modelling of buoyant effluents discharged into a cross flow

This study focuses on physical modelling of turbulent vertical buoyant jets, discharged into a transversal current and interacting with localized background turbulence. The physical model was developed in the Coastal Engineering Laboratory of the Technical University of Bari. The physical model consists of a sophisticated system that allows to monitor and adjust all the characteristic parameters of both the channel flow (e.g. discharge, flow depth) and the buoyant jets (e.g. flow rate, temperature, salinity). Positively and negatively buoyant jets are realized by discharging water respectively at a temperature and salinity higher than that of the receiving environment. Due to the complexity of the jet-current hydrodynamic phenomena, a set of sophisticated instruments to measure the jet spreading within the cross flow is used. The average jet dilution is measured by (i) four different Resistance Temperature Detectors (RTD) for the positively buoyant jet, and (ii) a MicroScale Conductivity Temperature Instrument (MSCTI) of high resolution for the negatively buoyant jet. Whereas, a Nortek Acoustic Doppler Velocimeter (ADV) system is used to measure the field flow velocities, together with CollectV software for data acquisition and ExploreV software for data analysis. The measured scalar and vector fields will be illustrated in this paper, with the aim to emphasize that a well-set physical model is able to explain the behavior of buoyant jets in an open channel with ambient factors, such as cross flow and vegetation.

Semi enclosed basin monitoring and analysis of meteo, wave, tide and current data: Sea monitoring

The present paper aims to show and discuss the long term and continuous recordings of both meteorological and hydrodynamic data collected in a semi enclosed sea. The site in question is composed by the Mar Grande and Mar Piccolo basins (Southern Italy), which are mutually connected. In turn, the Mar Grande is joined to the Ionian Sea by means of two openings. Therefore, the system shows features typical of a lagunar environment, which is also affected by coastal heavy industry and anthropic pressure, thus being highly vulnerable. A monitoring of its hydrodynamics could be useful, allowing both to check the real-time status of the basin and promptly intervene when accidents occur and to create a dataset necessary to calibrate and validate modelling systems providing forecasts. To this, in the framework of the Italian flagship Project RITMARE, a meteo-oceanographic station, a wave-current meter and a tide gauge have been installed in the area, since December 2013. In detail, measurements of wind, waves, tides and current profiles, are acquired on site with different sampling frequencies and are transmitted on a web cloud by a router 3G, where they are stored, thus being available for download by remote users. The data acquisition and processing is managed by the research group of the Department of Civil, Environmental, Building, Engineering and Chemistry (Technical University of Bari). All the acquired data are archived in monthly time-series, examined and discussed. The analysis of currents is made in two different measuring stations for the whole year 2015, as well as the analysis of wave data. On the contrary, tide data have been assessing only recently, since August 2015. Comparisons with available recordings of the year 2014 are examined. Also spatial and temporal correlations of both waves and currents are discussed. Finally, tidal trends are shown, consistent with current inversions.